<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>Chlorpyrifos
(CAS No: 2921-88-2)
Health-based Reassessment of Administrative Occupational Exposure Limits
Committee on Updating of Occupational Exposure Limits,
a committee of the Health Council of the Netherlands
No. 2000/15OSH/067, The Hague, 22 september 2003
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<pre>Preferred citation:
Health Council of the Netherlands: Committee on Updating of Occupational
Exposure Limits. Chlorpyrifos; Health-based Reassessment of Administrative
Occupational Exposure Limits. The Hague: Health Council of the Netherlands,
2003; 2000/15OSH/067.
all rights reserved
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<pre>1     Introduction
      The present document contains the assessment of the health hazard of
      chlorpyrifos by the Committee on Updating of Occupational Exposure Limits, a
      committee of the Health Council of the Netherlands. The first draft of this
      document was prepared by AAE Wibowo, Ph.D. and MM Verberk, Ph.D.
      (Coronel Institute of the Academic Medical Centre, University of Amsterdam,
      Amsterdam, the Netherlands).
          The evaluation of the toxicity of chlorpyrifos has been based on the reviews
      published in the ‘Handbook of pesticide toxicology’ (Gal91) and by the
      American Conference of Governmental Industrial Hygienists (ACG99) and
      Richardson (Ric95), Cochran et al. (Coc95), and Minton and Murray (Min88).
      Where relevant, the original publications were reviewed and evaluated as will be
      indicated in the text. In addition, in April 2000, literature was searched in the on-
      line databases Medline, Embase, and Chemical Abstracts, starting from 1966,
      1988 and 1970, respectively, as well as in the CD-ROM versions of the databases
      Poltox (1990-1994) HSELINE, CISDOC, MHIDAS, and NIOSHTIC (from 1998
      backwards), and using the following key words: chlorpyrifos, dursban, and 2921-
      88-2. The Hazardous Substances Data Bank (HSDB) was also consulted
      (NLM02). Data of unpublished studies were generally not taken into account.
      Exceptions were made for studies that were summarised and evaluated by
      international bodies such as the Food and Agricultural Organization/World
      Health Organization (FAO/WHO: Joint Meeting of the FAO Panel of Experts on
      Pesticides Residues on Food and the Environment and the WHO Expert Group
      on Pesticides Residues - JMPR) (FAO00) and the Health Effects Division (HED)
      of the US Environmental Protection Agency (EPA) as part of its hazard
      identification assessment review (EPA00).
          In October 2002, the President of the Health Council released a draft of the
      document for public review. Comments were received from the following
      individuals and organisations: PA Watson (Dow AgroSciences, Abingdon,
      England) and J Soave (Health and Safety Executive, London, England). These
      comments were taken into account in deciding on the final version of the
      document.
          An additional search in Toxline and Medline in April 2003 did not result in
      information changing the committee’s conclusions.
067-3 Chlorpyrifos
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<pre>2     Identity
      name                      :      chlorpyrifos
      synonym                   :      phosphorothioic acid, O,O-diethyl O-(3,5,6-trichloro-2-pyridinyl) ester;
                                       O,O-diethyl O-(3,5,6-trichloro-2-pyridinyl) phosphorothioate; Dursban 4E;
                                       Detmol U.A.; Lorsban; Dowco 79
      molecular formula         :      C9H11Cl3NO3PS
      structural formula        :
      CAS number                :      2921-88-2
3     Physical and chemical properties
      molecular weight          :     350.59
      boiling point             :     162oC (decomposes)
      melting point             :     42.5-43oC
      vapour pressure           :     at 25oC: 2.5 x 10-3 Pa
      solubility in water       :     practically insoluble (at 25oC: 0.14 mg/100 mL)
      Log Poctanol/water        :     4.96 (experimental); 4.66 (estimated)
      conversion factors        :     not applicable
      Data from: ACG99,FAO73, Gal91, NLM03, http://esc.syrres.com.
      Pure chlorpyrifos is a colourless to white, crystalline solid with a mild mercaptan
      odour. Chlorpyrifos is stable under normal storage conditions. The half-life in
      aqueous methanolic solution at pH 6 is 1930 days; at pH 10, it is 7.2 days
      (Gal91).
4     Uses
      Chlorpyrifos is used to control many types of insect pests in a wide range of
      crops and ornamentals. It is also used to control mosquitoes, flies, and household
      pests, including termites (Ric95, Rob99, Sul82). The compound is applied in
067-4 Health-based Reassessment of Administrative Occupational Exposure Limits
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<pre>      emulsifiable concentrates, dust, flowable pellet, spray and granular wettable
      powder (NLM02).
           According to the database of the Dutch Pesticide Authorisation Board
      (CTB)*, chlorpyrifos is at present registered in the Netherlands for its use as an
      active ingredient in a few formulations controlling crawling insects (in specific
      spaces), moths (protecting woollen fabrics), and cabbage maggots (on cabbage
      varieties).
5     Biotransformation and kinetics
      Human data
      Six male human volunteers were given an oral dose of chlorpyrifos of 0.5
      mg/kg bw. Approximately 70% of the administered dose was excreted in the
      urine as the metabolite 3,5,6-trichloro-2-pyridinol (3,5,6-TCP), most of it within
      24 hours after dosing. Peak 3,5,6-TCP concentrations in the blood were reached
      after 6 hours. No unchanged chlorpyrifos could be detected. Four weeks later, 5
      of the same men received a single dermal dose of 5 mg/kg bw of chlorpyrifos
      dissolved in dipropylene glycol methyl ether. The amount of the dose excreted in
      the urine as 3,5,6-TCP within 120 hours after dosing was 1.3%. Peak
      concentrations of the metabolite in blood were reached 24 hours after dosing.
      The half-life of elimination of 3,5,6-TCP from the blood and the urine was 27 h
      following both oral and dermal doses (Nol84). Chlorpyrifos metabolites diethyl
      phosphate (DEP) and diethyl phosphorothioate (DEPT) were found in the urine
      of spray workers, who had been exposed to 8-hour time-weighted average
      airborne chlorpyrifos concentrations ranging from 0.012 to 0.145 mg/m3. The
      authors claimed that most of these urinary alkylphosphates were derived from
      dermal absorption rather than from inhalation of chlorpyrifos (Sun89). In another
      exposure monitoring study, detectable concentrations of 3,5,6-TCP were
      measured in the urine of 6 termite control workers. Levels were lower than 0.10
      mg TCP/g creatinine in the off-season period; however, they increased markedly
      in the beginning of the termite control season and reached peak levels ranging
      from 0.07 to 4.18 mg TCP/g creatinine in the busy spraying season. A significant
      negative correlation was found between urinary TCP levels and plasma
      cholinesterase activities (Jit89). Chlorpyrifos in serum and urinary metabolites
      DEP and DEPT were measured in 3 subjects following ingestion of a
      chlorpyrifos formulation. The subjects were admitted to hospital 2 to 5 hours
*     at: http://www.ctb-wageningen.nl/geel.html.
