<b>Bijsluiter</b>. De hyperlink naar het originele document werkt niet meer. Daarom laat Woogle de tekst zien die in dat document stond. Deze tekst kan vreemde foutieve woorden of zinnen bevatten en de opmaak kan verdwenen of veranderd zijn. Dit komt door het zwartlakken van vertrouwelijke informatie of doordat de tekst niet digitaal beschikbaar was en dus ingescand en vervolgens via OCR weer ingelezen is. Voor het originele document, neem contact op met de Woo-contactpersoon van het bestuursorgaan.<br><br>====================================================================== Pagina 1 ======================================================================

<pre>HELLE
Health effects of low level exposures
Gezondheidseffecten van lage blootstellingniveaus
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<pre></pre>

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<pre>Aanbiedingsbrief Nederlands</pre>

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<pre></pre>

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<pre>Aanbiedingsbrief Engels</pre>

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<pre></pre>

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<pre>HELLE
Health effects of low level exposures
Gezondheidseffecten van lage blootstellingniveaus
Health Council of the Netherlands
Gezondheidsraad
to
the Minister of Housing, Spatial Planning, and the Environment
aan
de Minister van Volkshuisvesting, Ruimtelijke Ordening en Milieubeheer
Nr 1998/18, Den Haag, 26 November 1998
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<pre>Preferred citation:
Health Council of the Netherlands: HELLE; Health effects of low level exposures. The
Hague: Health Council of the Netherlands, 1998; publication no. 1998/18.
Deze publicatie kan als volgt worden aangehaald:
Gezondheidsraad: HELLE; Gezondheidseffecten van lage blootstellingniveaus. Den
Haag: Gezondheidsraad, 1998; publicatie nr 1998/18.
all rights reserved
auteursrecht voorbehouden
ISBN: 90-5549-242-6
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<pre>  Contents
1 The nature of the problem 11
2 The state of knowledge 13
3 The implications for risk assessment 17
  Annexes 19
A Drafting of the report 21
B Brief explanation of a number of concepts 23
C Conference report 25
D The debate about low levels of exposure 45
E Note for the conference 59
F Conference programme 67
G Participants and Scientific Advisory Committee 71
  Dutch translation of the report 75
  Nederlandse vertaling van het advies 75
9 Contents
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<pre>10 HELLE</pre>

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<pre>Chapter 1
        The nature of the problem
        The Health Council is closely involved in establishing the scientific foundation of
        exposure limits for substances and radiation in order to protect public health. Through
        the years, the Council has contributed to the formulation of principles and procedures,
        both for carcinogenic and for noncarcinogenic agents. As a rule, the discussion with
        regard to the derivation of health-based recommended exposure limits centres around
        the appropriateness of extrapolation methods (What can be inferred from data on high
        exposure levels and on experimental animals?). Generally speaking, there is a lack of
        direct information on the health effects of low levels of exposure. Effects at these levels
        cannot usually be detected by means of traditional animal experiments or
        epidemiological research. The capacity of these analytical instruments to distinguish
        between ‘signal’ and ‘noise’ is inadequate in most cases. Annex B of this report
        contains a brief outline of the difficulties and the established methods for tackling this
        problem.
            In spite of this, the hope exists that the posited weak signals, if they are indeed
        present, can be detected by other means. The search will have to take place on a deeper
        level. In other words, an effort must be made to discover what occurs at underlying
        levels of biological organization when organisms are exposed to low doses of radiation
        or substances. Molecular and cell biology provide various methods and techniques
        which give an insight into the processes within the cell. This results in an increase in
        the knowledge about the molecular and cellular effects of exposure to agents, or stated
        differently, the working mechanisms which form the basis of the health effects. Last
        year, the Health Council considered that the time was ripe to take stock of the state of
11      The nature of the problem
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<pre>   knowledge in this field. To this end, an international working conference was held from
   19 to 21 October 1997, entitled ‘Health Effects of Low Level Exposures: Scientific
   Developments and Perspectives for Risk Assessment’.
       The central question was the extent to which the sometimes fast-growing
   knowledge about molecular and cellular effects offers the desired basis for
   extrapolation. Against this setting, a number of more specific questions which have
   been hotly debated for some time were also addressed. One of the primary questions
   concerned the traditional but increasingly questioned division between stochastic and
   non-stochastic working agents, and the corresponding division between exposure-effect
   relations without a threshold and with a threshold (see Annex B for a concise
   explanation). Thoughts were also exchanged on what is often referred to as hormesis:
   the notion that low levels of exposure could actually improve health. For the purpose of
   illuminating the many aspects of these issues, experts from a number of areas were
   invited. In addition to this, three agents were selected to serve as points of
   crystallization for the general debate: ionizing radiation, ultraviolet (UV) radiation and
   dioxins.
       The present report calls attention to a selection of issues which emerged during the
   discussions on the above-mentioned central topic. Various more detailed questions and
   the wider context of the points considered are described at greater length in the
   enclosed conference report (Annex C) and in the background documents attached to the
   report (Annexes D and E). What follows is a series of considerations regarding the
   scientific basis for the derivation of recommended exposure levels, viewed in the light
   of current procedures and against the background of the work of the Health Council. In
   the preparation of the following comments and recommendations, various Dutch
   experts have been consulted (see Annex A).
12 HELLE
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<pre>Chapter 2
        The state of knowledge
        The participants were of the opinion that it would not be possible to formulate a general
        answer to the central question of the conference. Relatively speaking, a great deal is
        known about the working mechanisms of some agents, including ionising radiation,
        UV radiation and dioxins. Even with this knowledge, numerous problems stand in the
        way of achieving a far-reaching quantitative model for exposure-effect relations based
        on molecular and cell biology. For example, there is still only limited insight into how
        diseases and disorders which may partly be caused by the agents referred to come
        about. Even in cases where the molecular-biological foundation has been discovered to
        a large extent, some stages in the process from events within the cell to the
        manifestation of health problems remain a mystery. During the working conference, it
        was pointed out on several occasions that, in order to achieve a sound understanding,
        various levels of biological organization must be taken into account. As crucial as the
        study of molecular-biological processes may be, research with a more physiological
        orientation is just as important. On the other hand, the above-mentioned problems
        concerning the causes of disease can be put into perspective by realizing that it need
        not be essential to know all the details of every step of the process. Imagine that certain
        molecular or physiological biomarkers (more of which later) could be clearly linked to
        certain forms of exposure and health effects. In such a case, a detailed description of
        the intermediate process would be superfluous in terms of risk assessment. This is not
        to say that such information will not be extremely valuable for other purposes, such as
        the development of medical interventions.
13      The state of knowledge
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<pre>        If one examines pathogenetic processes from the perspective of risk factors, further
   difficulties present themselves. The conference participants pointed out that a whole
   range of phenomena can occur within a cell under the influence of xenobiotic agents.
   These include changes in gene expression, mutations and death of the cell due to
   apoptosis (programmed cell death) or necrosis (other types of cell death). It is also
   possible that such interactions do not leave clear traces behind. In many cases, not
   enough is known about the changes which can be brought about by specific agents.
   Consequently researchers often find themselves in the dark as to the possible health
   effects of these alterations, whether this means an increased risk of cancer, the
   acceleration of the ageing process or the disturbance of certain organ functions.
   Meanwhile there is yet another obstacle which affects this entire process: the usually
   unknown relationship between cellular processes (and their resulting effect) and the
   degree and rate of exposure.
   There are therefore a multitude of questions, but we generally possess very little
   information with which to formulate answers, even in cases where general information
   on molecular and cellular processes is readily available. Mechanistic modelling of
   exposure-effect relations looks like remaining an unattainable goal in the immediate
   future, or at least modelling of the entire range of pathogenetic processes. However, an
   ongoing stream of knowledge about certain subprocesses of exposure to specific
   xenobiotic agents is available. As already indicated, this can sometimes be sufficient
   for assessing risk.
        Interesting developments are taking place in such areas as toxicokinetics and
   toxicodynamics. By finding out how substances behave when absorbed into the body, it
   is possible to obtain a clearer picture of the biologically relevant (effective) exposure.
   With the help of so-called PBPK/PD models (PBPK/PD: physiologically based
   pharmacokinetic and pharmacodynamic) toxicologists are attempting to describe
   explicitly and systematically the associated distribution and metabolic processes. In
   the last few years, for example, a great deal of research has been done with dioxins.
   The validation of these kinds of models is generally recognized as a research priority
   and it is expected that approach will reduce such problems as ‘animal to human’
   extrapolation.
        An extension of the developments just outlined is the promising research into
   biomarkers for internal exposure and for high susceptibility. This could mean that
   people with certain genetic characteristics could be adversely affected by exposure to
   certain agents at an earlier stage or to a greater extent. If such biomarkers are traced
   and used successfully in phenomenological research, this will increase the statistical
   power of these analyses. Biomarkers for early effects, that is to say effects which
   precede the manifestation of health problems, will probably be longer in coming.
14 HELLE
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<pre>        Trends in the field of transgenesis (the transplantation of desired hereditary
   characteristics into the genome of a laboratory animal) also look very promising. By
   deactivating target genes in mice, it is possible to disengage one or more specific
   cellular processes, such as DNA repair and metabolism of chemical substances. In this
   way it is possible to analyze how such processes influence the effect of exposure to
   xenobiotic agents. The strong increase in the susceptibility of some mutated mice to
   certain substances also enables the direct measurement of the effect of low dosages. In
   addition to this, transgenic mice with sensitive systems for the detection of mutations
   may be able to increase our insight into the effect of exposure to genotoxic agents in
   the near future.
        Another important consideration is the ongoing progress in the field of information
   technology which facilitates simplification and refinement in the analysis and synthesis
   of all kinds of data. The conference heard that possibly too little use is still being made
   of the existing opportunities in this field. The statistical processing of the findings of
   animal experiments and epidemiological research, where possible enriched with
   information on working mechanisms, can provide further indications on the degree of
   uncertainty governing the establishment of exposure-effect relations and the derivation
   of recommended exposure limits. Such methods of analysis can sometimes offer a
   definitive answer as to the probable existence of a threshold dose for the occurrence of
   certain effects.
   The actual circumstances in which exposure takes place can be seen as a separate issue.
   In practice there is always a combined influence of a broader or narrower spectrum of
   endogenous and exogenous factors, with partly corresponding working mechanisms.
   One example is the production of so-called free radicals (certain reactive molecules) by
   the normal oxygen metabolism and by exposure to ionising radiation. At the
   conference, two questions were raised with regard to this issue.
        Firstly, it may be worth calling into question whether it is useful or meaningful to
   derive exposure-effect relations for individual agents. This immediately gives rise to a
   second question, namely which basic principles should be brought to bear when
   determining the relations between combination-exposure and health effects. The
   deliberations at the conference failed to produce a theoretical direction in which a
   solution should be sought.
        The second question concerns the possibility of hormesis. The idea that exposure to
   specific agents under certain circumstances mobilises certain reaction mechanisms
   which reduce the net damage caused by combination-exposure is not discounted by
   some researchers, while others even consider it plausible. In the general view at the
   conference, however, there was as yet no conclusive evidence to support this claim.
15 The state of knowledge
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<pre>16 HELLE</pre>

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<pre>Chapter 3
        The implications for risk assessment
        The arguments above lead to the conclusion that the present system of assessment can
        be refined on certain points and under certain conditions. Developments with regard to
        elements such as PBPK/PD modelling, biomarkers for variations in susceptibility and
        modelling methods, present opportunities in terms of a firmer foundation for elements
        or modules which occupy a place within the current system. These might take the form
        of better founded safety or extrapolation factors, which would for instance allow
        possible differences between and within species to be taken into account. Ideally, such
        factors can be replaced by models or sub-models which provide an explicit description
        of the variations. More generally, it is to be expected that the relationship between the
        components of the so-called integral toxicity profile, as described in the Health Council
        report ‘Toxicology-based recommended exposure limits’ (1996/12), can be mapped out
        in greater detail with the help of the analyses referred to here.
             As regards the circumstances under which deeper analyses appear justified,
        attention should be paid to the efficiency of the risk assessment. Thorough analyses
        like those alluded to are labour-intensive and costly to carry out and it is worth
        considering their initial application to agents with a societal priority by way of a trial.
        Criteria such as the plausibility of harmfulness at exposure levels expected in real life
        situations, the size of the population exposed, the seriousness of the effects, the
        possibility of risk reduction and the extent of the economic interests involved, could all
        be useful elements in the selection of these agents. A selection process of this kind is
        also of importance for other reasons: the risk assessment of existing chemicals on the
17      The implications for risk assessment
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<pre>   European market is proceeding rather slowly. Time and resources will need to be set
   aside in order to bring about improvements in this respect.
        When addressing the depth and appropriateness of risk assessment issues, the
   well-orchestrated input of various experts is a crucial factor. Examining each case in
   turn, and in close consultation with each other, they will have to consider which model
   best describes the whole picture formed by the available details. At this stage, it is
   difficult to make a definitive statement about the general relevance of such models. The
   conference sessions on ionising radiation, UV radiation and dioxins illustrated this
   problem (see Annex C). In any case, it is not possible at present to produce general
   recommendations with regard to basic principles for mechanistic modelling or with
   regard to the influence of homeostatic control processes. In short, experience will teach
   us the ways in which and the speed with which the present system of assessment will
   lend itself to refinement.
        In the Health Council’s 1999 Working Programme, five topics closely related to
   the issues outlined above are included under the heading ‘Principles for health-based
   recommended exposure limits’: (1) the drawing up of an integral toxicity profile; (2)
   the use of epidemiological data in the drawing up of such a profile; (3) the application
   of a so-called benchmark dose approach (the benchmark dose is the lower statistical
   confidence limit of an exposure level corresponding to a specified response level); (4)
   the use of safety margins and (5) dealing with combination exposure. In the
   Netherlands, research into a number of these topics is taking place at university
   departments, in independent and governmental research laboratories, and in industry.
   The often intensive cooperation between these institutions and their equivalent
   organizations abroad will certainly benefit the quality and the efficiency of risk
   assessment in the Netherlands.
18 HELLE
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<pre>A  Drafting of the report
B  Brief explanation of a number of concepts
C  Conference report
D  The debate about low levels of exposure
E  Note for the conference
F  Conference programme
G  Participants and Scientific Advisory Committee
   Annexes
19
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<pre>20 HELLE</pre>

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<pre>Annex A
      Drafting of the report
      The present report has been prepared by Eert Schoten, scientific secretary at the Health
      Council, after consultation of the following experts.
         dr B Brunekreef; professor of health studies; Agricultural University of
         Wageningen; The Netherlands
         dr VJ Feron; professor of biological toxicology; University of Utrecht; The
         Netherlands
         dr JHJ Hoeijmakers; professor of molecular biology; Erasmus University
         Rotterdam; The Netherlands
         dr PHM Lohman; professor of radiation genetics and chemical mutagenesis; Leiden
         University Medical Center; The Netherlands
21    Drafting of the report
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<pre>22 HELLE</pre>

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<pre>Annex B
      Brief explanation of
      a number of concepts
      For the purposes of convenience, the term exposure-effect relation is used throughout
      this report. Strictly speaking, the consequences of exposure can be divided into effect
      and response. The report ‘Toxicology-based recommended exposure limits’ (1996/12)
      defines an effect as the specific reaction of an organism upon exposure to a xenobiotic
      agent, while the term response refers to the fraction of organisms in the exposed
      population in which a certain effect occurs.