067-5 Chlorpyrifos
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<pre>      after ingestion and initially treated with atropin and then during the first 5 days of
      hospitalisation with pralidoxime. The maximum levels of chlorpyrifos in serum
      and of urinary DEP and DEPT were measured on the day of admission.
      Chlorpyrifos was eliminated from the serum with a half-life of 1.1-3.3 h. The
      half-life of excretion of total urinary DEP + DEPT was initially 6 h, followed by
      a slower phase with a half-life of 80 h. The chlorpyrifos oxygen analogue
      chlorpyrifos oxon was not detected. There was no correlation between the
      maximum concentration of total urinary DEP + DEPT metabolites normalised to
      creatinine and the initial inhibition of blood cholinesterase activities (Dre93,
      Vas92).
      Animal data
      The inhalation absorption of chlorpyrifos vapours has been studied in 4 female
      F344 rats, following nose-only exposure to a concentration of 201 µg/m3 for
      6 hours. The average amount of 3,5,6-TCP and conjugates excreted in the urine
      during the first 48 hours after the beginning of exposure was 0.019 µg per 1
      µg/m3 of airborne chlorpyrifos exposure. From these data, the authors calculated
      that 72% of the inhaled vapours of chlorpyrifos was absorbed (Nol86).
          Female Dublin ICR mice received a single dermal application of 1 mg/kg bw
      of 14C-ring-labelled chlorpyrifos dissolved in acetone. The half-life of skin
      absorption (i.e., disappearance of radioactivity from the application site) of the
      chemical was 20.6 min. At 8 hours post-application, the percentage of skin
      penetration was 73.9%. At that time point, the radioactivity recovered in the
      urine and faeces was 13% and 26%, respectively, while that in carcass, blood,
      intestine, and liver amounted to 24%, 2.7%, 1.9%, and 1.8% of the applied dose,
      respectively (Sha81). To assess the dermal absorption of chlorpyrifos in rats, 0.2,
      1.0, and 5.0% solutions of unlabelled compound in ethanol were applied under
      occlusion to the tail of Wistar rats for 4 hours. The amounts of chlorpyrifos found
      in tail tissues were 1.8, 2.4, and 2.5% of the applied dose, respectively (Tos94).
          The oral absorption of chlorpyrifos was assessed in ICR mice that were given
      a single oral dose of 1 mg/kg bw of 14C-ring-labelled chlorpyrifos by stomach
      needle. The half-life of absorption from the gastrointestinal tract into the body
      was 78 min. At 60 minutes post-application, the percentage of absorption was
      47.2%, with 13.4 % of the radioactivity recovered in the urine (Ahd81). The
      percentage of absorption of the compound through ligatured stomach of ICR
      mice at 60 minutes post-application was only 11% (Ahd82).
067-6 Health-based Reassessment of Administrative Occupational Exposure Limits
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<pre>          In a metabolism and tissue distribution study, groups of male and female
      F344 rats were given single oral doses of 0.5 or 25 mg/kg bw of 14C-labelled
      chlorpyrifos or 15 daily doses of 0.5 mg/kg bw unlabelled compound followed
      by one dose of 0.5 mg/kg bw of 14C-labelled chlorpyrifos. During 72 hours,
      essentially all radioactivity had been recovered, mainly in the urine (84-92% of
      the administered dose), with 6-12% found in the faeces and less than 0.2% in the
      tissues (mainly perirenal fat, liver and ovaries) and the carcass. Repeated dosing
      induced a 6-7% increase in urinary excretion as compared with the single dose
      of 0.5 mg/kg bw. The major urinary metabolites were 3,5,6-TCP (12%), its
      glucuronide (80%), and, tentatively, a sulphate conjugate (Nol87). In another
      study, 12 male rats were given a single oral (gavage) dose of 14C-ring-labelled
      chlorpyrifos of 5 mg/kg bw. Within 48 hours, 88% of the dose was excreted in
      the urine. At least 6 metabolites were detected, 3 of which accounted for 97% of
      the urinary radioactivity. These were identified as the glucuronide of 3,5,6-TCP
      (80%), a glucoside of TCP (4%), and freeTCP (12%) (Bak76). In a subsequent
      study with 4 female rats, given a single oral dose of 14C-chlorpyrifos of 0.93
      mg/kg bw, 79% of the dose was excreted in the urine and 15% in the faeces
      within 48 hours after the administration. Conjugated 3,5,6-TCP metabolites
      excreted in the urine would yield free 3,5,6-TCP on acid hydrolysis, indicating
      that the amount of total 3,5,6-TCP can be used to determine the amount of
      chlorpyrifos absorbed (Nol86).
          Activation of chlorpyrifos by oxidative desulphuration to the active
      cholinesterase inhibitor, i.e., chlorpyrifos oxon, also occurs in animals but the
      compound is often not detected owing to its rapid rate of hydrolysis. The bulk of
      this bioactivation of chlorpyrifos occurs in the liver, and detoxification of
      chlorpyrifos and its oxon analogue in the liver, and plasma, producing hydrolysis
      products DEP, DEPT, and 3,5,6-TCP in free and conjugated form (Ric95,
      Rob99). Extensive work on chlorpyrifos biotransformation with mouse liver
      microsomes and perfused intact mouse liver indicates that the free oxon does not
      escape from the liver (Sul88, Sul91).
6     Effects and mechanism of action
      Human data
      Human data of acute and long-term health effects reported to be related to
      exposure to chlorpyrifos have been summarised in several reviews (Blo97,
      FAO00, Shu97).
067-7 Chlorpyrifos
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<pre>      Sensitisation
      The prevalence of allergic contact dermatitis due to pesticide application was
      assessed by comparing 39 occupationally exposed subjects and 21 unexposed
      subjects by patch testing. Concerning chlorpyrifos, one out of 39 workers
      showed a positive reaction vs. none in the control group (Oma95).