           Whether we are dealing with exposure-effect relations or exposure-response
      relations, the nature of these functional links is increasingly difficult to detect the lower
      the exposure level becomes. Expressed in more concrete terms: if the strength of the
      ‘signal’ decreases linearly in relation to exposure, the size of the population studied has
      to increase quadratically to allow ‘signal’ to be distinguished from ‘noise’. In practice
      this entails that capacity for detection — the statistical power — of animal experiments
      and epidemiological research will be inadequate from a certain point. Statements about
      the relations in question must therefore be based on certain assumptions: in other
      words, it becomes necessary to extrapolate. One of the primary assumptions concerns
      the working mechanisms which underpin the health effects. In the case of a
      non-stochastic or deterministic working mechanism, it is assumed that the health effect
      will only occur above a certain level of exposure (threshold dose), above which the
      damage increases as the exposure increases. In cases where the agent has a stochastic
      working, it is generally assumed that such a threshold dose cannot be established and
      that the chance of an effect from the zero point will increase as the exposure increases.
23    Brief explanation of a number of concepts
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<pre>       This distinction is being called into question more and more. According to some
   experts, agents with a non-stochastic working can sometimes be best characterized by
   relations without a threshold: certain effects on the nervous system provide good
   examples of such cases. Others feel that there is evidence to support the possibility of
   hormesis: that health improves under certain exposure processes. These and other
   related questions were the motivation behind the organization of the ‘Health Effects of
   Low Level Exposures’ conference.
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<pre>Annex C
      Conference report
      Introduction
      The Health Council of the Netherlands, which informs the government and parliament
      on the current level of knowledge in the field of public health, has a long tradition of
      assessing the health risk of low levels of exposure to physical and chemical agents. The
      Council frequently discusses how scientific data can be utilized to shed light on the
      possible effects of these low level exposures. Epidemiological research into the health
      status of populations exposed to low levels of radiation or chemical substances is in
      principle the most direct approach for making the risk assessments in question. But
      often its results, if available, have insufficient power. Therefore, one must resort to
      other, less direct but potentially relevant, types of evidence, such as information on
      biochemical processes and experimental animal data. Explicit, or sometimes tacit,
      answers to the questions involved result in recommendations to use particular
      extrapolation models, which make it possible to deduce what happens at low levels of
      exposure in the absence of direct data.
          Because standard setting and protection measures frequently depend on the chosen
      extrapolation models, it is of great societal importance that these models be based on
      the best available evidence. This raises the question of when and how new scientific
      insights should be incorporated in the process of risk assessment. Particularly,
      developments in molecular and cell biology generate a continuously increasing set of
      data on the modes of action of agents. Many experts hold that risk analysts should
      develop techniques to take these data into account.
25    Conference report
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<pre>        In order to explore the possibilities and limitations of what may be termed
   ‘evidence based risk assessment’, the Health Council decided to organize an
   international working conference, entitled ‘Health Effects of Low Level Exposures:
   Scientific Developments and Perspectives for Risk Assessment’ . Co-sponsors were the
   Dutch Ministry of Housing, Spatial Planning, and the Environment and the European
   Commission. The meeting took place in Lage Vuursche, the Netherlands, from 19 till
   21 October 1997. This report summarizes the introductions given by participants and
   subsequent discussions.
        The working conference focused on three broad topics:
        the state of knowledge about deleterious effects and defence mechanisms occurring
        at low levels of exposure
        the implications of those insights for risk assessment procedures, and
        the types of research needed on a priority basis
   Three cases served as crystallizing points for the exchange of ideas: ionising radiation,
   UV radiation, and dioxins. The main rationale for this choice was that relatively much
   is known about their effects and modes of action. Participants were asked, however, to
   discuss issues concerning these cases with a view to the general questions mentioned
   above.
        The report also contains two background documents which were distributed in
   advance. In the first (see Annex D) a general survey was given of the major scientific
   and societal aspects of the ongoing debate about low level exposures. The second (see
   Annex E) provided some topical information for the sessions of the working
   conference and contained a number of additional, and more specific, questions related
   to its main theme. The conference programme is reproduced as Annex F. The members
   of the Scientific Advisory Committee and the conference participants are listed in
   Annex G.
        On the basis of the findings of this conference the Health Council will advise the
   Dutch government on principles of risk assessment for low level exposures.
   Session I Heuristic overture
   The introductory contributions of this session provide some general ideas and
   methodological considerations which may help develop a conceptual framework for
   assessing the possible health effects of exposure to low levels of environmental
   stressors. A recurring theme and major issue is how to integrate the influence of a
   particular agent and the effects of simultaneously operating endogenous and exogenous
   (‘background’) factors. Evolutionary biology and biogerontology seem to be among the
   disciplines attempting to give an answer to this problem. At least they tend to favour
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<pre>   systemic or homeostatic concepts in stead of reductionist or linear cause effect
   schemes. Some integrative principles of order and control are briefly touched upon. In
   this connection one of the important and sometimes hotly debated topics is a class of
   phenomena referred to as hormesis: a beneficial response to low doses.
   Opening address (Knottnerus)
   Dr Knottnerus, vice president of the Health Council, opens the working conference and
   welcomes the participants. After briefly sketching the mission and organizational
   structure of the Council and the background of the conference, he draws attention to
   some topics for discussion. Nowadays chemical and radiation risk assessment mainly
   relates to low levels or low rates of exposure. One usually faces major methodological
   difficulties in determining the — potential — health effects of such types of exposure.
   For example, there may be very little contrast between normal (’background’) exposure
   levels and additions due to a specific practice. A similar problem may hold for the
   measurement of potential increments in response, e.g. in incidence or prevalence rates.
   Moreover, some effects can only be detected after many years of observation. Cancer
   and genetic disorders are a case in point. The possible influence of other exogenous and
   endogenous stressors further complicates things. Dealing with these difficulties is a
   challenging task, especially given the fact that susceptibility to agents varies across the
   population and repair mechanisms and adaptive responses play a role as well. Dr
   Knottnerus hopes that the participants, with their different fields of expertise, are able
   to make useful recommendations on the key topics of the conference.
   Aim and structure of the conference (Schoten)
   Mr Schoten presents an overview of the relevant points in the two background
   documents (see Annexes D and E). The aim of the conference is to discuss the
   possibilities and limitations of evidence based risk assessment. What counts as
   evidence? How can it be utilized in risk assessment? And what kind of evidence is
   most needed? With respect to these and similar questions he points to a number of
   problems. On the one hand, there is an ever increasing set of data on the effects of all
   kinds of agents, relating to various levels of biological organisation (molecular,
   cellular, intercellular, organismal). Yet integrating these various types of evidence into
   a single exposure effect model appears to be far from easy. Often only part of the
   available information is used in deriving exposure effect relationships. A major issue is
   whether generic recommendations can be made concerning methods of extrapolation
   given this variety of data. On the other hand, risk managers and governmental
   organisations show a regular interest in faster risk assessment procedures. That wish
27 Conference report
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<pre>   may be at right angles to an in-depth analysis and synthesis of the available evidence.
   Reflecting upon this somewhat paradoxical situation, Mr Schoten advocates a
   differentiated approach to the development of principles for risk assessment. For
   instance, the thoroughness of an assessment could be made to depend on the societal
   impact of the decision in question. Also, ways of grading evidence could be useful for a
   decision support system.
   Low level effects from a theoretical perspective (Doucet)
   Hormesis is a term used to describe an hypothesized phenomenon, namely that at low
   exposure toxic agents turn out to be beneficial to organisms in some respects. Dr
   Doucet examines this phenomenon at a theoretical and general level. He is interested in
   the kinds of mechanisms which can produce non-monotonic exposure-effect curves.
   One may expect that hormesis is the outcome of some sort of control action. A
   feedback system, where the body’s counteraction is triggered by the effects caused by
   the toxicant, seems to be the most plausible candidate from a biological point of view.
   If one thinks along these lines, one comes across a paradox. Consider a toxic agent
   whose only effect is to kill particular cells. Exposure to this toxicant will generate a
   response from the homeostatic control system involved, but the new equilibrium at
   which the cell population settles, will inevitably be below the original level. However,
   Dr Doucet thinks this argument disregards other possibilities. He wants to demonstrate
   that hormesis is possible in the presence of feedback control, provided that certain
   conditions are satisfied. To this end he formulates a model with two state variables, viz.
   a population of physiologically active target cells and a population that produces these
   target cells. Again it is assumed that the toxicant itself has only a negative effect. This
   excludes situations where hormesis can be attributed to a shifting balance between
   beneficial and deleterious effects. Dr Doucet’s analysis shows that under particular
   conditions for the intrinsic growth rate of the producer cell population, the model
   system achieves the required behavior, i.e. hormesis and stability.
   Low level effects from the perspective of evolutionary biology (Kooijman)
   According to Dr Kooijman it is essential, but far from easy, to test purely theoretical
   ideas like those of Dr Doucet’s against observational data. He gives a brief introduction
   to the so-called Dynamic Energy Budget (DEB) theory, which attempts to do just that.
   The DEB model quantifies the fluxes of energy through organisms as they change
   during life history. Three stages are distinguished: the embryo (which does not eat,
   although it does consume), the juvenile (which eats but does not reproduce), and the
   adult (which eats and reproduces). Energy is used for competing physiological
28 HELLE
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<pre>   processes, such as growth, maintenance, and reproduction. The rules for uptake and use
   of food provide an explanation for a variety of suborganismal phenomena and for
   effects on populations and ecosystems. Dr Kooijman thinks the DEB model can also be
   used to specify in a quantitative way how the energetics interferes with the uptake and
   effects of non-essential or toxic compounds. He mainly focuses on ecological risk
   assessment, but assumes that some considerations may be relevant to human toxicology
   as well. Most ecologists tacitly accept that organisms can cope with varying
   concentrations of any particular stressor, because they evolved in a chemically varying
   environment. The boundaries of this tolerance range appear to differ, however,
   depending on the stressor and on the physiological process. For instance, the upper
   boundary may be zero, which implies that even very small exposures, or rather tissue
   concentrations, have an effect or induce an effect with a certain probability. As a first
   approximation, the effect size is a linear function of the tissue concentration when it
   exceeds the tolerance range. Dr Kooijman supposes that for most compounds the upper
   boundary is positive and the lower boundary equals zero, because they are not
   necessary for life. Essential nutrients are an exception, with lower boundaries
   exceeding zero. The poorly understood process of aging is only of secondary relevance
   to the DEB model in its present form. Yet, descriptions of survival data where aging
   can be assumed to be the major cause of death seem to call for an extra integration step,
   which points to DNA. Dr Kooijman suggests that free radical activity (which seems to
   cause partly irreparable damage to DNA) could provide the clue to the relationship
   between age specific survival probability, life span, and energetics. The way aging is
   treated within the DEB model closely links up with mutagenic effects, particularly if
   the free radical mechanism is correct. According to dr Kooijman, mutagenic
   compounds have about the same effect on organisms as free radicals. As a
   consequence, mutagenic effects might be studied by changing aging acceleration.
   Low level effects from the perspective of biogerontology (Vijg*)
   Dr Vijg makes some comments on theories of aging, in particular on the role of
   oxidative damage to macromolecules as a mechanism of aging. This idea has a history
   that goes back some forty years, with papers on the commonality of mechanisms of
   oxygen toxicity and X-irradiation. Today techniques and assays are available to
   investigate the relationship between mutagenesis and aging at the molecular level.
   Studies indicate that protein and DNA oxidative damage substantially increases during
   aging. According to Dr Vijg, this phenomenon is most probably due to an increase in
   the rates of oxidant generation. No consensus has emerged as to whether or not the
*  Dr Vijg was willing to substitute for Dr Kirkwood who had to cancel his participation at the last moment.
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<pre>   efficiency of antioxidant defences and DNA repair declines during aging. Neither has it
   as yet been determined whether oxidative damage is a somewhat random phenomenon
   or whether there are specific and critical targets of such damage. The second
   mechanism might be a contributory factor, given the predictable nature of age-related
   physiological changes and life spans of different species.
   Possibilities and impossibilities of environmental epidemiology
   (Brunekreef)
   Compared with experimental research in the field of toxicology or biogerontology,
   epidemiology has both limitations and advantages. Dr Brunekreef starts by mentioning
   some of its major problems. It is difficult to eliminate or minimize measurement errors
   and the influence of confounding factors, especially in the case of environmental
   epidemiological investigations, where the potential effects under analysis are small and
   confounders are numerous. On the other hand, epidemiologists directly study the
   endpoints that matter most, such as physiological effects and various disease outcomes.
   In his opinion, utilizing biomarkers offers good prospects for diminishing the gap
   between epidemiological and experimental research, but similar methodological
   difficulties will remain. Still, epidemiological studies may have sufficient power to
   detect small effects due to low levels of exposure to environmental stressors. Dr
   Brunekreef takes air pollutants as a case in point. Time series analyses of the
   correlation between fluctuations in the daily concentration of major air pollutants (such
   as particulate matter and ozone) and the daily mortality rates reveal a linear
   exposure-effect relationship without any apparent threshold, let alone a beneficial or
   hormetic response.
   Discussion
   Synthesis of biological subdisciplines with partially comparable approaches to
   explanatory descriptions or experimental methods, e.g. developmental and evolutionary
   biology or oncology and biogerontology, is gaining ground. Although the participants
   at the conference expect that such scientific trends in the long term enlarge the set of
   analytical instruments for risk evaluations, they believe that at present there are hardly
   any opportunities for risk analysts to benefit from these developments. No clear-cut
   guiding principles exist to incorporate ideas like ‘homeostatic control’ and ‘reserve
   capacity’ into chemical and radiation risk assessments. Neither can controversies about
   the possibility or plausibility of phenomena such as hormesis be solved with an appeal
   to these generally fuzzy concepts. Most of the participants believe that there is no hard
   evidence to support the idea of hormesis.
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<pre>        The discussion also makes it clear that there is no simple and uniform correlation
   between events at suborganismal levels and physiological or pathological effects. It is
   true that particular biochemical shifts might be predictive of disease outcomes, and
   molecular biology shows impressive success, but the physiological approach has its
   own merits and should not be neglected. Some participants point out that, moreover,
   the results of traditional epidemiological investigations may sometimes be quite
   adequate to risk assessment and risk management.
   Session II Ionising radiation
   Introduction (Bridges)
   Relatively much is known about the biological and health effects of ionising radiation
   and about its mode of action, at least in comparison with most chemical agents. In
   addition, ionising radiation affects cells in a simple fashion, because metabolic
   processes do not play a role. Despite this favorable situation, however, many problems
   with respect to the effects of low doses and low dose rates still await a solution.
   According to Dr Bridges, emerging data are continuously shaking the radiobiological
   community out of complacency. On the one hand, there is experimental evidence that
   radiation may stimulate particular repair mechanisms. On the other hand, some studies
   indicate that one energy loss event can trigger more than one negative effect in cells. It
   is important to determine when such data are sufficient to take them into account in
   radiological protection.
   Significance of data on repair mechanisms
   Potential contributions from biomarker epidemiology (Cox)
   Dr Cox considers the basic tumorigenic processes and the stages where ionising
   radiation appears to act, taking colon carcinogenesis as his example. Animal
   experiments and evidence from biochemical, cytogenetic and molecular studies suggest
   that neoplastic initiation is the key stage that is targeted by low doses of ionising
   radiation. The data are consistent with a monoclonal mechanism of tumor development
   that does not differ in a discernable fashion from that of a ‘spontaneous’ tumor.
   Tumor-suppressor gene loss is likely to be a factor of major importance in radiation
   oncogenesis. Other types of mutations may be contributing factors as well. According
   to Dr Cox, even a single radiation track traversing the nucleus of a target cell can
   generate a tumor initiating mutation, albeit at a very low frequency. In his view this
   implies that, at the level of DNA damage, there is no basis for the existence of a
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<pre>   threshold dose below which the risk of tumor induction will be zero. To defend this
   conclusion, he addresses the other side of the picture, viz. the influence of repair
   mechanisms. He makes a distinction between single strand and double strand DNA
   damage. The former arises spontaneously at a high frequency in the cell due to
   endogenous metabolic processes. Many experts argue that this form of DNA damage is
   repaired in an error-free fashion and does not make a significant contribution to cancer
   risk. However, double strand breaks, which appear to be a very minor component of
   spontaneous damage but can be efficiently induced by ionizing radiation, will not all be
   repaired correctly. Even at low doses some residual damage should be anticipated at the
   molecular and organismal level.
       Next, Dr Cox discusses other potentially protective processes relating to the
   various stages of carcinogenesis and their corresponding levels of biological
   organization. There is some evidence that low dose radiation may induce or activate
   cellular defence systems: the so-called adaptive response. Three possible mechanisms
   have been suggested: additional DNA repair, induction of radical scavenging pathways,
   and subtle effects on cell cycle progression, which facilitate repair processes. Dr Cox
   thinks the last possibility is the most probable. But data relating to these responses and
   their relevance to neoplastic processes are insufficiently developed and understood to
   provide a sound basis for the judgement that carcinogenic response at low doses and
   low dose rates is likely to have a non-linear component, which might result in a dose
   threshold at the organismal level. In his opinion similar considerations apply to
   programmed cell death (apoptosis), terminal differentiation (to a non-dividing state),
   and immune surveillance: they have yet to be adequately described and remain
   contentious scientific issues with respect to their effects on carcinogenic response at
   low doses of radiation. Dr Cox concludes his introduction by making a remark on the
   prospects for molecular epidemiology and on individual cancer susceptibility. The
   development of molecular biomarkers is based on the mechanisms of action of the
   agent in question. Currently, it is difficult to determine the role of tumorigenic agents
   through mutational signatures present in a given tumor. However, genetic marker
   studies may be expected to improve and refine the ability to identify cancer susceptible
   populations by searching for specific germline mutations. In principle this approach
   could increase the power of certain epidemiological investigations.
   Insights into adverse effects (Bridges)
   Adaptive responses receive a lot of attention these days, but other unconventional
   effects should be taken into account as well. Dr Bridges discusses two of them. Both
   are forms of what may be termed effect amplification. It is known for some time from
   cytogenetic research that the number of germline mutations caused by exposure to
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<pre>   ionising radiation can increase during successive generations. More recent data suggest
   that, besides this ‘amplification in time’, there may exist an ‘amplification in space’.
   Genetic studies of animal populations show that acute doses of gamma-radiation cause
   a large increase in so-called minisatellite mutation rates, much larger than can be
   explained by the number of energy loss events involved (minisatellite loci are repeated
   units of short DNA fragments). The mechanism of this high sensitivity is not known at
   present. It may be that radiation first triggers instability of the genome, which then
   operates on the hypersensitive locus to change the repeat number. According to some
   researchers, minisatellite mutation rates are also unusually high in exposed populations
   after the Chernobyl accident. Dr Bridges thinks the human data are still unclear,
   because of various methodological problems with the analysis. Nevertheless, such
   biomarkers and analytic techniques promise new insights into the way in which
   radiation interacts with living organisms. Apart from this, the relevance of these
   phenomena to human health is a matter of debate. The same holds for the adaptive
   response. Such effects should at least make one cautious with respect to modifcations
   of dose-effect relationships and regulatory decisions.
   Discussion
   In their introductions the speakers stated that new studies add interesting dimensions to
   the understanding of the actions of ionising radiation, but that the present evidence
   does not justify a readjustment of the conceptual framework for risk assessment. The
   participants at the conference agree. But they conclude as well that evidence like this
   should always be carefully evaluated in choosing risk models for radiation protection
   purposes, also if the judgement is that the findings cannot be quantitatively taken into
   account.
        It is argued that only (multidisciplinary groups of) experts are in a position to
   decide when and how to adjust the framework of analysis on the basis of such
   mechanistic evidence. To this end, precise analytical tools should be developed for
   comparing and coupling experimental and human data. Usually researchers will first
   attempt to unravel events, and their dose dependencies, in experimental systems (the
   easier task). Next they should examine to which degree this information corresponds
   with epidemiological data. Agreement between mechanistic data and the broad
   predictions from epidemiology may then allow more confident judgements on cancer
   risks at low doses.
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<pre>   Session III UV radiation
   Introduction (Van der Leun)
   From a risk assessment perspective UV radiation differs from ionising radiation in a
   number of respects. Firstly, the range of doses to which one may be exposed lies
   approximately within only one order of magnitude: outdoor workers in the Netherlands
   receive a mean of 300 MED per year (MED: Minimal Erythema Dose, the dose causing
   a just visible reddening in the average white skin), indoor workers about 100 MED per
   year. The maximum UV dose from sunlight in the Netherlands is about 2000 MED per
   year. Secondly, for a sufficient production of vitamin D3 in the skin, about 50 MED
   per year is required. So beneficial effects of UV exposure have to be considered as
   well. Thirdly, problems of extrapolation appear to be smaller: over the range of 10 to
   300 MED a clear dose-response has been observed for some types of skin cancer.
   Cellular effects and repair mechanisms (Mullenders)
   Dr Mullenders follows with a brief presentation on aspects of DNA damage and repair.
   UV-B radiation (with relatively short wavelengths) induces predominantly direct
   lesions in DNA, such as pyrimidine dimers, whereas exposure to UV-A radiation (with
   longer wavelengths) enhances indirect oxidative damage to DNA bases. Considerable
   progress has been made in understanding the repair systems organisms have developed
   for coping with these forms of damage. Principal defence mechanisms are base
   excision repair and nucleotide excision repair. Dr Mullenders pays special attention to
   the latter, which has two different pathways. Transcription coupled repair only takes
   place in actively transcribed DNA and it seems to occur to a comparable extent in mice
   and men. Experimental research suggests that the mechanism depends on dose, low
   doses inducing a relatively better repair than high doses. A second pathway operates
   for genomic regions that are non-coding. This form of repair is higher in men than in
   mice. Dr Mullenders mentions a number of topics for further investigation: the
   connections between deficient DNA repair, genomic instability, and cancer risk; the
   nature of the relationship between decreased DNA repair and enhanced apoptosis; the
   influence of dose on various cellular processes; and the comparability of data on mouse
   and man.
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<pre>   Intercellular effects and repair mechanisms (Ullrich)
   There are three intervention points in which protective mechanisms stop the cascade of
   steps leading from UV exposure to skin cancer: melanogenesis (the production of
   melanin, which shields the skin from additional UV damage), DNA repair (the theme
   of the previous introduction), and immune surveillance. Dr Ullrich presents some data
   on the effects of UV radiation on the skin immune system. He discusses in particular
   the apparent relation between DNA damage and immunosuppression. Recent
   experimental findings suggest which pathways may be involved. DNA damage caused
   by UV radiation induces the release of cytokines, which act to stimulate carcinogenesis
   by blocking immune surveillance. Unrepaired pyrimidine dimers could be the trigger of
   shift in immune response from an active to a suppressive mode. Moreover, because
   cytokines mediate communication between cells, DNA damage in one cell can alter
   gene expression in undamaged cells. When analysing these phenomena and
   hypotheses, it is important to note some limitations and uncertainties. For example, the
   effects in question have not yet been studied in man. Neither is it clear how they
   depend on dose. In Dr Ullrich’s opinion, the relation will probably turn out to be
   non-linear.
   Combining epidemiological and mechanistic information (De Gruijl)
   Dr De Gruijl addresses partly the same issues as Drs Mullenders and Ullrich.
   Moreover, he touches upon the comparison of data on experimental
   photocarcinogenesis with results of epidemiological investigations. As to the latter,
   there is a clear dependence of skin cancer incidence rates on geographical latitude,
   pointing to an influence of UV radiation. With some adjustments the
   dose-time-response relation for mice can be fitted to human data. The model obeys
   Weibull statistics. Lack of data on tumor progression precludes using biologically
   based models at the moment. Dr De Gruijl also considers the perspectives of molecular
   epidemiology, which investigates associations between certain molecular or cellular
   changes and risk of disease. Such studies are only promising when insights into
   pathogenetic processes are substantial. Researchers have identified particular mutations
   in the p53 tumor suppressor gene that appear to be related to UV exposure, and that are
   consistently found to be frequent in human skin tumor cells. However, the p53
   pathway, which is likely to be important to tumor progression, may become
   dysfunctional through other alterations as well. So caution is warranted in using p53
   mutations as biomarkers. Still, increasing understanding of UV carcinogenesis might
   provide a set of relevant biomarkers, e.g. with respect to individual susceptibility.
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<pre>   Discussion
   The major topic of discussion during this session is the importance of mechanistic
   modelling to risk assessment. The cellular and intercellular response to UV exposure
   has been studied relatively well and the processes involved have been described in
   considerable detail. In fact, various questions concerning mechanistic aspects can at
   least be partially answered. Yet the participants do not quite agree on the urgency of
   developing mechanistic models for UV radiation. Some argue that one should always
   use all available data in construing risk models. Others feel that, for assessing and
   managing risks, the marginal returns of — continuously — incorporating mechanistic
   data in exposure effect models will be small when there is a reasonably large set of
   phenomenological data on the effect of low level exposure, as is the case for UV
   radiation. Statistical models may then suffice.
        On the other hand, there appear to be no substantial differences of opinion about
   research questions relating to cellular events and their possible interactions. Dose
   dependencies are a major issue. In addition most participants advocate the development
   of increasingly sophisticated mouse models to help clarify the links between various
   levels of biological organization, to pinpoint variabilities in susceptibility, and to
   identify similarities and differences between mice and men.
   Session IV Dioxins
   Introduction (Neumann)
   The term ‘dioxins’ stands for the large group of polychlorinated dibenzo-p-dioxins, the
   most toxic of which is TCDD. A great deal of research has been done on the many
   adverse effects of this agent and on its modes of action. Recently IARC issued a 700
   page report which presents an overview of the experimental and human data. Mainly
   on the basis of mechanistic insights TCDD has been classified as a so-called class 1
   carcinogen (proven carcinogenic in humans). The other dioxins have been put in class
   3 (not classifiable as to its carcinogenicity in humans) due to a lack of data. Dr
   Neumann brings up two topics for discussion. The first concerns the discrepancy
   between risk evaluations of different organizations. For reasons of scientific and
   administrative transparency it is important to pinpoint where different courses are open
   to risk analysts. Among other things, choices have to be made regarding the ranking of
   endpoints and the methods of extrapolation. Secondly, the IARC classification system
   might provide clues for grading evidence with respect to risk modelling decisions.
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<pre>   Using in vitro and in vivo data on carcinogenicity (Van den Berg)
   Dr Van den Berg, who was a member of the IARC working group on dioxins, focuses
   on the arguments resulting in a class 1 assignment to TCDD. Experimental research
   shows that this chemical is a multisite and transspecies carcinogen. Tumors are found
   primarily in skin, liver, and lungs. Epidemiological investigations among highly
   exposed workers (including an IARC multicountry study) confirm these experimental
   findings to a considerable extent. In the Seveso cohort study, however, different tumors
   have been observed. The difference might be due to latency period, exposure
   circumstances, and the possible influence of other agents. The mechanism of action has
   been extensively discussed in the IARC working group. Various studies suggest that
   the so-called Ah receptor is involved in the process of carcinogenesis. In this
   connection particular hormones may play an important role as well. Currently a number
   of ideas about relevant mechanisms are being explored. In summary, IARC classified
   TCDD as a class 1 carcinogen on the following grounds. High exposure increases
   overall cancer mortality rates. TCDD is a multisite carcinogen both in experimental
   animals and in humans. In addition toxicokinetic evidence points to parallel cellular
   processes, at high exposure levels, in experimental animals and humans: IARC
   members agreed that the Ah receptor has similar functions in these species. However,
   the functions seem not to be fully identical, because quantitative differences have also
   been observed. According to IARC, questions about the shape of the exposure effect
   curves at low levels of exposure can presently not be answered with any confidence.
   Although TCDD is considered not to be genotoxic, it is not clear whether a threshold or
   a non-threshold model is more appropriate for risk assessment. Other endpoints, such
   as developmental and reproductive effects, should then be carefully studied as well, in
   particular because they might be more sensitive than cancer incidence or mortality
   rates.
   Using in vitro and in vivo data on developmental effects
   Combining epidemiological and mechanistic information (Silbergeld)
   Put simply, molecular epidemiology may make the ‘black box’ between exposure and
   disease more transparent. Many scholars assume that events measured at the molecular
   level are relevant to and predictive of events in more complex systems, like human
   beings. Dr Silbergeld believes that these new epidemiological techniques may be
   particularly useful in studies of low dose effects. Firstly, the events associated with low
   exposure to environmental stressors are likely to be best observed at the cellular level.
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<pre>   Secondly, the increased precision of biochemical measures allows for more sensitive
   detection of effects. Before addressing the latest insights into the biological and health
   effects of low level exposure to dioxins, Dr Silbergeld outlines the main results,
   opportunities, and limitations of this field of research. Molecular epidemiology has
   been of greatest assistance in refining exposure measurement. In fact the measurement
   of toxicants in blood or other compartments has in quite a few cases been the standard
   for defining exposure for several decades. More recent developments relate to the
   identification of markers which are intermediate between exposure and preclinical
   pathophysiology. The best described, and still most frequently utilized, set of such
   markers concerns carcinogens and cancer risk. Sensitive methods have been developed
   to detect interactions of chemicals with DNA or proteins. However, the interpretation
   of DNA or protein adducts and their relevance to risk assessment completely depend on
   the quality of the pathogenetic understanding, which is often still in its infancy.
   Molecular epidemiology has also been used to define effects more precisely and to
   examine host factors that modulate the relationships between exposure and effect. For
   example, analysis of the types and locations of p53 mutations might become
   increasingly important to the study of chemical carcinogenesis. The identification of
   so-called ‘susceptibility genes’ is another major area of research. Although
   susceptibility may involve many events other than genotype, genetic differences within
   populations are likely to be informative when one attempts to explain the variability in
   human response to exposures.
        What have molecular biology and molecular epidemiology to offer to analysts
   assessing the risks of dioxins? In Dr Silbergeld’s opinion, notwithstanding the large
   literature on the mechanistic toxicology of dioxins, the gap between the increasing
   knowledge of the early mechanistic events and the major toxic manifestations of dioxin
   exposure (reproductive dysfunction, birth defects, cancer, immune suppression)
   remains large, and the usefulness of molecular tools to the epidemiologist unclear. Yet,
   some links of the exposure effect chain are reasonably well understood. The highly
   toxic dioxins and related chemicals, especially the PCBs, act in a manner similar to
   hormones, by binding to the Ah receptor. This receptor appears to affect the
   transcription of particular genes, such as the estrogen receptor, keratins, and growth
   factors. At present it is not clear that there are any exposure markers (e.g. induction of
   CYP450 enzymes) more informative than direct measurements of dioxins in human
   serum and adipose tissue. Neither have susceptibility markers as yet been clearly
   identified, despite the existence of substantial species differences. Since dioxins are not
   appreciably metabolized, it is not likely that genotypic variations in metabolizing
   enzymes play an important role. Recent data suggest that differences in response may
   be due to variations in the so-called Ah receptor nuclear translocator protein. It is
   possible that some of the target genes for dioxin action through the Ah receptor are
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<pre>   polymorphic. Dr Silbergeld concludes her contribution by considering early outcome
   markers, which most interest risk analysts, especially because many of the low dose
   effects of dioxin probably increase the risks of chronic diseases. Enzyme induction is a
   relatively sensitive response, but it is highly variable among individuals and is not
   specific to dioxins. Work done by various research groups indicates that changes in
   growth factor pathways might predict later events associated with both developmental
   effects and cancer. However, these changes mainly occur in tissues which are not
   accessible to the epidemiologist. Dioxins also have a range of effects on the immune
   system. But in view of the complexity of the events and given the unclear relationship
   between immunotoxicity and health effects of interest, such as cancer or reproductive
   dysfunction, immunologic markers can currently not be used as predictors of disease.