      Acute toxicity
      In a case report of poisoning of a 42-year-old man with chlorpyrifos at an
      estimated dose of 300 mg/kg bw, inhibition of neuropathy target esterase (NTE)
      activity in lymphocytes was observed prior to the development of
      polyneuropathy. NTE is considered a molecular target in the nervous system for
      organophosphate-induced delayed polyneuropathy (OPIDN). Following
      ingestion of chlorpyrifos, cholinergic signs such as respiratory insufficiency,
      lachrymation, salivation, and fasciculations lasted for 17 days. During this
      period, extensive therapy with atropine and pyridine 2-aldoxime methane
      sulphonate (PAM) and artificial respiration was given. Thirty days after
      intoxication, the clinical and electrophysiological examination of the peripheral
      nervous system was normal, but plasma ChE, red blood cell acetylcholinesterase
      (AChE), and NTE activity were still inhibited by approximately 90%, 50%, and
      60%, respectively. On day 43, weakness and paraesthesia in the legs were noted
      which became more severe over time. Clinical examination, electrophysiology,
      and a nerve biopsy revealed signs of a peripheral polyneuropathy with mild distal
      axonopathy, consistent with OPIDN (Lot84, Lot86). In another case report, a
      38-year-old man drank an undefined quantity of a 25% chlorpyrifos solution.
      Cholinergic effects were stupor, respiratory distress, and complete inhibition of
      serum ChE activity. The man was successfully treated with extensive antidote
      therapy and artificial respiration and was discharged from the hospital one month
      after the incident. His serum ChE activity was still undetectable (She88). In a
      third case, a 21-year-old man who ingested an unknown quantity of chlorpyrifos
      showed cholinergic effects comprising pupillary constriction, excess secretions,
      tachycardia, and impaired consciousness. He was treated with atropine and PAM
      and received artificial respiration. Twenty-eight days after exposure, bilateral
      vocal cord paralysis developed which was considered to be indicative of OPIDN.
      The patient was asymptomatic within 4 weeks of the onset of symptoms (Sil94).
      In a fourth case, a pesticide applicator who had reportedly been exposed to
      chlorpyrifos in a closed environment for 6 months showed cholinergic signs,
067-8 Health-based Reassessment of Administrative Occupational Exposure Limits
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<pre>      including lachrymation, muscle twitching, and inhibition of red blood cell AChE.
      In addition, paraesthesias and numbness were reported. Neurological evaluation
      6 weeks later revealed sensory loss of all modalities as well as mild distal
      weakness and decreased reflexes in the lower extremities, without fasciculations
      or tremors. Changes in neurological examination were consistent with
      polyneuropathy. Follow-up after 1 year revealed normalisation of the
      neurological examination and remission of all symptoms (Kap93).
          In a human volunteer study, 6 male subjects were given a single oral dose of
      chlorpyrifos of 0.5 mg/kg bw. Four weeks later, 5 of the same men received a
      single dermal dose of 5 mg/kg bw of chlorpyrifos dissolved in dipropylene
      glycol methyl ether. No signs or symptoms of toxicity were reported in any of the
      men during the study. After oral administration, plasma ChE activity was
      maximally inhibited by 64-85% (mean 70%), 12 to 24 hours post-exposure, and
      red blood cell AChE by 11-52% (mean 27%), 4 days post-exposure. Plasma ChE
      activity had returned to more than 80% of the mean baseline level at day 30 after
      oral treatment. Men dermally exposed to 5 mg/kg bw exhibited a peak plasma
      ChE inhibition of 64-85% on post-exposure day 3 and returned to baseline values
      by day 40 of the study. Red blood cell AChE was not significantly inhibited after
      dermal dosing (Nol82, Nol84). In another human volunteer study, the effects of
      single oral doses of chlorpyrifos (purity: 99,8%) on red blood cell AChE activity
      was studied. Groups of 6 fasted men and women, aged 18-55 years, received
      single oral doses of 0, 0.5, 1, or 2 mg/kg bw in lactose powder. Blood samples for
      red blood cell AChE determination were collected pre-exposure and 2, 4, 8, 12,
      24, 36, 48, 72, 96, 120, 144, and 168 hours after treatment. Treatment had no
      effect on general health or on clinical chemical or haematological parameters
      measured 7 days after dosing. At 2 mg/kg bw, one woman exhibited a peak red
      blood cell AChE inhibition of 26% at 24 hours. The inhibition was 20% at 48
      hours after treatment and no further samples were taken. The mean red blood cell
      AChE activity was not significantly different among treated groups and between
      treated groups and the 2 control groups at any time. The NOAEL for inhibition
      of red blood cell AChE following an acute oral dose is 1 mg/kg bw on the basis
      of significant inhibition in one of 12 subjects, and the NOAEL for clinical signs
      or symptoms was 2 mg/kg bw (Kis99).
      Changes in the peripheral immune system were studied in 12 individuals, one to
      4.5 years following exposure to chlorpyrifos after treatment of their homes or
      workplaces by licensed operators. In one subject, exposure took place by an
      accidental spill. No quantitative exposure measurements were reported. Control
067-9 Chlorpyrifos
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<pre>       groups comprised volunteer groups of 28 students and 29 home dwellers. A
       higher rate of atopy, antibiotic sensitivities, elevated CD26 cells (activated T
       cells), and autoimmunity was observed in the exposed group when compared to
       the control groups. The autoantibodies were directed toward smooth muscle,
       parietal cells, brush border, thyroid gland, myelin, and antinuclear antibody. The
       authors concluded that chlorpyrifos, as used in pesticide sprays, should be
       examined more closely as a probable immunotoxin (Thr93). In a subsequent
       study comprising 29 non-smoking individuals (10 males, 19 females), the results
       of the former study were confirmed. This group had chronic health complaints
       with symptoms such as flu-like illness, headache, loss of memory, dizziness,
       gastrointestinal disturbance, arthralgia, menstrual irregularities, fatigue, and
       heightened olfactory sensitivity to low concentrations of chemicals, and were
       diagnosed by their physicians as having multiple chemical hypersensitivity. The
       results of analyses of the peripheral blood markers peripheral lymphocyte
       phenotypes, autoantibodies, and mitogenesis to phytohaemagglutinin and
       concanavillin of this group were compared with the 2 volunteer unexposed and 1
       exposed groups from the former study. Compared to the unexposed groups, there
       were changes in lymphocytes phenotypes, including an increase in CD26
       expression and a decrease in percentage of CD5 phenotype, an increased
       frequency of autoantibodies, and a decreased mitogenesis in response to
       phytohaemagglutinin and concanavillin. These alterations in peripheral blood
       markers were unaffected by medication, age, sex, or season (Thr02).
       Short-term toxicity
       In a human volunteer study, 6 subjects were exposed to aerosols from a 63%
       concentrate of Dowco 179 in xylene by using a thermal fogging machine. Three
       subjects were exposed to an average air concentration of 2.7 mg/m3, for 3 to 8
       minutes, and 3 subjects to an average air level of 102 mg/m3, for 1 to 4 minutes.
       Two subjects had a temporary decrease in plasma cholinesterase (ChE) activity
       (15 and 16% respectively) at 24 hours post-treatment. Activities, however, were
       already recovered at 96 hours post-exposure. No change in red blood cell
       acetylcholinesterase (AChE) activity was measured in any subject (Lud70).
            In a dermal dose study, 5 men received single doses of chlorpyrifos ranging
       from 94 to 620 mg/kg bw (intermediate levels not given), for 12 hours. No skin
       irritation was recorded. Plasma ChE was significantly inhibited at the 2 highest
       doses, but no changes were observed in red blood cell AChE activity (Kil70).
067-10 Health-based Reassessment of Administrative Occupational Exposure Limits
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<pre>           In an oral study, male volunteers (n=4/dose) were given tablets of technical
       chlorpyrifos at doses of 0, 0.014, 0.03, or 0.1 mg/kg bw/day for 48, 27, 20, and 9
       days respectively. After 9 days of treatment with 0.1 mg/kg bw/day, the mean
       plasma ChE activity of the 4 volunteers was decreased by 66% (range: -36 to -
       82% of baseline values) with respect to their pre-exposure values, while a mean
       increase of 20% was seen in the concurrent control group. The mean red blood
       cell AChE activity was increased by 5% (range: +2 to +9% of baseline values),
       while a mean increase of 14% was seen in the concurrent controls. One man
       developed blurred vision, runny nose, and a feeling of faintness, probably
       unrelated with exposure. No changes in clinical chemical and haematological
       analyses were found. Exposure of this group was discontinued because of the
       severe plasma ChE depressions, which returned to pre-exposure levels at 28 days
       after cessation of exposure. Men exposed to 0.03 mg/kg bw/day for 20 days
       showed a mean plasma ChE inhibition of 29% (range: -16 to -51% of baseline
       values), while the level in the concurrent control group was increased by 7%. A
       steady state inhibition of plasma ChE was reached after about 16 days of
       exposure. Plasma ChE activities had been returned to pre-exposure values at 28
       days after cessation of treatment. The mean red blood cell AChE activity was
       increased by 1.5% (range: -5 to +6% of baseline values), compared with an
       increase of 3% in the concurrent control group. Even at 0.014 mg/kg bw/day, the
       mean plasma ChE activity of the men was still decreased by 6.5% (mean: -20 to
       +21% of baseline values), accompanied by a 6.5% increase in the concurrent
       controls. However, the difference was not statistically significant. Mean red
       blood cell AChE activity was increased by 2.5 % (range: -10 to +10% of baseline
       values) (Cou72, Gri76). According to the committee, the NOAEL for inhibition
       of plasma ChE after 28 days of oral exposure to chlorpyrifos is 0.014 mg/kg
       bw/day. The NOAEL for red blood cell AChE is greater than 0.014 mg/kg
       bw/day.
           Several field studies were carried out to assess the health implications
       following application of chlorpyrifos. Five out of 7 spray workers, applying a
       0.5% emulsion or suspension of water-wettable powder for control of pest
       mosquitoes, showed decreases in plasma ChE of 50-70% of baseline values
       within 2 weeks after beginning the work. No inhibition of red blood cell AChE
       was observed at any time and there were no signs or symptoms of illness (Eli69).
       In other studies, plasma ChE or red blood cell AChE were measured in pest
       control operators, applying a range of organophosphorus pesticides, among
       which chlorpyrifos. The principle mechanism of toxicity for all these compounds
067-11 Chlorpyrifos
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<pre>       is inhibition of cholinesterase activity, so that the effect of a single compound,
       e.g., chlorpyrifos could not be assessed (Hay80, Yea93).
       Long-term toxicity
       In a cohort study, the prevalence of selected illnesses and symptoms was
       compared between 175 employees potentially exposed to chlorpyrifos and 335
       matched controls with no history of exposure to organophosphorus pesticides.
       Chlorpyrifos workers had held jobs between 1 January 1977 and 31 July 1985.
       Control subjects were matched on age, race, sex, starting date of employment,
       and pay status. Morbidity data were abstracted from company medical records.
       Subjects were subdivided into 3 exposure groups (high, moderate, low) on the
       basis of job title and air monitoring data. The time-weighted average (times not
       given) airborne exposures to chlorpyrifos, measured by personal air sampling
       during manufacture and formulation over the period 1979 to 1983, were between
       <0.03 and 0.12 mg/m3. However, peak exposures as high as 1.1 mg/m3 were
       measured. No information was given on air exposure levels in the 3 groups.
       Plasma ChE activity was measured in workers at monthly intervals as an
       indicator of exposure to chlorpyrifos. In comparison with pre-employment
       levels, mean plasma ChE activities of workers subdivided into the 3 exposure
       categories were inhibited by 32% (high), 32% (moderate), and 19% (low). No
       significant differences in illness or prevalence of symptoms between the exposed
       and control groups, or among the 3 exposure subgroups were observed.
       Potentially exposed employees did report symptoms of dizziness, malaise, and
       fatigue relatively more often than subjects from the control group. Further
       analysis by estimated exposure level, process area, or time did not support a
       relation with exposure. No cases of peripheral neuropathy were seen among the
       exposed workers (Bre89). In a follow-up to this study, the data were updated to
       include the period 1987-1994, and additional medical disorders (e.g., disorders
       of the central nervous system and of the eye and adnexa and diseases of the ear
       and the mastoid process) were considered. The updated study comprised 496
       potentially exposed employees and 911 controls matched for age, race, sex, pay,
       year of hire, and smoking and drinking habits. Workers were subdivided into 4
       categories of chlorpyrifos exposure: ‘high’ (average air levels >0.2 mg/m3, or
       high dermal exposure), ‘moderate’ (average air levels between 0.03 and 0.2
       mg/m3, or moderate dermal exposure), ‘low’ (average air levels between 0.01
       and 0.03 mg/m3, or low dermal exposure) or ‘negligible’ (average air levels<0.01
       mg/m3, or negligible dermal exposure). Plasma ChE activity was measured in
067-12 Health-based Reassessment of Administrative Occupational Exposure Limits
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<pre>       workers at monthly intervals as an indicator of exposure to chlorpyrifos. In
       comparison with pre-employment levels, mean plasma ChE activities of workers
       subdivided into the 4 exposure categories were inhibited by approximately 45%
       (‘high’; n=1), 20% (‘moderate’; n=324), and 5% (‘low’; n=110) while an
       increase of 20% in plasma ChE activity was seen in the ‘negligible’ exposure
       group (n=29). The results of morbidity showed that the prevalence of peripheral
       neuropathy was not significantly increased among the group of employees
       potentially exposed to chlorpyrifos. Significantly increased prevalence odds
       ratios were identified for 5 diagnostic categories: diseases of ear and mastoid
       process; acute respiratory infections; other diseases of the respiratory system;
       general symptoms, signs, and ill defined conditions; and symptoms, signs, and ill
       defined conditions involving the digestive system. Analyses by exposure
       classification or mean plasma ChE activity did not show a dose response
       (Bur98).