   Communication between toxicologists and epidemiologists is indispensable for
   elucidating those processes that are most needed in opening the ‘black box’ between
   exposure and disease.
   Discussion
   One reason why dioxins were selected as a case for this conference is that, in contrast
   with ionising radiation and UV radiation, they are not only carcinogenic but also have a
   range of other biological and health effects at low levels of exposure. Cancer risk was
   until recently the main variable for which exposure effect models, including
   mechanistic ones, have been developed. The study of other endpoints has often been
   limited to establishing so-called no observed adverse effect levels. The participants
   argue that this classical dichotomy, which has frequently been supposed to correspond
   in broad outline with the difference between non-threshold and threshold effects, is
   getting obsolete. Progress in the understanding of mechanisms seems to call for more
   refined systems of classification and more detailed principles of description.
        Techniques used in mechanistic cancer risk modelling might contribute to
   determining quantitative exposure effect relationships for sensitive endpoints such as
   developmental and reproductive effects. Some participants point out that the scientific
   literature contains a wealth of data on basic cellular processes, e.g. on how cell cycles
   are controlled. Yet initiatives to design models which interpret the available data for
   purposes of risk assessment have so far been remarkably scarce.
        A major problem is to identify valid, sensitive measures (biomarkers) which are
   predictive of clearly adverse effects or disease outcomes. For it is possible that some
   biological effects are nothing more than normal physiological adaptive responses.
   However, according to a number of experts even such effects might sometimes be
   relevant to risk assessment, because of potential variations in physiological resiliency
   between individuals.
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<pre>   Session V Scientific possibilities and limitations
   In this session general issues in risk modelling and in the biology of low level
   exposures are addressed. Comments and discussion have been moved up to the
   concluding session of the working conference.
   Remark on classifying carcinogens (Neumann)
   Continuing his introductory remarks on categorizing evidence in the previous session,
   Dr Neumann outlines a new classification of carcinogens in Germany. The German
   MAK-Kommission, which proposes health-based occupational exposure limits,
   recently drew up a new scheme. It consists of five groups, the first three corresponding
   to those of the EU: (1) substances carcinogenic to humans; (2) substances carcinogenic
   in experimental animals; (3) substances suspected to be carcinogenic; (4) substances
   with carcinogenic potential for which genotoxicity plays no or at most a minor role. No
   significant contribution to human cancer risk is expected, provided that the MAK value
   is observed. (5) substances with carcinogenic and genotoxic potential, the potency of
   which is considered to be so low that, provided the MAK value is observed, no
   significant contribution to human cancer risk is to be expected. Regulation of chemicals
   in categories (4) and (5) will thus be based on mechanistic information —
   nongenotoxic versus genotoxic — and the possibility to assess the carcinogenic
   potency at low doses.
   Synthesis with a view to modelling (Portier)
   New techniques, new methods, and new data emerge constantly, but it is not always
   clear how they can be useful to risk assessment: there is no simple arrow going from
   science to policy. Having said this, Dr Portier notes that usually only part of the
   information on toxicity is incorporated in risk models. However, toxicological
   evaluations of chemical agents should no longer be simply based on outcomes of
   bioassays or epidemiological studies. There is a, sometimes considerable, increase in
   information on the effects of an agent on processes like signal transduction, gene
   expression, endocrine signalling, cellular proliferation, and DNA interactions. Dr
   Portier is a champion of an integrative, yet at the same time pragmatic approach: one
   should develop a variety of models and test them against all available data. If a
   particular model makes sense in terms of these data, it can be used in risk assessment.
   At the US National Toxicology Program methods of experimentation and analysis are
   developed to strengthen the scientific foundation of risk evaluations. This includes
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<pre>   advancing collaboration between researchers with different professional backgrounds.
   Dr Portier elucidates his position by presenting exposure effect models for TCDD. One
   model attempts to integrate all experimental animal data. It describes the kinetics and
   dynamics of TCDD in rats and it encompasses about 100 equations and 200
   parameters. A similar model for humans could not be developed due to a lack of data.
   Less sophisticated approaches using statistical models generally involve much larger
   uncertainties with regard to extrapolation procedures. They are especially useful for
   assessing risks in populations exposed to levels approximately within the range of
   observations. When mechanistic data are few, these models may be the best we have.
   Dr Portier uses a formula with a shape parameter to evaluate, for a large number of
   endpoints, whether the available data on TCDD are consistent with threshold or
   non-threshold exposure effect curves. About fifty percent of the endpoints fits a
   threshold model and about fifty percent a non-threshold model. Weighing the relevance
   to health of the endpoints under analysis and combining the corresponding data may
   then be the best approach for risk assessment. It might at least shift the edge of
   extrapolation downwards.
   Significance of cellular and intercellular processes (Trosko)
   Dr Trosko emphasizes that there are more things in a cell than DNA, and that
   carcinogenesis involves more than mutagenesis. It is important to take the evolutionary
   context into account. During evolution multicellular organisms survived by adaptive
   responses to both endogenous oxidative metabolism and exogenous chemicals and low
   level radiation. The defence repertoire exists at all levels of the biological hierarchy.
   Roughly speaking, three levels of communication can be distinguished: extracellular
   (‘large distance’) signalling (e.g. hormone action), intercellular (‘short distance’)
   signalling, and intracellular signalling. Dr Trosko pays special attention to intercellular
   events. He contends that so-called gap junctional intercellular communication is of
   crucial importance to many fundamental biological processes, from early
   embryogenesis to regulation of cell growth later in life. Modulation of gap junctional
   communication, by the action of e.g. cytokines and growth factors, is likely to play a
   significant role in the process of carcinogenesis: various studies indicate that blockage
   of these communication channels may act as an endogenous tumor promoter.
   Conversely, it is possible that intercellular signalling mechanisms provide protection of
   any cell hit by e.g. a radiation track through the sharing of reductants and by triggering
   apoptosis. Dr Trosko has developed a tissue culture system in which the effects of low
   doses on gap junction intercellular communication can be examined. These
   experiments may help predict the effects of low level exposures on complex organisms.
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<pre>   Session VI Perspectives for risk assessment: final discussion and
   recommendations
   The debate focuses on possibilities to make more effective links between science and
   policy. As to the scientific side of that relationship, the participants critically review
   promising approaches and note a number of issues in need of clarification. Opening the
   black box between exposure to a particular agent and its health effects is seen as the
   major route to progress. Various types of research can shed light on various parts of
   such a black box, generating a diversity of biomarkers. It is argued that comprehension
   of low level effects will evolve iteratively from application of a variety of biomarker
   variables relating to different levels of biological organization. At present, only a few
   markers are available that can, for instance, be effectively used in epidemiological
   studies, but applications will no doubt increase. Future usefulness of biomarkers
   strongly depends on the rate at which problems with respect to their validity, reliability,
   and generalizability will be overcome. The participants endorse the general paradigm
   of human-animal parallellism. Systematically comparing animal and human data,
   ranging from results of in vitro methods to outcomes of in vivo approaches, is critical
   for determining fundamental links of exposure-effect chains and for identifying
   uncertainties with regard to interspecies extrapolation and exposure-effect modelling.
   Some participants have high expectations of the development and use of transgenic
   mice, which can be tailored to study the influence of particular molecular events on
   physiological variables. This includes the examination of gene environment
   interactions and of variations in susceptibility. However, it is remarked that some
   endpoints, e.g. neoplastic lesions, may lend themselves more to comparison than
   others, e.g. particular neuropsychological phenomena. When it comes to formulating
   guidelines to create a framework for risk modelling, the participants recommend to be
   pragmatic and open minded: a variety of models can be useful for risk assessment. In
   the final analysis, experts should decide on an ad hoc, case-by-case basis. Consensus
   emerges that analysts should keep themselves informed about technical advances in
   risk assessment methodology. Researchers have been developing new modelling
   techniques which attempt to utilize more of the available scientific knowledge and
   expertise. In this connection it is essential that scientists with different backgrounds
   collaborate. In fact the risk assessment enterprise can be structured as a modular
   activity: when the experts feel that the evidence concerning particular processes is solid
   enough, models can be tied to it. During the conference Dr De Vries Robbé showed
   how communication and collaboration between professionals can be promoted by using
   so-called cognitive maps, which attempt to make explicit the ideas and conceptual
   frameworks taking root in various biological subdisciplines.
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<pre>        Quite a different matter is whether sophisticated modelling activities are
   worthwhile from the perspective of risk management. Many participants emphasize
   that the problems under analysis should always be put in a societal context. This entails
   examining actual options for reducing exposures, evaluating the costs and benefits
   involved, and performing sensitivity analyses with regard to the modelling of risk as a
   function of exposure. Parties who are affected by the risk management problem should
   help frame the questions for risk assessment, e.g. which endpoints should be
   considered. Basically, they should determine how high the stakes are and how deep the
   analysis should be, knowing that it is not practical to crack a nut with a sledgehammer.
   Other participants add that different perspectives can come up, which may be referred
   to as ‘agent-orientated’ and ‘health-orientated’. The former point of view more or less
   coincides with prevailing or legally required methods of risk assessment, whereas the
   latter addresses the usually multifactorial nature of health problems and tries to
   determine the influence of exposure to one particular agent against the ‘background’ of
   many other contributing risk factors. It is concluded that techniques should be
   developed for analysing the risks of combined exposures and for establishing the
   contribution of individual stressors. Some participants state that sophisticated
   modelling will become the easier and less time consuming, the more the experience
   with handling the analytical instruments increases. As a matter of fact it can be
   expected that many basic modelling components apply to a large number of agents: the
   wheel does not have to be reinvented time and again. Whether the application of these
   modelling techniques should be accompanied by statements detailing the degree of
   evidence, is a topic that, according to the participants at this conference, may warrant
   another workshop.
   Some afterthoughts
   Given the diverse set of participants it is noteworthy that areas of agreement were
   large. The participants appeared to express similar views on many recurrent themes:
        Although hormesis cannot be excluded on theoretical grounds, there is at present
        no hard evidence for it.
        It is true that insights into molecular and cellular effects of exposure to physical
        and chemical agents are sometimes rapidly increasing, but many questions remain
        to be answered. It is generally poorly understood how exposure timing and
        exposure dose could influence the potential cellular effects, such as ‘no change’,
        mutations, cell death by necrosis or apoptosis, or altered gene expression.
        Furthermore, there is usually insufficient scientific knowledge to establish
        precisely how each of these potential effects at the cellular level could contribute to
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<pre>       various physiological or disease outcomes, such as cancer, developmental effects,
       or reproductive dysfunctions.
       Progress can be made by systematically comparing mechanistic and
       phenomenological information on the one hand, and animal and human data on the
       other hand. However, there are no clear cookery-book procedures for combining
       data with respect to low level risk assessment: in the final analysis, experts should
       decide on a case-by-case basis.
   The conference left open the question of whether it is always appropriate to use
   sophisticated modelling techniques. Many participants felt that such techniques should
   only be used when the societal stakes are high. Others believed that practice makes
   perfect.
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<pre>Annex D
      The debate about low levels of exposure
      Eert Schoten
      This note concerns scientific research into the effect on the human body of low doses
      of physical and chemical agents, and the significance of various research data for health
      assessments of such exposures. It is an extremely general exploration of an issue that
      will be the subject of a working conference to be held next year, under the auspices of
      the Health Council of the Netherlands, and is intended to serve as background
      information for working out the details of the conference programme.
1     The background to the problem
      Our insight into the effect of radiation and chemical substances on health is less than
      we would wish. We know that exposure to high doses of these agents can damage
      organs but, leaving aside the case of accidents, there are questions about their potential
      harmfulness at the relatively low exposure levels that occur in the physical or working
      environment. Do high and low levels of exposure only differ in the strength or
      frequency of what are, for the rest, similar effects, or are there more likely to be
      essential differences in the reactions? If there are, what should be considered as ‘high’
      and ‘low’? It is not possible to get around these questions when standardizing
      exposure levels to protect or promote the health of the public.
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<pre>        The Health Council has a long tradition of assessing scientific data that can be used
   to support the environmental and occupational exposure limits in question. A thread
   that runs through the recommendations of the Council is the fact that, if available,
   epidemiological data on the effect of low levels of exposure usually reveal very little.
   In general, it is impossible to exclude any particular negative effect, such as additional
   harm of any description to the people exposed vis-à-vis people in the control group, or
   the reverse of this, i.e., any positive effect, or the absence of any effect at all.
        It is therefore necessary to resort to other sources of information to express an
   opinion on the possible health consequences of exposure conditions of this kind. A
   whole range of data can then qualify for consideration, such as the results of
   epidemiological studies into the effect of high levels of exposure, the outcome of
   animal tests (usually also at high doses), and information about the way in which the
   molecules of an agent interact with those of a cell.
        But these indirect approaches are not without problems. Because of their indirect
   character, we cannot avoid specifying how data of this kind can shed light on the effect
   of low doses in the human body. Each of the sources of information referred to presents
   us with just as many extrapolation problems in terms of what ‘high’ implies for ‘low’
   (the question already raised in the introductory paragraph), what an ‘animal’ can reveal
   about ‘human beings’ and what a ‘molecule’ can tell us about an ‘organ’? Answers to
   these questions result in recommendations to use particular extrapolation models.
   Using these, it is possible to deduce what effects can be theoretically expected for low
   levels of exposure, even if the effects are not demonstrated directly as a manifestation
   of disease symptoms, at the level of organs.
2  Discussion of models
   The models that have been used or proposed have been a point of discussion right from
   the start. Besides covering the scientific aspects (How strong is the empirical evidence
   for certain hypotheses that form the basis of the models? How can these hypotheses be
   tested and further specified?), the debate is also concerned with questions that have a
   normative or, put another way, political tint (How should the uncertainties be dealt
   with? Are simplifications necessary from the administrative point of view?). In recent
   years, the discussion seems to have been getting more heated. This is partly because of
   the rapidly advancing developments in cell and molecular biology, and partly because
   of a growing and more frequently expressed scepticism about the reasonableness of
   various standard setting procedures. Many people are asking themselves whether the
   balance is right between the costs of all kinds of laws and rules, on the one hand, and,
   on the other, the supposed benefits, viz. the prevention or reduction of damage to
   health? The question from the scientific point of view is whether the opinion about the
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<pre>   damage caused by exposure to low doses can be maintained, in the light of the most
   recent insights into the biochemical machinery that cells have for adapting themselves
   to stimulation from outside.
        This issue is also frequently discussed by the Health Council. Some time ago, the
   Standing Committee on Radiation Protection discussed the question of whether there is
   a possibility that cellular defence mechanisms induced by exposure to background
   radiation (ionising radiation that occurs naturally) might actually indirectly reduce the
   risk of cancer (see section 5). This is contrary to the current standpoint on radiation
   protection which is that background radiation increases the risk of cancer. The
   discussion gave the former chairperson of the Health Council cause to have a further
   exchange of ideas with a number of council members about this possible ‘effect
   compensation’ and, more in general, about the significance of data on the mode of
   action, in terms of estimating the risks of exposure to low doses. Besides the Council’s
   chairperson, Drs Blok, Feron, and Lohman took part in the discussions. Below, the
   comments of the aforementioned council members are placed in the context of a
   number of scientific and social trends.
3  The message of the critics
   When articles appear in scientific literature under titles such as ‘The triumph of
   theology over science: the non-threshold effects model’ (Sag94) and ‘Cancer risk
   assessment: the science that is not’ (Gor92), it is obvious that we are dealing with a
   topic that will continue to be a point of discussion for some time to come. These
   authors believe that making assessments of the risks of exposure to low doses of
   carcinogenic agents has a lot in common with making a declaration of faith. According
   to them, these agents only appear to be carcinogenic at high doses, and then often only
   in animals. Simply assuming that exposure to low doses increases the risk of cancer,
   albeit to a relatively limited degree, fails to pay sufficient heed to indications that, in
   reality, matters are a lot more involved. In particular, the rigorous application of the
   linear no-threshold hypothesis should be blamed for this. It is a dual hypothesis: (1)
   even the lowest doses of these agents (‘absorbed dose’ in the case of ionising radiation,
   and ‘ingested amount per unit of body weight’ in the case of substances) can cause
   irreversible damage to cells and thereby increase the risk of cancer; and (2) in the case
   of low levels of exposure, the risk increases proportionately with the dose. Within the
   scope of this, the qualification ‘low’ is usually simply described as ‘not high’. High
   doses are those that result in acute organ damage.