           In a recent unpublished prospective cohort study, neurobehavioural
       examinations and neurological examinations that included evaluation of central
       and peripheral nervous system function were conducted on 53 workers of the
       same chemical company, engaged in the manufacture of chlorpyrifos. A group of
       60 workers, not occupationally exposed to chlorpyrifos or potential neurotoxic
       agents, was chosen as a referent population. Data on years of employment of
       workers were not given. Subjects were examined on 2 occasions at 1-year
       intervals (autumn 1999 and autumn 2000). Data collected were a general medical
       examination, a neurological examination, an electrodiagnostic evaluation, a
       psychological interview, neurobehavioural testing, and measurement of red
       blood cell AChE activity. In addition, monthly measurements of plasma ChE
       activity were conducted. Industrial hygiene measurements of airborne
       chlorpyrifos concentrations were conducted to evaluate potential worker
       exposure during the study period, and overall chlorpyrifos absorption into the
       worker’s body was assessed by the measurement of timed overnight urinary
       3,5,6-TCP excretion on 4 occasions. Urine was collected at the time of the 1999
       and 2000 clinical examination, in autumn 2000 during the chlorpyrifos
       maintenance shutdown, and in autumn 2000 during normal plant operations. The
       mean cumulative airborne chlorpyrifos exposure during the study period (about
       225 days) was 6.13 mg/m3•day among chlorpyrifos workers, corresponding with
       an average daily exposure of approximately 0.027 mg/m3, compared with 0.0
       mg/m3•day in the referent group. The mean cumulative historic exposure was
       64.2 mg/m3•day among chlorpyrifos workers, compared with 0.7 mg/m3•day
       among the referent group. The 1-year weighted average urinary 3,5,6-TCP
067-13 Chlorpyrifos
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<pre>       concentration of chlorpyrifos workers (n=51) was 192 µg/g creatinine (range: 9
       to 1536 µg/g creatinine), compared with 6.2 µg/g creatinine (range: 1.5 to 12
       µg/g creatinine) among the referent group (n=60). The cumulative airborne
       chlorpyrifos exposure during the study was significantly correlated with urinary
       3,5,6-TCP concentration, suggesting that skin absorption was minimal. The
       average plasma ChE level was statistically significantly decreased by 13%
       among chlorpyrifos workers, compared with the referent group. There were
       statistically significant inverse correlations between airborne chlorpyrifos or
       urinary 3,5,6-TCP concentrations and plasma ChE activity. The authors
       suggested that no effect on plasma ChE occurred at ambient exposures below
       0.11 mg/m3 or urinary 3,5,6-TCP concentrations below 12 µg/g creatinine. No
       statistically significant difference was found in the average red blood cell AChE
       activity among chlorpyrifos and control workers, and no correlation was found
       between urinary 3,5,6-TCP concentration and red blood cell AChE activity.
       According to the committee, this implies that at 1-year weighted average 3,5,6-
       TCP excretions, ranging from 9 to 1536 µg/g creatinine, corresponding to
       absorbed chlorpyrifos doses of approximately 0.03 to 3.7 mg/day (or 0.0003 to
       0.043 mg/kg bw/day), no significant inhibition of red blood cell AChE activity
       occurs. With regard to the neurobehavioural examinations, no dose-related
       subclinical or clinically evident adverse neurobehavioural effects were found. No
       statistically significant changes in central and peripheral nervous system function
       were found in chlorpyrifos workers compared to the referent group (Alb02).
       From these data the committee concludes that the NOEL for inhibition of plasma
       ChE is between 0.01 and 0.11 mg/m3 chlorpyrifos. However, from data generated
       in the above short-term human volunteer study and the recent prospective cohort
       study, the committee considers the NOAEL for inhibition of red blood cell AChE
       to be substantially higher than the NOEL for inhibition of plasma ChE. In
       addition, the committee does not exclude that the observed plasma ChE
       inhibitions may be partly due to dermal absorption rather than to inhalation of
       chlorpyrifos, as demonstrated in other exposure monitoring studies (Jit89,
       Sun89).
       Developmental toxicity
       Four cases of birth defects allegedly associated with exposure to chlorpyrifos
       were reported. The children were found to have defects, including the brain,
       eyes, ears, palate, teeth, heart, feet, nipples, and genitalia. Brain defects were
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<pre>             present in the ventricles, corpus callosum, choroid plexus, and septum
             pellucidum, and genital defects included the testes (not descended), microphallus
             and fused labia. All children had growth retardation and 3 had hypotonia and
             profound mental retardation. The mothers of the affected children had reportedly
             been exposed to chlorpyrifos, one of them at the workplace and the others at
             home during pregnancy. The exposure patterns were poorly characterised and it
             was not indicated whether it was severe or sustained during organogenesis.
             Medical records of the cases indicated that the same effects in some of the
             children were consistent with a diagnosis of an autosomal recessive birth defect
             syndrome of the brain and eye (Gib96, She96).
             Animal data
             Irritation and sensitisation
             Chlorpyrifos is slightly irritating to the eyes and the skin of rabbits, using
             standard tests (FAO00). In a 4-day dermal study in rabbits, slight erythema in 2/4
             females was observed at 1 and 10 mg/kg bw/day (Cal88, Cal89). The compound
             was not a skin sensitiser in the standard Magnusson-Kligman or Buehler tests
             (FAO00).
             Acute toxicity
             Results of acute lethal toxicity tests are summarised in Table 1.
Table 1 Summary of acute toxicity studies for chlorpyrifos in mammals.
exposure route conditions                    species (strain)          sex          LC50 or LD50   reference
           a                                                                                 3
inhalation      vapour; 4 h; nose only       rat (Sprague-Dawley)      male, female >36 mg/m       Bla94
                undiluted; 4 h; nose only;   rat (Sprague-Dawley)      male, female >230 mg/m3     And95
                powder
                aerosol; undiluted; 4 h;     rat (Wistar)              male, female >1020 mg/m3    Ken87
                whole body
                aerosol; undiluted; 4 h;     rat (Wistar)              male, female 560 mg/m3      Fre96
                whole body
dermal          in xylene                    rat (Sherman)             male         202 mg/kg bw   Gai69
                in arachis oil               rat (Sprague-Dawley)      male, female >2000 mg/kg bw Dre94b
                in polyethylene glycol       rat (Sprague-Dawley)      male.female  >2000 mg/kg bw Lac85
                undiluted                    rat (Sprague-Dawley)      male.female  >2000 mg/kg bw Jac94
067-15       Chlorpyrifos
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<pre>                 undiluted                      rat                             male, female   >2000 mg/kg bw     Nis84a
                 undiluted                      rat (F344)                      male, female   >2000 mg/kg bw     Jef86
                 in saline                      rat (Sprague-Dawley)            male, female   >5000 mg/kg bw     Buc81b
                 in water                       rabbit (Himalayan)              male, female   1233 mg/kg bw      Fre95a
                 undiluted                      rabbit (New Zealand White)      male, female   1580-1801 mg/kg bw Hen80
oral             in peanut oil                  rat (Sherman)                   male           155 mg/kg bw       Gai69
                 in peanut oil                  rat (Sherman)                   female         82 mg/kg bw        Gai69
                 in corn oil                    rat (Sherman)                   male           118, 135 mg/kg bw  McC74
                 in corn oil                    rat (Sherman)                   female         155 mg/kg bw       McC74
                 in corn oil                    rat (Dow-Wistar)                male           163, 245 mg/kg bw  McC74
                 in corn oil                    rat (Dow-Wistar)                female         135 mg/kg bw       McC74
                 in soya bean oil               rat (Sprague-Dawley)            female         169 mg/kg bw       Ber78
                 in arachis oil                 rat (Sprague-Dawley)            male           276 mg/kg bw       Dre94a
                 in arachis oil                 rat (Sprague-Dawley)            female         350 mg/kg bw       Dre94a
                 in maize oil                   rat (Sprague-Dawley)            male           264 mg/kg bw       Wil94
                 in maize oil                   rat (Sprague-Dawley)            female         141 mg/kg bw       Wil94
                 in maize oil                   rat (Sprague-Dawley)            male           475 mg/kg bw       Buc81a
                 in maize oil                   rat (Sprague-Dawley)            female         337 mg/kg bw       Buc81a
                 in corn oil                    rat (Sprague-Dawley)            male           221 mg/kg bw       Nis84b
                 in corn oil                    rat (Sprague-Dawley)            female         144 mg/kg bw       Nis84b
                 in corn oil                    rat                             male           163 mg/kg bw       Tay63
                 in corn oil                    rat                             female         135 mg/kg bw       Tay63
                 in vegetable oil               rat (Wistar)                    male, female   134 mg/kg bw       Fre95b
                                                rat (Sprague-Dawley)            male           205-270 mg/kg bw   Hen80
                                                rat (Sprague-Dawley)            female         96-174 mg/kg bw    Hen80
                 in aqueous gum tragacanth      mouse (Swiss-Webster)           male           102 mg/kg bw       Cou71
                 in soya bean oil               mouse (NAMRU)                   female         152 mg/kg bw       Ber78
                 in vegetable oil               mouse (Swiss albino)            male, female   109 mg/kg bw       Tay63
                 in corn oil                    guinea pig                      male           504 mg/kg bw       McC74
a
  The inhalation studies were carried out at the highest attainable vapour or aerosol concentrations.
              Single oral doses of chlorpyrifos (in corn oil) of 500, 1000, or 2000 mg/kg bw
              caused mortality in 0/2, 0/2, and 2/2 New Zealand White rabbits (sex: not
              reported), respectively (McC74).
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<pre>       From the results in Table 1, the committee concludes that there is considerable
       variation in the susceptibility of different species. Mice are the most sensitive
       and rabbits the less sensitive species. Females are more sensitive than males.
       In adult rats receiving single oral doses of 0.5, 1, 5, 10, or 100 mg/kg bw
       chlorpyrifos (purity: 89-99%), NOAELs of 10 and 0.5 mg/kg bw were found for
       inhibition of brain AChE and plasma ChE, respectively (Men98). When rats
       were given chlorpyrifos by subcutaneous injection at a dose of 280 mg/kg bw,
       marked inhibition of AChE activity (>90%) in the cerebral cortex and corpus
       striatum was measured, which did not recover within 14 days after dosing. The
       changes in AChE activity were accompanied by reductions in muscarinic
       receptor binding sites in the cortex and striatum (Cha93).
            In an acute neurotoxicity study, rats given single oral doses of 0, 10, 50, or
       100 mg/kg bw chlorpyrifos (purity: 98.2%) exhibited decreased body weight,
       motor activity, and clinical signs of intoxication at 50 and 100 mg/kg bw. The
       NOAEL for systemic effects was 10 mg/kg bw (Wil92).
            In a neurobehavioural study, as determined by observational tests of function
       and motor activity, adult male Long-Evans rats (n=10/group) that received single
       oral (gavage) doses of 0, 20, 50, and 100 mg/kg bw chlorpyrifos (purity: 99.5%)
       showed autonomic activity, e.g., salivation, lachrymation, miosis, and mild
       tremors, altered neuromuscular function (gait changes), and decreased motor and
       sensorimotor activity. The most sensitive effect was a change in motor activity,
       which was depressed at 20 mg/kg bw. The rats recovered from the effects at 72
       hours after dosing (Mos95). In a subsequent neurobehavioural study by the same
       author, adult male Long-Evans rats (n=20/group) were given single oral (gavage)
       doses of 0, 10, 30, 60, or 100 mg/kg bw of chlorpyrifos (purity: 99.5%). Clinical
       and neurobehavioural signs, i.e., increased autonomic and convulsive activity
       and decreased motor, neuromuscular, and sensorimotor activity were observed at
       doses of 30 mg/kg bw and above, at 3.5 hours after dosing. At these doses, brain
       AChE and whole blood ChE activities were inhibited by approximately more
       than 60% and 80%, respectively. At 24 hours, there was partial recovery from
       behavioural effects, i.e., considerably fewer behavioural endpoints were affected
       and most of them were seen in the 2 higher dose groups only, with little or no
       corresponding recovery of ChE activity. Body temperature, gait, and motor
       activity appeared to be the most sensitive measures. At 10 mg/kg bw, the lowest
       dose tested, brain AChE activities were inhibited by approximately 30 and 40%
       3.5 and 24 hours after dosing, respectively, and whole blood ChE activities by
       about 80 and 35%, respectively, but no behavioural effects were observed.