        The aforementioned critics and their supporters object to this position. Most
   experts now think that carcinogenic agents cannot all be placed in the same category.
   Meanwhile, the opinion is fairly widespread that there are so-called genotoxic and
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<pre>   non-genotoxic agents. According to this opinion, the first category is capable of
   damaging cell nuclei permanently, the second appears to particularly affect cell
   division and cell proliferation, often by means of a reversible process. The implication
   of this is that the harmful effect in the second case does only occur above a certain
   threshold dose, whereas in the first case it does not. Health Council committees have
   also subscribed to this view (GR78, GR94).
   However, this differentiation is not the main concern of the critics, although they
   appreciate the focus on the way in which agents affect cells. In their view, it is more
   important that the ability of agents to damage DNA molecules in cell nuclei or to affect
   cell division is not the whole story, as other processes that can interfere with those
   mentioned above also occur in cells. If the developments in molecular biology and cell
   biology have taught us anything, it is that a complex combination of actions takes place
   with numerous possible reactions. We are learning increasingly more about the
   cascades of biochemical reactions in and between cells, both in healthy tissue and in
   tumours (Kar95, Spo96, Var93). For example, it has been known for some time that the
   p53 tumour-suppressor gene can protect DNA molecules by temporarily blocking cell
   division after damage, thereby enabling repair mechanisms to do their work. Recently
   there have also been some indications that p53 itself can trigger certain repair
   mechanisms (Mar94). This information increases the insight into what happens in cells
   after they receive an external stimulus, for example. However, this also presents new
   questions about the precise relationship between the various subprocesses and the
   results of their interaction under different conditions. There is still no clear answer to
   these questions.
4  The BELLE initiative
   It is still too early to make any definite statements but, in terms of estimating and
   assessing risks, the interest in the importance of new insights is growing. An example
   of this is the initiative in the United States in the early nineteen nineties, known by the
   acronym BELLE: Biological Effects of Low Level Exposures. This group of
   researchers set themselves the task of examining more systematically the various
   processes that occur in cells and organs when they are ‘hit’ by radiation or chemical
   substances. Two symposium collections have been published thus far under the
   auspices of BELLE (Cal91, Cal94). They also publish a regular newsletter.
        The articles in the collections and the newsletter are quite varied, with reports on
   observational studies and theoretical considerations appearing alongside each other,
   together with toxicological and epidemiological discussions and contributions on
   carcinogenic and non-carcinogenic agents. However, they have in common that they
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<pre>   are often concerned with dose-response and dose-effect relationships that are in some
   way different from what is currently taken to be the case. As already mentioned briefly,
   in many circles it has become the established view that there is a fundamental
   difference between genotoxic and non-genotoxic agents. The harmful effect of the
   former is considered to be characterized by dose-response relationships without a
   threshold. (There are differences of opinion about the precise shape of the relevant
   curves, whether they are linear or non-linear for example.) The second category,
   regardless of whether they are carcinogens, are supposed to only result in a harmful
   change above a certain threshold dose, below which nothing happens. This is a clear
   picture, which organizations that have to set standards can work with easily. The
   BELLE group is illuminating the picture’s cracks and unevenness: genotoxic agents for
   which there are nevertheless indications of a threshold, and also various agents that
   first have a favourable (or at least not unfavourable) effect as exposure increases, but
   then later show an unfavourable effect.
        If we assume that these findings cannot generally be ascribed to incorrect research
   techniques, but that they indicate processes that may well be difficult to reveal but that
   nevertheless exist, there do, indeed, appear to be reasons for examining the
   aforementioned standard picture more critically than in the past. However, the question
   concerns how that standard picture may need to be changed, especially bearing in mind
   standard setting procedures. It is one thing to point to the multitude of favourable,
   unfavourable or neutral processes that occur in and between cells, but it is a completely
   different matter to describe those processes quantitatively as a function of the dose,
   both separately and in their interactions.
5  The possibility of effect compensation
   The exchange of ideas between the council members mentioned in section 2 was
   mainly concerned with the fundamental scientific preconditions in this whole area: are
   ‘different’ dose-response or dose-effect relationships possible according to present
   physical and biochemical insights? In concrete terms, the discussion focused on the
   effect of low doses of ionising radiation and the possibility of ‘effect compensation’.
   The passage of ionising radiation through cells involves the release of energy packets
   that are considerably larger than those exchanged between molecules during the normal
   functioning of cells. This can result in what appears to be unfavourable damage to the
   DNA in the cell nucleus. However, it can also result in extra biochemical repair
   reactions that can partially undo the harmful effects of other, possibly stronger,
   genotoxic agents, to which such a cell is also exposed (see section 6.1). It is therefore
   conceivable that the net result of all these interactions could be positive for a given
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<pre>   dose range; in other words, the risk of developing cancer can, on balance, be reduced
   (effect compensation).
        On the basis of a rough calculation, council member Blok explained that for an
   absorbed dose of 10 mGy per year — a little more than that resulting from background
   radiation — a random body cell would be hit about once every two days and a random
   cell nucleus would be hit about once every quarter. According to him, it is possible to
   conclude from this that any effect compensation is more likely to be caused by the
   induction of protection mechanisms in the cytoplasm than in the cell nucleus. In the
   latter case, any such mechanisms would in fact have to remain operational for several
   months after the cell had been affected. The effect of chemical genotoxic agents can
   also be looked at from two points of view: the damage caused to cell nuclei following
   exposure and the defence mechanisms that are mobilized. The interactions mentioned
   and their dependence on the dose may be different for each agent. The present level of
   knowledge precludes the possibility of making any more detailed statements about this,
   let alone formulating any particular laws.
   The lack of insight is also clear from the language used in scientific literature on the
   subject. It is often full of metaphors: researchers talk about ‘stimuli and reins’ or about
   ‘invaders and defenders’, complete with ‘rules’ according to which the battle between
   both proceeds. This is all based on the idea that a system of balance (homeostasis)
   exists inside and between cells, which can take an occasional blow. However, as
   indicated, it is difficult to provide any quantitative substantiation of these processes, or
   they have to involve simulations. In this regard, if one takes into account the
   mobilization of the repair mechanisms or the increase in cell death, both of which are
   assumed to be saturable, model calculations are instructive in showing how the risk of
   cancer first decreases and then increases as a function of the dose, after exposure to a
   fictitious genotoxic agent (Ste94).
6  Trends in scientific research
   However, calculation exercises of this kind are of little use if they are not properly
   related to empirical data. Unfortunately, in all scientific advances, the picture of the
   biochemical machinery in and between cells is still very diffuse and fragmented.
   However, work continues to reduce the knowledge gaps in the various, partially
   overlapping, fields of research. These fields include the following.
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<pre>   The research into DNA repair
   Various mechanisms in the cell nucleus help maintain the DNA structure by, as far as
   possible, removing defects that occur during replication and damage that results from
   exposure to stimuli in the environment. There is a whole range of repair processes.
   Some involve a single step, whereas others involve a series of steps. Some focus on the
   entire genome but others are concerned with specific genes in the genome (Boh95,
   Cle94, Han95). The last few years in particular have produced some rapid
   developments in techniques for tracing and further delineating repair mechanisms in
   genes.
       The discovery of what are termed nuclear factors in the cytoplasm of mammalian
   cells was also of major importance. These are proteins that are activated by
   extra-cellular signals. They subsequently penetrate the cell nucleus as a transcription
   factor, where they cause expression of particular genes, so that certain defence proteins
   are produced. An example of this is the nuclear factor NF-kB (Sch92, Tha95). Many
   stimuli (viruses, bacteria, cytokines (intercellular signalling molecules), UV radiation,
   ionizing radiation and certain chemical agents) appear to activate this factor (and
   related factors). The number of target genes in the cell nucleus appears to be even
   larger. However, still little is known about the function of the defence proteins that are
   produced. They include cytokines, which are able to produce a high state of alert in
   neighbouring cells that may not have been affected. A possible effect compensation
   would therefore be able to spread like ripples in a pool.
   The research into oxidants and anti-oxidants
   However, a real battle takes place before mechanisms that repair damaged DNA can
   become active. The battle is between, amongst other things, what are referred to as
   oxidants and anti-oxidants. Oxidants are substances that can damage all kinds of
   macromolecules, including DNA, through an oxidation process. This involves reactive
   interim products (radicals) in the reduction of oxygen to water. They occur in cells after
   penetration by, for example, ionising radiation, but also especially as a result of normal
   metabolic processes. Anti-oxidants impede or delay these oxidation reactions in
   biomolecules. Some examples of oxidation inhibitors are vitamin E, vitamin C and
   carotenoids, which occur in vegetables and fruit. Anti-oxidants do not offer any generic
   protection against oxidation reactions; the effectiveness of the protection is highly
   dependent on the nature of the threat (the radical type), the structure of the molecules
   under threat and the mechanism by which the anti-oxidant works. (Are radicals
   blocked? Is their formation impeded? Is the damage repaired?) (Hal95).
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<pre>        Both issues, oxidative stress and defence possibilities, are receiving increasing
   attention in the search for the causes and pathogenesis of degenerative diseases, such as
   cancer, cardiovascular diseases and diseases of the nervous system and the immune
   system (Ame93, Ame95, Bor93, Shi94). The interesting thing about this development
   is that attempts are being made to consider cellular processes in cohesion and to place
   them in perspective. A couple of salient points (Ame93, Ame95) within the scope of
   this are: the oxidative damage caused by exposure to synthetic chemicals and radiation
   is very much less than that which results from the normal metabolism in cells;
   consequently, a reduction or deferment of the damage and the associated degenerative
   diseases may be more readily attained through a reduction of the metabolic rate (eating
   less on a daily basis) and by following a diet that is rich in anti-oxidants (a lot of
   vegetables and fruit), rather than through extremely stringent environmental protection
   standards.
        Some people are calling for anti-oxidizing vitamins to be added to foods or for
   these substances to be taken in addition to the normal diet. According to the Health
   Council’s last annual report, at present the health benefits of this have not been
   sufficiently documented (GR95). The Council will soon be looking at this issue in
   more detail (GR96).
   The research into toxicokinetic and toxicodynamic processes
   Traditionally, in studying the consequences of exposure to chemicals, a distinction has
   been made between the toxicokinetic phase, which is concerned with the absorption,
   distribution, biotransformation and excretion of a substance, and the toxicodynamic
   phase, in which the interaction of the molecules of the agent with those in the cells is
   central. In setting standards, it is important to know which part of the dose (defined as
   the amount taken per unit of body weight) is ‘biologically effective’. Some trends
   clearly seem set to continue over the coming years (Fre95, Men95). One such trend is
   the growing interest in PBPK/PD models (PBPK/PD stands for Physiologically Based
   Pharmacokinetic and Pharmacodynamic). Using these models, researchers want to
   provide the most explicit and systematic description possible of the aforementioned
   kinetic and dynamic processes. The predictive power of PBPK/PD models leaves a lot
   to be desired at the moment; it is not usually possible to deduce any valid dose-effect
   relationships from them. Their usefulness is more their instrumental value in testing
   hypotheses. (Why is one organ damaged and another not? How is it that some species
   of animals are unsusceptible?) The potential carcinogenicity of methylene chloride
   provides an example of the latter question (Kai96). Exposure to this substance can
   cause tumours in mice, owing to a metabolic product formed in the nuclei of lung and
   liver cells, which damages DNA locally. In rats and humans, this metabolic process
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<pre>   occurs outside the cell nucleus, which seems to cancel the carcinogenic potential.
   Given the present level of knowledge, one of the priorities of the research is to validate
   PBPK/PD models. Another is the standardization of the methods of determining the
   many process parameters that are intended to give shape to the models.
        A second trend is the study of structure activity relationships (SARs). These
   relationships indicate how the physical and chemical properties of an agent are
   connected to its toxicity. The need for SARs mainly arises from the major lack of direct
   data on the toxic potential of many chemicals. Toxicologists are expected to
   increasingly attempt to get round this lack of knowledge by using SARs in PBPK/PD
   models.
   The research into biomarkers
   Whereas traditional epidemiology concentrates on finding causal connections between
   exogenous factors and diseases, molecular epidemiology is interested in the links of
   these cause and effect chains. Exposure and the manifestation of the disease only mark
   the beginning and end of a continuum of events. In between, all kinds of measurable
   changes take place, which can provide an insight into the pathogenesis and, in
   principle, can provide a point of application for prevention or early treatment.
   Biomarkers is the term researchers use for these changes, as well as for cell structures
   that are relevant from this point of view. DNA adducts (covalently bonded complexes
   between DNA and a mutagenic agent) are one example. In molecular epidemiology, a
   relationship is sought between these biomarkers and diseases, such as the possible link
   between the number of DNA adducts and the risk of certain types of cancer. Although
   this type of research is generally considered to hold a lot of promise, opinions tend to
   differ about how long it will be before useful results are available (Cuz95, McM94,
   Per96a, WHO95). However, refinements in research techniques are expected to enable
   the effect of exogenous factors to be better subdivided into the endogenous
   susceptibility of people, in particular into their genetic characteristics, which can also
   be considered as biomarkers (Dol96, Per96b, San95, Sch95).
   The research into ageing processes
   Many of the aforementioned research topics come together in the study of the driving
   forces behind the origin of age-related diseases and ageing in general. Because
   (chronic) low exposure to agents is associated with — an acceleration or alternatively a
   delay in — the onset of degenerative processes of this kind, biogerontology seems to
   be a branch of science that is especially capable of providing a coherent description of
   the effect of such exposure. A lively debate is underway at present about the
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<pre>   significance of genetic factors for the ageing process and about their interaction with
   environmental determinants (Jaz96, Kir92, Lit96, Mar96, Par93, Soh96). The context is
   usually that of evolution biology: is ageing programmed or does it arise as a trade off
   between increased chances of reproduction and a shortening of life? Is that trade off
   optimal or could it be improved? The attractive aspect of the evolution perspective is
   that it takes into account the time dimension of the effect of exposure to agents,
   something which has been put forward on several occasions by the Health Council.
7  An information handling problem
   As illustrative and limited as the descriptions and examinations given in the preceding
   sections may be, one thing is clear: there are signs in various branches of science that
   the method of classification and extrapolation used thus far for estimating the risks
   associated with exposure to low doses of agents fails to consider some potentially
   interesting questions. According to the council members, in the light of what is known
   at present, it is at least possible to say that phenomena such as effect compensation
   cannot be excluded, even if it is not possible to say much about their plausibility.
        The aforementioned signs have not gone unnoticed by politicians, administrators
   and lobbyists. Particularly but not only in the United States, there are growing
   objections to the current methods of assessing risks; the estimates, which are associated
   with uncertainties, are considered to err too much on the safe side. As an extension of
   this, the procedures for managing risks are under attack; exposure standards are often
   determined without paying sufficient attention to the costs involved and without regard
   to the importance of setting priorities (Abe93, Arr96, Mac96, Par95). The call for more
   attention to be paid to cost-benefit considerations has been heard, insofar as a body like
   the EPA (Environmental Protection Agency) recently drew up guidelines for methods
   of risk assessment, in which data on the mode of action of agents have to be
   considered. This expresses the growing realization that it would be better to consider
   the uncertainty of the benefits along biological lines, rather than express them in
   aspecific extrapolation principles (Kai96, Men95, Sto95).
        However, for the time being the expectations of many experts are tempered by the
   lack of insight into all the processes that play a role in the interaction between agents
   and cells. The council members also believe it will only be possible to say more about
   the effect of low doses, including what ought to be understood by ‘low’, when the
   scientific research in the field becomes more systematic and is more advanced.
   However, an interim solution must be offered, because policy-makers are unable to
   wait for the results of this. In fact, this concerns an information handling problem
   (which is incidentally not unique to environmental protection and occupational safety
   but also applies to ‘evidence-based medicine’ and other complex assessment
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<pre>      processes): how can the various types of data, with all their deficiencies, be ordered
      (and assessed) so that policy-makers can make optimal use of them in making their
      decisions?
           As far as the Health Council is concerned, this general question should set the
      course for the working conference. In which case, participants at the conference would
      have to address problems such as: how can data on the effect and repair mechanisms be
      incorporated in the dose-effect curve models? Is it possible to make any generic
      statements about this or are we practically compelled to make agent-specific
      recommendations? Can we produce assessment methods that flexibly incorporate new
      scientific insights that lead to appreciable results? What methods do we have available
      for giving uncertainties a specific place in the risk assessments? Questions like these
      should be discussed on the basis of a limited number of clear-cut and well-documented
      case studies.
8     References
Abe93 Abelson PH. Pathological growth of regulations. Science 1993; 260: 1859.
Ame93 Ames BN, Shigenaga MK, Hagen TM. Oxidants, antioxidants, and the degenerative diseases of ageing.
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Ame95 Ames BN, Gold LS, Willett WC. The causes and prevention of cancer. Proc Natl Acad Sci USA 1995; 92:
      5258-65.
Arr96 Arrow KJ, Cropper ML, Eads GC, a.o. Is there a role for benefit-cost analysis in environmental, health,
      and safety regulation? Science 1996; 272: 221-2.
Boh95 Bohr VA. DNA repair fine structure and its relations to genomic instability. Carcinogenesis 1995; 16:
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Bor93 Borek C. Molecular mechanisms in cancer induction and prevention. Environ Health Perspect 1993; 101
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Cal91 Calabrese EJ (ed). Biological effects of low level exposures to chemicals and radiation. Boca Raton, 1991.
Cal94 Calabrese EJ (ed). Biological effects of low level exposures: dose-response relationships. Boca Raton,
      1994.
Cle94 Cleaver JE. It was a good year for DNA repair. Cell 1994; 76: 1-4.
Cuz95 Cuzick J. Molecular epidemiology: carcinogens, DNA adducts, and cancer - still a long way to go. J Natl
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Dol96 Doll R. Nature and nurture: possibilities for cancer control. Carcinogenesis 1996; 17: 177-84.
Fre95 Frederick CB. Summary of panel discussion on the ‘advantages/limitations/uncertainties in the use of
      physiologically based pharmacokinetic and pharmacodynamic models in hazard identification and risk
      assessment of toxic substances’. Toxicology Letters 1995; 79: 201-6.
Gor92 Gori GB. Cancer risk assessment; the science that is not. Regul Toxicol Pharmacol 1992; 16: 10-20.
55    The debate about low levels of exposure
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<pre>GR78   Gezondheidsraad. Advies inzake de beoordeling van carcinogeniteit van chemische stoffen.
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GR94   Gezondheidsraad. Risk assessment of carcinogenic chemicals in the Netherlands. Regul Toxicol
       Pharmacol 1994; 19: 14-30.
GR95   Gezondheidsraad. Jaaradvies Gezondheidszorg 1994-1995. Gezondheidsraad. Publikatie nr 1995/13. Den
       Haag, 1995.
GR96   Gezondheidsraad. Werkprogramma 1996-1997. Gezondheidsraad. Publikatie nr A96/02. Den Haag, 1996.
Hal95  Halliwell B. Antioxidant characterization. Methodology and mechanism. Biochem Pharmacol 1995; 49:
       1341-8.
Han95  Hanawalt PC. DNA repair comes of age. Mutat Res 1995; 336: 101-13.
Jaz96  Jazwinski SM. Longevity, genes, and ageing. Science 1996; 273: 54-9.
Kai96  Kaiser J. New data help toxicologists home in on assessing risks. Science 1996; 272: 200.
Kar95  Karp JE, Broder S. Molecular foundations of cancer: new targets for intervention. Nature Medicine 1995;
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Kir92  Kirkwood TBL. Comparative life spans of species: why do species have the life spans they do? Am J Clin
       Nutr 1992; 55: 1191S-5S.
Lit96  Lithgow GJ, Kirkwood TBL. Mechanisms and evolution of ageing. Science 1996; 273: 80.
Mac96  Macilwain C. Risk: a suitable case for analysis? Nature 1996; 380: 10-1.
Mar94  Marx J. New link found between p53 and DNA repair. Science 1994; 266: 1321-2.
Mar96  Martin GM, Austad SN, Johnson TE. Genetic analysis of ageing; role of oxidative damage and
       environmental stresses. Nature Genetics 1996; 13 (mei): 25-34.
McM94  McMichael AJ. Invited commentary - “molecular epidemiology”: new pathway or new travelling
       companion? Am J Epidemiol 1994; 140: 1-11.
Men95  Menzel DB. Extrapolating the future: research trends in modeling. Toxicology Letters 1995; 79: 299-303.
Par93  Partridge L, Barton NH. Optimality, mutation, and the evolution of ageing. Nature 1993; 362: 305-11.
Per96a Perera FP. Uncovering new clues to cancer risk. Scientific American 1996; 274(5): 40-6.
Per96b Perera FP. Molecular epidemiology: insights into cancer susceptibility, risk assessment, and prevention. J
       Natl Cancer Inst 1996; 88: 496-509.
Por95  Portney PR, Harrington W. Health-based environmental standards: balancing costs with benefits.
       Resources 1995 (summer): 7-10.
Sag94  Sagan L. The triumph of theology over science: the non-threshold effects model. Int J of Occup Med and
       Toxicol 1994; 3: 161-72.
San95  Sankaranarayanan K, Chakraborty R. Cancer predisposition, radiosensitivity and the risk of
       radiation-induced cancers. I Background. Radiat Res 1995; 143: 121-43.
Sch92  Schreck R, Albermann K, Baeuerle PA. Nuclear Factor kB: an oxidative stress responsive transcription
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Sch95  Schatzkin A, Goldstein A, Freedman LS. What does it mean to be a cancer gene carrier? Problems in
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<pre>Shi94 Shigenaga MK, Hagen TM, Ames BN. Oxidative damage and mitochondrial decay in ageing. Proc Natl
      Acad Sci USA 1994; 91: 10771-8.
Soh96 Sohal RS, Weindruch R. Oxidative stress, caloric restriction, and ageing. Science 1996; 273: 59-63.
Spo96 Sporn MB. The war on cancer. Lancet 1996; 347: 1377-81.
Ste94 Stevenson DE, Sielken RL, Bretztaff RS. Challenges to low-dose linearity in carcinogenesis from
      interactions among mechanistic components as exemplified by the concept of ‘ invaders’ and ‘defenders’.
      BELLE Newsletter 1994; 3(2): 1-8.
Sto95 Stone R. A molecular approach to cancer risk. Science 1995; 268: 356-7.
Tha95 Thanos D, Maniatis T. NF-kB: a lesson in family values. Cell 1995; 80: 529-32.
Var93 Varmus H, Weinberg RA. Genes and the biology of cancer. New York, 1993.
WHO95 WHO Regional Office for Europe. Guiding principles for the use of biological markers in the assessment
      of human exposure to environmental factors: an integration approach of epidemiology and toxicology.
      Report on a WHO consultation. Toxicology 1995; 101: 1-10.
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<pre>Annex E
      Note for the conference
1     Introduction
      Following recommendations in the background document ‘The debate about low levels
      of exposure’ (Annex D) the working conference ‘Health Effects of Low Level
      Exposures: Scientific Developments and Perspectives for Risk Assessment’ will be
      structured around three cases and three broad questions. These cases are
      1 ‘ionising radiation’,
      2 ‘UV radiation’, and
      3 ‘dioxins’.
      The questions are
      1 What is the state of knowledge about deleterious effects and defence mechanisms
           following low levels of exposure?
      2 What are the implications of those insights for risk assessment procedures?, and
      3 Which types of research should be prioritized to promote evidence-based risk
           assessment?
      Together with the background document this note provides some topical information
      for the sessions of the conference. It highlights a number of insights, research
      questions, or controversial issues which play a more or less prominent role in
      discussions about the impact of low level exposures. Among the recurring themes in
      the scientific debate are:
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<pre>        the relative importance of intracellular and intercellular processes,
        the relative importance of adverse and beneficial effects,
        the difference between early and late health effects, and
        the significance of subclinical effects.
    The next sections briefly touch upon one or more of these issues. Section 2 provides
    background information on the three cases of the working conference. Section 3 draws
    attention to the importance of conceptual frameworks and to the possible contributions
    from various biological disciplines. The main objective of these sections is to indicate
    what kind of phenomena and problems the Health Council would like to see addressed
    by speakers and other participants. To this end subsections 2.4 and 3.2 contain
    specifications of the three broad questions listed above.
2   Topics in low level risk assessment
2.1 Issues with respect to ionising radiation
        In recent years much discussion has been devoted to so-called adaptive responses:
        a low priming dose of ionising radiation appears to protect cells from damage of a
        subsequent high dose. The deleterious effects in question are chromosomal changes
        and gene mutations (Mut96, NRP95, UNS94). Radiobiologists try to find out what
        kind of mechanisms may be responsible for these experimental findings. In
        addition, and more importantly, they are investigating whether similar adaptive
        responses or protective mechanisms may lead to a reduction in tumour incidence
        rates following low level exposure. This still remains a controversial issue, with
        potentially far reaching implications for risk assessment and radiological protection
        measures.
        In contrast to the adaptive responses, there is recent evidence for at least two
        mechanisms by which the effects of ionising radiation can be extended beyond the
        immediate consequences of energy loss events. While there is currently no direct
        evidence that such dose amplification effects have health implications, the fact that
        they exist suggests that the implications of the adaptive responses ought not to be
        considered in isolation in the context of low dose effects.
        Ionising radiation is one of the few environmental stressors for which relatively
        detailed mechanistic dose-response models have been developed (Bog97, Lue96,
        Moo90). Models like these give a mathematical description of cellular processes of
        carcinogenesis, such as (various types of) cell transformation, cell proliferation,
        cell killing, and cell replacement. Many experts hold that in principle mechanistic
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<pre>        models are better tools for risk assessment than statistical ones. However, so far
        only the latter have been used for policy purposes.
2.2 Issues with respect to UV radiation
        Quite a lot is known about the way in which UV radiation can induce skin cancer
        (Gru96). It generates mutagenic DNA photoproducts, leading to dysfunctional
        genes and malignant transformations, and also downregulates immune responses
        which can eliminate such transformed cells. Although there exists a whole battery
        of defence mechanisms, from radical scavengers and repair enzymes to apoptosis
        and immune surveillance, protection is generally assumed not to be perfect. Yet,
        with very low daily exposures, a threshold for tumour induction in experimental
        animals seems to appear, probably because induction times become longer than the
        lifespan. Another interesting finding is the apparent connection between DNA and
        cytokines (Yar96). Unrepaired DNA photoproducts cause the release of particular
        cytokines which stimulate carcinogenesis, whereas repair of DNA lesions checks
        the release and expression of these cytokines.
        Not only resistance against tumour induction may be affected by exposure to UV
        radiation, other immune functions are also at risk. Studies indicate that this immune
        modulation might influence the incidence and severity of allergic, infectious, and
        autoimmune diseases. However, data necessary to quantitate these risks still seem
        to be lacking (Sel97).
2.3 Issues with respect to dioxins
        Recent years have seen a growing interest in early subclinical effects of low levels
        of exposure to dioxin and dioxin-like chemicals (Bir95, Koh96). One of the areas
        of investigation is the increase in UGT enzymatic activity subsequent to dioxin
        exposure. This phenomenon is considered to be useful as a biomarker for
        tumorigenic changes in thyroid hormone levels. So-called physiological dosimetric
        models are developed to formulate a quantitative dose-effect relationship for this
        biomarker. However, the connection between biomarker values and tumour
        incidence rates remains to be clarified. A second effect which occupies the
        attention of toxicologists and which might have something in common with the
        first relates to subtle influences on the early development of organisms. Its
        significance for human health in the longer term is unclear.
        The mode of action of dioxins and dioxin-like compounds has been extensively
        studied over the past decades. Investigations have provided a fairly clear picture of
        the relevant signalling pathways (Sch96). Ah receptor-mediated responses are
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<pre>        usually classified as either adaptive, involving the upregulation of genes encoding
        xenobiotic-metabolizing enzymes, or toxic, involving effects which are
        inconsistent with an adaptive response and appear to have a negative impact. Only
        potent Ah receptor agonists seem to be able to elicit these toxic responses.
2.4 Specified questions for discussion following the introductory contributions
    concerning ‘ionising radiation’, ‘UV radiation’, and ‘dioxins’
    1   Which intracellular and intercellular processes qualify for consideration in
        assessing the health effects of low levels of exposure?
    2   How should ‘exposure’ be defined with respect to these processes, and which
        exposures or exposure rates are to be taken as ‘low’?
    3   When does the evidence base suffice to incorporate such processes into scientific
        or risk assessment models, and how should this be done?
    4   How are models taking modes of action into account to be fitted to epidemiological
        or experimental data, e.g. what are the perspectives for biomarker epidemiology?
    5   Can a priority list be given of issues in need of clarification?
3   The importance of conceptual frameworks
3.1 Biological complexity and ideas for dealing with it
    The more detailed and refined our insights into the toxicokinetic and toxicodynamic
    properties of a particular environmental stressor become, the better will be our
    predictions of what happens at low levels of exposure, or rather the less uncertain we
    will feel about them. But this truism involves two major drawbacks. Firstly, there may
    be a very large number of facts about the stressor in question which in principle qualify
    for consideration in predicting its low level effect. In the final analysis that may lead to
    very complicated and time consuming descriptions and derivations. Secondly, there
    exists a large variety of stressors, which may considerably differ in their biochemical
    characteristics and mutual interactions. This might entail, among other things, that
    predictions of the effect of low level exposure to an individual stressor given the
    simultaneous influence of other stressors pose even greater difficulties.
    So, in order to keep things manageable simplifications are unavoidable.
    Radiobiologists and toxicologists are engaged in selecting mechanisms and processes
    to be included in the descriptions of the (potential) effects of low level exposures.
    Often the available data about an individual environmental stressor are the be-all and
    end-all of the analysis. Approaches attempting to integrate the mode of action of a
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<pre>   particular stressor and the gamut of simultaneously operating endogenous and
   exogenous (’background’) factors seem to receive relatively little attention. Other
   branches of biology might be helpful here. For instance, they might provide evidence
   for the existence of a set of global constraints on possible interactions of diverse
   mechanisms, irrespective of some of the individual characteristics of a particular
   stressor. Such constraints could be important for other reasons as well. Data for many
   stressors are scarce or fragmentary rather than numerous or detailed. Global principles
   of description might then be a scientifcally justified way to compensate for this lack of
   data. They might also be used to suggest and defend research priorities.
   In theoretical biology interactions between deleterious and defensive mechanisms are
   studied from a very general perspective (Dou96). An example is what kind of systems
   are able to produce phenomena which are characterized by non-monotonic
   exposure-effect curves. Evolutionary biology has as one of its basic ideas that humans
   like any other organism have a history of adaptation and natural selection. From this
   point of view it may be argued that there exists a system of checks and balances
   (homeostasis), but also that with toxic stress physiological costs are enhanced (For96).