067-17 Chlorpyrifos
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<pre>       Relating incidence of behavioural effects to the extent of ChE inhibition (instead
       of by dose) in individual rats suggested that single dosing of chlorpyrifos did not
       induce clinical signs when brain AChE and whole blood ChE activities were
       inhibited by less than 60 and 80%, respectively (Nos97). Sprague-Dawley rats
       (n=5/sacrifice group; sacrifices 2, 4, 6, and 12 weeks post-treatment) that
       received a single subcutaneous dose of 279 mg/kg bw (estimated to be the
       maximum tolerated dose; see Pop91) of chlorpyrifos (purity: 98%) showed
       inhibition of AChE activity in cerebral cortex and corpus striatum by
       approximately 95% and 60% at weeks 2 and 6 after dosing, respectively. AChE
       activities had returned to normal values at 12 weeks after dosing. Tests of
       locomotor activity showed hypoactivity in treated rats for 2-3 days after
       treatment (Pop92). Long-Evans rats, trained to perform an appetitive test of
       memory and motor function, received a single subcutaneous injection with 0, 60,
       125, or 250 mg/kg bw chlorpyrifos (purity: not given). Seven days post-
       treatment, statistically significant, dose-related reductions in AChE activities
       were observed in cerebral cortex, hippocampus, corpus striatum, and
       hypothalamus. Apart from the activity in the hypothalamus at 60 mg/kg bw,
       these AChE activities were still statistically significantly lower when compared
       to controls 21 days post-treatment although partial recovery had occurred. AChE
       activity in whole blood was inhibited 4 days after treatment. Activity slowly
       returned to control levels in the low-dose animals by day 53 and in the mid-dose
       animals by day 74. In the high-dose group, there was still a small, but statistically
       significant decrease 53 days post-treatment. At the 2 highest dose levels, effects
       on working memory and motor function appeared within 2 days after dosing, but
       returned to normal within 3 weeks (Bus93).
           The potential of chlorpyrifos to inhibit neuropathy target esterase (NTE) was
       studied in Fischer 344 female rats that were given single oral (gavage) doses of
       0, 1, 5, 10, 50, or 100 mg/kg bw of chlorpyrifos (purity: 98.1%). NTE was not
       inhibited at any dose. The NOAEL was 10 mg/kg bw for inhibition of brain
       AChE, and 1 mg/kg bw for inhibition of red blood cell AChE and plasma ChE
       (Dit97).
           Chlorpyrifos was tested for acute delayed neurotoxicity in several studies
       with hens. In all studies, chlorpyrifos was given together with atropine and/or
       PAM to protect the animals from the acute effects of the compound. In hens
       treated with a single dose of 110 mg/kg bw of chlorpyrifos (purity: 96.8%), no
       clinical signs of delayed neurotoxicity and no microscopic lesions of nerve
       tissues characteristic of OPIDN were found (Rob87). Hens that were given oral
       doses of chlorpyrifos (purity: 99%) ranging from 60 to 150 mg/kg bw exhibited
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<pre>       inhibition of 59-97% of brain NTE and of 75-100% of brain AChE activity, 4-6
       days after dosing. Delayed polyneuropathy was already observed at 60-90 mg/kg
       bw, which corresponds to 4-6 times the LD50 (Cap91). In another study, hens
       receiving oral doses of chlorpyrifos (purity: 100%) at 0, 75, 150, or 300 mg/kg
       bw exhibited brain AChE depressions of 0, 58, 75, and 86% and brain NTE
       depressions of 0, 21, 40, and 77% respectively, 4 days after treatment (Ric93).
           The effect of chlorpyrifos on the production of hepatic and brain lipid
       peroxidation was studied in female Sprague-Dawley rats, following 2 equal oral
       doses of 41 mg/kg bw chlorpyrifos (purity: 97%) in corn oil at 21-hour intervals.
       A 4 to 5-fold increase in lipid peroxidation was measured in both hepatic and
       brain homogenates, probably resulting from enhanced production of reactive
       oxygen species (Bag95).
           The effect of age on the toxicity of chlorpyrifos has been reported in several
       studies in the peer-reviewed literature. Rats given subcutaneous injections of 2
       mg/kg bw chlorpyrifos on post-natal day 1, showed significant inhibition of
       DNA and protein synthesis (10 and 30%, respectively) in all brain regions within
       4 hours of treatment. Comparable inhibition of DNA synthesis was seen at 8
       days of age, at 11 mg/kg bw/day, but here regional selectivity was observed.
       Effects on protein synthesis was distinct from that on DNA synthesis, as it had
       diminished to <10% by 8 days of age and did not show regional selectivity. The
       authors argued that this effect was not secondary to systemic intoxication
       (Whi95). The maximum tolerated dose of chlorpyrifos in Sprague-Dawley rats
       injected subcutaneously was 45 mg/kg bw in 7-day-old pups and 279 mg/kg bw
       in adults (90-day old). Neonatal brain AChE inhibition was 80% on day 1 and
       45% on day 7 after treatment, whereas 90% inhibition was seen in adult rats,
       with little or no recovery on day 7 (Pop91). The committee concludes that the
       young rat will respond more to the anticholinesterase effects of chlorpyrifos than
       adult animals.
       Subacute and subchronic toxicity
       Exposure of Fischer 344 rats (numbers not given) that received chlorpyrifos
       (purity: 95%) by nose-only inhalation, 6 hours/day, for 5 days, at a concentration
       of 0.34 mg/m3, resulted in a significant decrease in plasma ChE activity in
       females, but no effects were seen in red blood cell or brain AChE activities
       (New88a). Nose-only exposure of female Fischer rats (numbers not given) to
       0.172 mg/m3 of chlorpyrifos for 2 weeks (6 hours/day, 5 days/week) did not
       result in mortality, cholinergic signs of toxicity, changes in clinical chemistry and
067-19 Chlorpyrifos
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<pre>       haematological parameters, or inhibition of plasma ChE, red blood cell AChE, or
       brain AChE (Lan86). In a 2-week whole-body inhalation study, Fischer 344 rats
       (numbers not given) were exposed to 0, 0.014, and 0.072 mg/m3 chlorpyrifos
       vapour (6 hours/day, 5 days/week). There were no treatment-related mortality,
       clinical signs of toxicity, or changes in body weight, and no treatment-related
       changes in organ weights or microscopic abnormalities. No effects were
       observed on brain and red blood cell AChE or plasma ChE activities (Str87). In
       another 2-week whole-body inhalation study, Wistar rats (numbers not given),
       exposed to chlorpyrifos at concentrations 0, 10, 94, and 388 mg/m3 (6 hours/day,
       5 days/week), showed high mortality at the high dose. At 388 and 94 mg/m3,
       cholinergic signs of toxicity, decreased body weights and increased adrenal
       weights were seen. There was no NOAEL for inhibition of cholinesterases in this
       study. At the lowest concentration, brain AChE was inhibited by 50% (males and
       females), red blood cell AChE by 78% in males and 60% in females, and plasma
       ChE by 78% in males and 91% in females (Ken88).