   The possible links between such costs, homeostasis, and adverse effects on health are
   intriguing. Biogerontology is a field of study where insights are being developed which
   for a number of reasons may be particularly useful in this respect (Ess95, Kow96).
   Because attention is focused on, usually multifactorial, physiological effects rather than
   on the influence of separate stressors, integrative approaches are a natural characteristic
   of this discipline. Moreover, ageing and late health effects, such as cancer, are often
   assumed to be tightly coupled processes: as maintenance and repair become
   increasingly ineffective with age, the incidence of chronic disorders strongly increases.
   Two interrelated phenomena just mentioned, viz. the simultaneous influence of various
   endogenous and exogenous stressors and the multifactorial character of many health
   effects, cause some experts to argue that linearity of exposure-effect curves might be
   the rule rather than the exception for environmental stressors (Hei97). This position
   seems to depend on a number of critical assumptions, in particular that the
   ‘background’ level of the effects under analysis is non-zero and that defensive
   countermeasures have already been overwhelmed. The validity of these assumptions is
   a matter of debate. Differences in susceptibility to environmental stressors between
   various groups within a population might turn out to be a factor of major importance in
   this context.
   Insights into signalling pathways between cells are rapidly growing. The existence of
   such types of cell-cell communication is often assumed to be a clear indication for the
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<pre>      defensive capabilities of the human body. But pathways like these might also be
      involved in pathogenetic processes. Two mechanisms might be a case in point. Firstly,
      gap junctions, which are formed by proteins called connexins, play a vital role in
      embryogenesis, cell differentiation, and the coordination of tissue responses. Emerging
      data gradually show that abnormalities in connexins can lead to various diseases
      (Pen96). Secondly, the nuclear factor NF-kB turns on genes involved in the body’s
      response to inflammation, infection, and stress (Bae96). Recent experiments have
      suggested that NF-kB might both block and mediate apoptotic cell death (Lip97). This
      illustrates the potential complexity of signalling cascades associated with inter- and
      intracellular processes and draws attention to the opposing forces which might be at
      work there.
3.2   Questions for discussion following the introductory contributions
      in sessions 1 and 5
      1    Is it possible to formulate some general constraints on descriptions of the effects of
           low level exposures, for example principles depending on the exposure rate, on the
           replicative capacity of the cell types under attack, and on the number and nature of
           stages involved in pathogenesis?
      2    How should the influence of simultaneously operating endogenous and exogenous
           (‘background’) factors be taken into account?
      References
Bae96 Baeuerle PA, Baltimore D. NF-kB: ten years after. Cell 1996; 87 : 13-20.
Bir95 Birgelen APJM van, Smit EA, Kampen IM, a.o. Subchronic effects of 2,3,7,8-TCDD or PCBs on thyroid
      hormone metabolism: use in risk assessment. Eur J Pharmacol 1995; 293 : 77-85.
Bog97 Bogen KT. Do U.S. county data disprove linear no-threshold predictions of lung cancer risk for residential
      radon? - a preliminary assessment of biological plausibility. Human and Ecological Risk Assessment
      1997; 3:157-86.
Dou96 Doucet PG. A feedback model for hormesis (mimeo, Amsterdam, 1996)
Ess95 Esser K, Martin GM (eds). Molecular aspects of ageing. Chichester, 1995.
For96 Forbes VE, Calow P. Costs of living with contaminants: implications for assessing low-level exposures.
      BELLE Newsletter 1996; 4: 1-8.
Gru96 Gruijl FR de. Photobiology of photocarcinogenesis. Photochem and Photobiol 1996; 63 : 372-5.
Hei97 Heitzmann M, Wilson R. Low-dose linearity: the rule or the exception? BELLE Newsletter 1997; 6(1) :
      2-8.
Koh96 Kohn MC, Sewall CH, Lucier GW, Portier CJ. A mechanistic model of effects of dioxin on thyroid
      hormones in the rat. Toxicol Appl Pharmacol 1996; 136 : 29-48.
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<pre>Kow96 Kowald A, Kirkwood TBL. A network theory of ageing: the interactions of defective mitochondria,
      aberrant proteins, free radicals and scavengers in the ageing process. Mutat Res 1996; 316 : 209-36.
Lip97 Lipton SA. Janus faces of NF-kB: neurodestruction versus neuroprotection. Nature Medicine 1997; 3 :
      20-1.
Lue96 Luebeck EG, Cutis SB, Cross FT, Moolgavkar SH. Two-stage model of radon-induced malignant lung
      tumors in rats: effects of cell killing. Radiat Res 1996; 145: 163-73.
Moo90 Moolgavkar SH, Cross FT, Luebeck G, a.o. A two-mutation model for radon-induced lung tumors in rats.
      Radiat Res 1990; 121: 28-37.
Mut96 Mutation Research. The adaptive response to very low doses of ionizing radiation. Special issue. Mutat
      Res 1996; 358 : 125-246.
NRP95 National Radiological Protection Board. Risk of radiation-induced cancer at low doses and low dose rates
      for radiation protection purposes. Chilton, Didcot, Oxon 1995.
Pen96 Pennisi E. Genes, junctions, and disease at cell biology meeting. Science 1996; 274 : 2008-9.
Sch96 Schmidt JV, Bradfield CA. Ah receptor signaling pathways. Annu Rev Cell Dev Biol 1996; 12 : 55-89.
Sel97 Selgrade MK, Repacholi MH, Koren HS. Ultraviolet radiation-induced immune modulation: potential
      consequences for infections, allergic, and auto-immune disease. Environ Health Perspect 1997; 105 :
      332-4.
UNS94 UNSCEAR. Annex B. Adaptive responses to radiation in cells and organisms, in: Sources and effects of
      ionizing radiation. 1994 Report to the General Assembly, with annexes. New York, 1994.
Yar96 Yarosh DB, Kripke ML. DNA repair and cytokines in antimutagenesis and anticarcinogenesis. Mutat Res
      1996; 350 : 255-60.
65    Note for the conference
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<pre>Annex F
      Conference programme
      October 19, Sunday
         16.00 - 17.00    Reception
      Session I. Heuristic overture
      (For background information see section 3.1 of the note (Annex E))
         17.00 - 18.30    Opening address
                          JA Knottnerus (chairman)
                          Aim and structure of the working conference
                          EJ Schoten
                          Low level effects from a theoretical perspective
                          P Doucet
                          Low level effects from the perspective of evolutionary biology
                          SALM Kooijman
                          Low level effects from the perspective of biogerontology
                          J Vijg
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<pre>                       Possibilities and impossibilities of environmental epidemiology
                       B Brunekreef
      18.30- 19.30     Discussion
                       (For questions see section 3.2 of the note)
      19.30 - 21.00    Dinner
   October 20, Monday
      09.00 - 09.05    Technical information
   Session II. Ionising radiation
   (For background information see section 2.1 of the note)
      09.05 - 10.15    Introduction
                       BA Bridges (chairman)
                       Significance of data on repair mechanisms
                       R Cox
                       Potential contributions from biomarker epidemiology
                       R Cox
                       Insights into adverse effects
                       BA Bridges
      10.15 - 11.30    Discussion
                       (For questions see section 2.4 of the note)
      11.30 - 12.30    Break, Lunch
   Session III. UV radiation
   (For background information see section 2.2 of the note)
      12.30 - 13.45    Introduction
                       JC van der Leun (chairman)
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<pre>                      Cellular effects and repair mechanisms
                      LHF Mullenders
                      Intercellular effects and repair mechanisms
                      SE Ullrich
                      Combining epidemiological and mechanistic information
                      FR de Gruijl
      13.45 - 15.00   Discussion
                      (For questions see section 2.4 of the note)
      15.00 - 16.00   Break
   Session IV. Dioxins
   (For background information see section 2.3 of the note)
      16.00 - 17.15   Introduction
                      HG Neumann (chairman)
                      Using in vitro and in vivo data on carcinogenicity
                      M van den Berg
                      Using in vitro and in vivo data on developmental effects
                      EK Silbergeld
                      Combining epidemiological and mechanistic information
                      EK Silbergeld
      17.15 - 18.30   Discussion
                      (For questions see section 2.4 of the note)
      18.30 - 20.00   Dinner
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<pre>   October 21, Tuesday
      09.00 - 09.05   Technical information
   Session V. Scientific possibilities and limitations
      09.05 - 10.00   Introduction
                      JE Trosko (chairman)
                      Synthesis with a view to modelling
                      CJ Portier
                      Significance of cellular and intercellular processes
                      JE Trosko
      10.00 - 12.00   Discussion on the basis of a handout made at the working
                      conference
                      (For questions see also section 3.2 of the note)
      12.00 - 13.30   Break, lunch
   Session VI. Perspectives for risk assessment
      13.30 - 15.30   Introduction
                      JA Knottnerus (chairman)
                      Concluding discussion about questions and statements in the handout
                      Closing address
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<pre>Annex G
      Participants and
      Scientific Advisory Committee
      The following experts participated in the working conference:
          dr M van den Berg; Research Institute of Toxicology; University of Utrecht; The
          Netherlands
          dr BA Bridges; Medical Research Council Cell Mutation Unit; University of
          Sussex; Brighton, United Kingdom
          dr B Brunekreef; Department of Health Studies; Agricultural University of
          Wageningen; The Netherlands
          dr KH Chadwick; Radiation Protection Research Unit; Directorate General XII of
          the European Commission; Brussels, Belgium
          dr R Cox; National Radiological Protection Board; Chilton, Didcot, United
          Kingdom
          dr P Doucet; Department of Theoretical Biology; Free University of Amsterdam;
          The Netherlands
          dr WH Farland; Environmental Protection Agency; Washington, United States
          dr VJ Feron; Toxicology Division; TNO Nutrition and Food Research Institute;
          Zeist, The Netherlands
          dr FR de Gruijl; Department of Dermatology; University Hospital of Utrecht; The
          Netherlands
          dr JHJ Hoeijmakers; Department of Cell Biology and Genetics; Erasmus University
          Rotterdam; The Netherlands
          dr SALM Kooijman; Department of Theoretical Biology; Free University of
          Amsterdam; The Netherlands
71    Participants and Scientific Advisory Committee
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<pre>       dr E Lebret; Department for Chronic Diseases and Environmental Epidemiology;
       National Institute of Public Health and the Environment; Bilthoven, The
       Netherlands
       dr JC van der Leun; Department of Dermatology; University Hospital of Utrecht;
       The Netherlands
       dr PHM Lohman; Department of Radiation Genetics and Chemical Mutagenesis;
       Leiden University Medical Center; The Netherlands
       dr LHF Mullenders; Department of Radiation Genetics and Chemical Mutagenesis;
       Leiden University Medical Center; The Netherlands
       dr HG Neumann; Department of Toxicology; University of Würzburg; Germany
       mrs MNEJG Philippens; Ministry of Housing, Spatial Planning, and the
       Environment; The Hague, The Netherlands
       dr CJ Portier; National Institute of Environmental Health Sciences; Research
       Triangle Park, United States
       dr W Seinen; Research Institute of Toxicology; University of Utrecht; The
       Netherlands
       dr EK Silbergeld; Department of Epidemiology and Preventive Medicine;
       University of Maryland at Baltimore; United States
       dr W Slob; Laboratory for Health Effects Research; National Institute of Public
       Health and the Environment; Bilthoven, The Netherlands
       dr JE Trosko; Department of Pedriatics and Human Development; Michigan State
       University; East Lansing, United States
       dr SE Ullrich; Department of Immunology; MD Anderson Cancer Center; Houston,
       United States
       dr PF de Vries Robbé; Department of Medical Informatics and Epidemiology;
       University of Nijmegen; The Netherlands
       dr J Vijg; Harvard Institute of Medicine; Harvard University; United States
       dr JD Wilson; Resources for the Future; Washington, United States
   Participants from the Health Council were:
       dr ASAM van der Burght
       dr JA Knottnerus, Vice President
       dr WF Passchier
       dr E van Rongen
       mr EJ Schoten, Scientific Secretary
       dr PW van Vliet
   Drs Van der Burght, Van Rongen, and Van Vliet helped draw up the minutes of the
   conference. Mrs MFC van Kan gave secretarial assistance.
72 HELLE
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<pre>   Members of the Scientific Advisory Committee for the conference were:
       dr BA Bridges; Medical Research Council Cell Mutation Unit; University of
       Sussex; Brighton, United Kingdom
       dr EJ Calabrese; Northeast Regional Environmental Public Health Center;
       University of Massachusetts; Amherst, United States
       dr JHJ Hoeijmakers; Department of Cell Biology and Genetics; Erasmus University
       Rotterdam; The Netherlands
       dr TBL Kirkwood; School of Biological Sciences and Department of Geriatric
       Medicine; University of Manchester; United Kingdom
       dr SALM Kooijman; Department of Theoretical Biology; Free University of
       Amsterdam; The Netherlands
       dr PHM Lohman; Department of Radiation Genetics and Chemical Mutagenesis;
       Leiden University Medical Center; The Netherlands
       dr HG Neumann; Department of Toxicology; University of Würzburg; Germany
       dr CJ Portier; National Institute of Environmental Health Sciences; Research
       Triangle Park, United States
       dr JE Trosko; Department of Pediatrics and Human Development; Michigan State
       University; East Lansing, United States
73 Participants and Scientific Advisory Committee
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<pre>74 HELLE</pre>

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<pre>   HELLE
   Gezondheidseffecten van lage blootstellingniveaus
1  De aard van het probleem 77
2  De stand van wetenschap 79
3  De implicaties voor risicoanalyse 83
   Bijlage 85
A  Totstandkoming van het advies 87
75 Nederlandse vertaling van het advies
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<pre>76 HELLE</pre>

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<pre>Hoofdstuk 1
          De aard van het probleem
          De Gezondheidsraad is nauw betrokken bij de wetenschappelijke onderbouwing van
          blootstellingsnormen voor stoffen en straling ter bescherming van de volksgezondheid.
          In de loop der jaren heeft de Raad bijgedragen aan de formulering van principes en pro-
          cedures, zowel voor carcinogene als voor niet-carcinogene agentia . Bij de afleiding
          van gezondheidskundige advieswaarden draait de discussie als regel om de vraag welke
          extrapolatiemethodes in aanmerking komen (wat valt te concluderen uit gegevens over
          hoge blootstelling en over proefdieren?). In het algemeen schort het namelijk aan recht-
          streekse gegevens over gezondheidseffecten bij lage niveaus van blootstelling. Effecten
          bij die niveaus laten zich zelden detecteren via het gangbare dierexperimentele of epi-
          demiologische onderzoek: daarvoor schiet het vermogen van deze analyse-instrumen-
          ten om ‘signaal’ van ‘ruis’ te onderscheiden meestal tekort. Annex B bij dit advies be-
          vat een korte schets van de moeilijkheden en van de ingeburgerde manieren om daar-
          aan het hoofd te bieden.
               Toch bestaat de hoop dat de veronderstelde zwakke signalen, indien aanwezig,
          langs andere weg kunnen worden opgevangen. Men zou dan dieper moeten graven, dat
          wil zeggen moeten trachten na te gaan wat zich op onderliggende niveaus van biologi-
          sche organisatie afspeelt wanneer organismen worden blootgesteld aan lage doses stra-
          ling of stoffen. De moleculaire en de celbiologie reiken diverse methodes en technieken
          aan waarmee processen in cellen in kaart gebracht kunnen worden. Als gevolg daarvan
          groeit het inzicht in de moleculaire en cellulaire effecten van blootstelling aan agentia,
          dat wil zeggen in de werkingsmechanismen die aan de gezondheidseffecten ten grond-
          slag liggen. De Gezondheidsraad achtte vorig jaar de tijd rijp voor een inventarisatie
77        De aard van het probleem
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<pre>   van de stand van wetenschap op dit terrein. Daartoe werd van 19 tot en met 21 oktober
   1997 een internationale werkconferentie georganiseerd met als titel: ‘Health Effects of
   Low Level Exposures: Scientific Developments and Perspectives for Risk Assessment’.
        Kernvraag was in hoeverre de soms snel groeiende kennis over moleculaire en cel-
   lulaire effecten het verhoopte houvast biedt voor extrapolatie. Verschillende deelvragen
   waarover al langere of kortere tijd een debat woedt, kwamen tegen dat decor aan de or-
   de. Een van de voornaamste kwesties betrof de gangbare, maar steeds vaker ter discus-
   sie gestelde tweedeling tussen stochastisch en niet-stochastisch werkende agentia, en
   het daarmee corresponderende onderscheid tussen blootstelling-effectrelaties zonder en
   met een drempel (zie Annex B voor een beknopte toelichting). Ook werd van gedach-
   ten gewisseld over wat dikwijls wordt aangeduid als hormese: lage blootstellingsni-
   veaus zouden de gezondheid kunnen bevorderen. Om de vele facetten van de thematiek
   belicht te krijgen, waren deskundigen uit diverse vakgebieden uitgenodigd. Verder wa-
   ren drie agentia als kristallisatiepunten gekozen voor het algemenere debat: ioniserende
   straling, ultraviolette (UV) straling en dioxinen.
        In het voorliggende signalement wordt aandacht gevraagd voor enkele zaken die
   tijdens de discussies over de zoëven aangeduide kernvraag naar voren kwamen. Diver-
   se detailkwesties en de bredere context van de beschouwingen worden uitvoeriger be-
   schreven in het bijgevoegde verslag van de conferentie (Annex C) en in aan het verslag
   gehechte achtergronddocumenten (Annexes D en E). Wat volgt is een reeks overwegin-
   gen met betrekking tot de wetenschappelijke basis voor de afleiding van advieswaar-
   den, bezien in het licht van de vigerende procedures en tegen de achtergrond van het
   werk van de Gezondheidsraad. Bij de voorbereiding van de hierna volgende opmerkin-
   gen en aanbevelingen zijn verscheidene Nederlandse deskundigen geraadpleegd (zie
   Bijlage A).
78 HELLE
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<pre>Hoofdstuk 2
          De stand van wetenschap
          Naar de mening van de deelnemers laat de kernvraag van de conferentie zich niet in al-
          gemene zin beantwoorden. Voor sommige agentia, zoals ioniserende straling, UV stra-
          ling en dioxinen, is verhoudingsgewijs veel bekend over werkingsmechanismen. Maar
          zelfs dan staan verscheidene problemen een verreikende kwantitatieve modellering van
          blootstelling-effectrelaties op moleculair en celbiologische grondslag in de weg. Zo is
          het inzicht in de ontstaanswijze van ziekten en aandoeningen die mede veroorzaakt
          zouden kunnen worden door de bedoelde agentia, over het algemeen nog erg beperkt.
          Ook als de moleculair-biologische fundamenten grotendeels zijn blootgelegd, kent men
          nog niet alle stappen in het traject van gebeurtenissen binnen cellen naar manifeste ge-
          zondheidsschade. Tijdens de werkconferentie werd er meermalen op gewezen dat voor
          een goed begrip verschillende niveaus van biologische organisatie in beschouwing ge-
          nomen moeten worden: onmisbaar als de bestudering van moleculair-biologische pro-
          cessen mag zijn, meer fysiologisch georiënteerd onderzoek speelt eveneens een belang-
          rijke rol. Anderzijds laten de zoëven bedoelde problemen met betrekking tot de ont-
          staanswijze van ziekten zich in zoverre relativeren, dat niet per se alle processtappen
          steeds haarfijn bekend hoeven te zijn. Gesteld men weet dat bepaalde moleculaire of
          fysiologische biomarkers — waarover straks meer — eenduidig samenhangen met be-
          paalde vormen van blootstelling en van gezondheidseffecten. Dan kan een nauwkeurige
          kartering van het tussengelegen traject voor de risicobeoordeling achterwege blijven.
          Overigens kan dergelijke informatie voor andere doeleinden, bijvoorbeeld voor de ont-
          wikkeling van medische interventies, wel heel waardevol zijn.
79        De stand van wetenschap
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<pre>        Als men vanuit de invalshoek van risicofactoren naar pathogenetische processen
   kijkt, doemen verdere moeilijkheden op. De conferentiedeelnemers gaven aan dat zich
   onder invloed van xenobiotische agentia een scala aan fenomenen in cellen kan voor-
   doen. Daartoe behoren veranderingen in genexpressie, mutaties en celdood via apopto-
   se (geprogrammeerde afsterving) of necrose (andersoortige afsterving). Ook is het mo-
   gelijk dat zulke interacties geen duidelijke sporen nalaten. Welke veranderingen speci-
   fieke agentia zoal teweeg kunnen brengen, is vaak onvoldoende bekend. Vervolgens
   tast men niet zelden in het duister over de mogelijke invloed van die veranderingen op
   de gezondheid, of het daarbij nu gaat om een verhoogde kans op kanker, om versnelde
   veroudering of om ontregeling van bepaalde orgaanfuncties. Bij dit alles komt nòg een
   obstakel: hoe cellulaire processen — en hun resultante — precies afhangen van de ma-
   te en het tempo van blootstelling, is grotendeels in nevelen gehuld.
   Er zijn dus vragen te over en meestal betrekkelijk weinig gegevens voor de beantwoor-
   ding daarvan, zelfs als algemene informatie over moleculaire en cellulaire processen in
   ruime mate ter tafel ligt. Mechanistische modellering van blootstelling-effectrelaties
   lijkt de eerstkomende tijd nog buiten bereik te blijven, althans modellering ‘over de he-
   le linie’, dat wil zeggen van complete pathogenetische processen. Maar kennis over be-
   paalde deelprocessen bij blootstelling aan bepaalde xenobiotische agentia komt wel de-
   gelijk in een aanhoudende stroom beschikbaar. Zoals gezegd kan dat voor risicobeoor-
   deling soms toereikend zijn.
        Interessante ontwikkelingen doen zich onder meer voor op het terrein van de toxi-
   cokinetiek en -dynamiek. Door na te gaan hoe stoffen zich bij opname in het lichaam
   gedragen, is het mogelijk meer zicht te krijgen op de biologisch relevante (effectieve)
   blootstelling. Met behulp van zogeheten PBPK/PD-modellen (PBPK/PD: physiologi-
   cally based pharmacokinetic and pharmacodynamic) proberen toxicologen de betrok-
   ken verdelings- en omzettingsprocessen expliciet en systematisch te beschrijven. Bij di-
   oxinen bijvoorbeeld wordt daar de laatste jaren nogal wat onderzoek naar gedaan. Men
   ziet validering van dit soort modellen alom als een onderzoeksprioriteit en verwacht dat
   bijvoorbeeld de problemen van extrapolatie ‘van dier naar mens’ zo verminderd kun-
   nen worden.
         In het verlengde van de zojuist geschetste ontwikkelingen ligt veelbelovend onder-
   zoek naar biomarkers voor inwendige blootstelling en voor verhoogde gevoeligheid.
   Zo zouden mensen met bepaalde genetische eigenschappen eerder of sterker dan ande-
   ren de nadelige gevolgen van blootstelling aan bepaalde agentia kunnen ondervinden.
   Komt men dergelijke biomarkers op het spoor en slaagt men erin daarvan in fenomeno-
   logisch onderzoek gebruik te maken, dan kan de zeggingskracht van de analyses toene-
   men. Biomarkers voor vroege, dat wil zeggen aan manifeste gezondheidsschade voor-
   afgaande, effecten laten waarschijnlijk langer op zich wachten.
80 HELLE
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<pre>        Ook trends op het terrein van de transgenese (het overbrengen van gewenste erfelij-
   ke eigenschappen naar het genoom van een proefdier) bieden perspectief. Door gerichte
   gen-inactivatie in muizen is het mogelijk om specifiek één of meer cellulaire processen,
   bijvoorbeeld DNA-herstel en metabolisme van chemische stoffen, uit te schakelen. Zo-
   doende kan men analyseren hoe dergelijke processen het effect van blootstelling aan
   xenobiotische agentia beïnvloeden. Vanwege de sterk verhoogde gevoeligheid van
   sommige muismutanten voor bepaalde stoffen is ook het effect van lage doseringen
   soms direct meetbaar. Verder zullen transgene muizen die voorzien zijn van gevoelige
   systemen voor de detectie van mutaties, in de nabije toekomst het inzicht in het effect
   van blootstelling aan genotoxische agentia kunnen vergroten.
        Dan is er nog de voortgaande opmars van de informatietechnologie, die de analyse
   en synthese van allerlei voorliggende gegevens kan vereenvoudigen en verfijnen. Tij-
   dens de conferentie viel te beluisteren dat de op dat gebied bestaande mogelijkheden
   misschien nog te weinig benut worden. De statistische bewerking van uitkomsten van
   dierexperimenteel en epidemiologisch onderzoek, waar mogelijk aangevuld met infor-
   matie over werkingsmechanismen, kan nadere aanwijzingen verschaffen over de mate
   van onzekerheid waarmee de bepaling van blootstelling-effectrelaties en de afleiding
   van advieswaarden verbonden zijn. Zulke analysemethodes kunnen soms uitsluitsel ge-
   ven over de waarschijnlijkheid van het bestaan van een drempeldosis voor het optreden
   van bepaalde effecten.
   Een vraagstuk dat aparte vermelding verdient, betreft de feitelijke omstandigheden van
   blootstelling. Steeds is in de praktijk sprake van een gecombineerde invloed van een
   breder of smaller spectrum van endogene en exogene factoren, met deels overeenkom-
   stige werkingsmechanismen. Een voorbeeld is de productie van zogeheten vrije radica-
   len (bepaalde reactieve moleculen) door het normale zuurstofmetabolisme en door
   blootstelling aan ioniserende straling. Op de conferentie kwamen twee kwesties aan de
   orde die met dat gegeven verband houden.
        Ten eerste kan men zich afvragen of het correct en zinnig is blootstelling-effectre-
   laties af te leiden voor afzonderlijke agentia. Die vraag roept echter onmiddellijk een
   wedervraag op, namelijk welke uitgangspunten dan zijn te hanteren bij de bepaling van
   relaties tussen combinatie-blootstelling en gezondheidseffecten. Een principiële oplos-
   singsrichting tekende zich tijdens de conferentie nog niet af.
        Ten tweede is er de mogelijkheid van hormese. Sommige onderzoekers sluiten niet
   uit, of vinden het zelfs plausibel, dat blootstelling aan een specifiek agens onder om-
   standigheden bepaalde reactiemechanismen mobiliseert die de netto schade van de
   combinatie-blootstelling verminderen. Naar het oordeel van de conferentiedeelnemers
   ontbreken daarvoor echter totnogtoe overtuigende aanwijzingen.
81 De stand van wetenschap
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<pre>Hoofdstuk 3
          De implicaties voor risicoanalyse
          Het voorgaande leidt tot de conclusie dat de huidige beoordelingssystematiek op be-
          paalde onderdelen en onder bepaalde omstandigheden verfijnd kan worden. Ontwikke-
          lingen met betrekking tot bijvoorbeeld PBPK/PD-modellering, biomarkers voor varia-
          ties in gevoeligheid en modelleringsmethodieken, bieden kansen voor een nadere on-
          derbouwing van elementen of modules die een plaats hebben binnen de vigerende sys-
          tematiek. Men kan denken aan beter gefundeerde veiligheids- of extrapolatiefactoren
          waarmee mogelijke verschillen in gevoeligheid tussen en binnen species verdisconteerd
          worden. Idealiter laten zulke factoren zich geheel vervangen door (deel)modellen die
          de bedoelde variaties expliciet beschrijven. Meer in het algemeen valt te verwachten
          dat de relaties tussen componenten van het zogeheten integraal toxiciteitsprofiel, zoals
          beschreven en toegelicht in het Gezondheidsraadadvies ‘Toxicologische advieswaarden
          voor blootstelling aan stoffen’ (1996/12), met behulp van de hier bedoelde analysetech-
          nieken beter in kaart gebracht kunnen worden.
               Wat betreft de omstandigheden waaronder diepere analyses gerechtvaardigd lijken,
          verdient de doelmatigheid van de risicobeoordeling aandacht. Doorwrochte analyses in
          de zojuist bedoelde zin zijn arbeidsintensief en kostbaar. Het valt te overwegen ze het
          eerst te beproeven bij maatschappelijk prioritaire agentia. Criteria als de plausibiliteit
          van schadelijkheid bij feitelijk te verwachten blootstellingsniveaus, de omvang van de
          blootgestelde populatie, de ernst van de effecten, de mogelijkheid van risicoverminde-
          ring en de grootte van de meespelende economische belangen zouden bij de selectie
          van die agentia behulpzaam kunnen zijn. Zo’n selectie is ook om een andere reden van
          belang: de risicobeoordeling van bestaande stoffen op de Europese markt verloopt nog-
82        HELLE
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<pre>   al traag. Er zullen ook tijd en middelen moeten worden gereserveerd om daarin verbe-
   tering te brengen.
        Als het bij een risicobeoordelingsvraagstuk om diepgang en maatwerk gaat, kan
   een goed geregisseerde inbreng van verschillende deskundigen niet worden gemist. Zij
   zullen per geval, in onderling overleg, moeten bezien welk model het geheel aan voor-
   liggende gegevens het best beschrijft. Op dit moment is het moeilijk een uitspraak te
   doen over de generaliseerbaarheid van zulke modellen. De conferentiesessies gewijd
   aan ioniserende straling, UV straling en dioxinen illustreerden dit probleem (zie Annex
   C). In ieder geval zijn nu nog geen algemene aanbevelingen mogelijk met betrekking
   tot uitgangspunten voor mechanistische modellering of met betrekking tot de invloed
   van homeostatische controleprocessen. Kortom, de ervaring zal moeten leren in welke
   zin en hoe snel de huidige beoordelingssystematiek zich laat verfijnen.
   In het Werkprogramma 1999 van de Gezondheidsraad zijn, onder de kop ‘Uitgangs-
   punten voor gezondheidskundige advieswaarden’, vijf thema’s opgenomen die nauw
   verband houden met het voorgaande: (1) het opstellen van een integraal toxiciteitspro-
   fiel; (2) het gebruik van epidemiologische gegevens bij het opstellen van zo’n profiel;
   (3) het toepassen van de zogeheten ‘benchmark dose’-benadering (de benchmark dose
   is de onderste statistische betrouwbaarheidsgrens van de blootstelling die behoort bij
   een bepaald responsniveau); (4) het gebruik van veiligheidsmarges; en (5) het omgaan
   met combinatie-blootstelling. In Nederland vindt ook onderzoek naar een aantal van
   deze onderwerpen plaats, bij universitaire vakgroepen, in onafhankelijke onderzoeksla-
   boratoria, bij de overheid en vanuit de industrie. De soms al intensieve samenwerking
   tussen deze en buitenlandse instanties zal de kwaliteit en doelmatigheid van de risico-
   beoordeling in ons land zeker ten goede komen.
83 De implicaties voor risicoanalyse
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<pre>A  Totstandkoming van het advies
84 HELLE
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<pre>   Bijlage
85
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<pre>Bijlage A
        Totstandkoming van het advies
        Het advies is voorbereid door Eert Schoten, secretaris bij de Gezondheidsraad, na raad-
        pleging van de volgende deskundigen:
            dr ir B Brunekreef; hoogleraar gezondheidsleer; Landbouwuniversiteit Wageningen
            dr VJ Feron; hoogleraar biologische toxicologie; Universiteit Utrecht
            dr JHJ Hoeijmakers; hoogleraar moleculaire biologie; Erasmus Universiteit, Rotter-
            dam
            dr ir PHM Lohman; hoogleraar stralengenetica en chemische mutagenese; Rijks-
            universiteit Leiden
86      HELLE
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<br><br>