           Two identical 13-week inhalation studies were conducted in Fischer 344 rats.
       In the first study, groups of 10 rats/sex/group were exposed nose-only to
       concentrations of 0, 0.072, 0.143, or 0.287 mg/m3 (the highest attainable) of
       chlorpyrifos vapour (purity: 100%), 6 hours/day, 5 days/week. There were no
       treatment-related signs of toxicity during the course of the study. No
       abnormalities were observed in urine analysis, haematology, clinical chemistry,
       organ weights, or upon gross and microscopic examination. No differences in
       plasma ChE, in red blood cell, and brain AChE activities were noted between
       any of the exposed groups and the control group (Cor89). In the second study,
       groups of 15 rats/sex/group received nose-only exposure to 0, 0.07, 0.14, or 0.28
       mg/m3 of chlorpyrifos (purity: 95%), for 13 weeks. There were no treatment-
       related mortality or ophthalmic, haematological, or clinical chemical changes. At
       the high dose, plasma ChE activity was inhibited by 23% in males. Brain AChE
       and red blood cell AChE were unaffected (New88b). The committee concludes
       that the NOAEL for inhibition of brain AChE or red blood cell AChE was
       greater than the highest attainable concentration of chlorpyrifos vapour, i.e.,
       >0.287 mg/m3.
           Fischer 344 rats (numbers not given) were treated dermally with chlorpyrifos
       (purity: 100%) at doses of 0, 1, 10, 100, or 500 mg/kg bw/day in corn oil, for 4
       days. No clinical signs of intoxication were observed. At 10 mg/kg bw, red blood
       cell AChE activity was inhibited by 16% and plasma ChE by 45%. The same
       authors conducted a 3-week dermal study with chlopyrifos in rats at 0, 0.1, 0.5,
       or 5 mg/kg bw in corn oil, for a total of 15 applications. No treatment-related
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<pre>       cholinergic signs were observed at any dose, and plasma ChE, red blood cell
       AChE, and brain AChE activities were not affected. Gross and microscopic
       examination did not reveal treatment-related changes. The NOAEL was 5 mg/kg
       bw, based on inhibition of red blood cell AChE (Cal88, Cal89).
           Groups of 20 Sprague-Dawley rats of each sex were given chlorpyrifos
       (purity: not given) in the diet at levels equivalent to 0, 0.03, 0.15, or 0.75 mg/kg
       bw, for 6 months. No treatment-related changes in body weight gain, food
       consumption, or haematological or clinical chemical parameters were observed.
       At 0.75 mg/kg bw/day, red blood cell AChE and plasma ChE activities were
       inhibited by 50% and 65%, respectively. Brain AChE activity was not inhibited
       at any dose level (Cou71). In a 13-week oral study, Sprague-Dawley rats (n=20/
       sex/group) were fed levels of chlorpyrifos (purity: 95%) equivalent to 0, 0.025,
       0.5, or 10 mg/kg bw/day. No deaths or clinical signs were observed at any dose
       and no treatment-related abnormalities were observed in clinical chemical
       parameters and upon ophthalmoscopic examination. However, at 10 mg/kg
       bw/day, reduced body weights, haemoglobin levels, and erythrocyte counts were
       measured. Brain and red blood cell AChE activities were not measured in this
       study. The NOAEL for systemic effects was 0.5 mg/kg bw/day (Cro88). In
       another 13-week feeding study, Fischer 344 rats were given chlorpyrifos (purity:
       96%) at doses equivalent to 0, 0.1, 1, 5, or 15 mg/kg bw/day. At 15 mg/kg
       bw/day, treatment-related effects included decreased body weight gain,
       decreased erythrocyte count, increased platelet counts, reduced serum protein,
       albumin and globulin levels, increased organ weights, and increased adrenal
       gland vacuolation. Plasma ChE and red blood cell AChE were inhibited at 1 mg/
       kg
       bw and above, and brain AChE in both sexes at 5 and 15 mg/kg bw/day. The
       NOAEL was 0.1 mg/kg bw/day, based on inhibition of red blood cell AChE
       (Sza88).
           In a 4-week neurotoxicity study, chlorpyrifos (purity: 98%) was fed to Long-
       Evans rats (n=10/sex/group) at doses equivalent to 0, 1, 3, or 10 mg/kg bw/day.
       Cholinergic signs of intoxication were observed at 10 mg/kg bw/day (salivation
       and tremors) and at 3 mg/kg bw/day (miosis). Brain AChE, red blood cell AChE,
       and plasma ChE activity were inhibited by more than 60% at 3 and 10 mg/kg
       bw/day. At the lowest dose, brain AChE was not significantly reduced (8%), but
       red blood cell AChE and plasma ChE were still inhibited by 56% and 68%,
       respectively. Neuropathy target esterase (NTE) was decreased by 6% in the high-
       dose group, but the committee did not consider this as toxicologically significant.
       Cognitive function, as measured by short-term memory and attention/encoding
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<pre>       deficits, was not impaired at any of the dose levels (Mat96). In a 13-week
       neurotoxicity study, Fischer 344 rats (n=10/sex/group) were fed chlorpyrifos
       (purity: 98%) at doses equivalent of 0, 0.1, 1, 5, or 15 mg/kg bw/day. A
       functional observational battery of tests (FOB), ophthalmic examination, and
       gross and microscopic examination of nerve tissues were conducted. FOB,
       conducted before dosing and at weeks 4, 8, and 13, consisted of observations of
       cholinergic signs, measurement of grip performance, and landing foot splay. A
       slight reduction in motor activity was observed at the top dose during week 4
       only, and increased urine incontinence was observed in several females in the 2
       highest dose groups at weeks 4, 8, and 13. According to the authors, these
       transient effects were not treatment-related. Neuropathological examination did
       not reveal any differences, which might be attributed to treatment.
       Cholinesterase activities were not measured. The NOAEL for neurotoxic effects
       was >15 mg/kg bw/day (Sha93).
           CD-1 mice (n=40/sex/group) were given chlorpyrifos (purity: 96%) in the
       diet at levels equivalent to 0, 2.7 (males), or 3.4 (females) mg/kg bw/day, for 4
       weeks. No mortality, clinical signs, or abnormalities in gross and microscopic
       examination were observed. Male body weight gain was reduced by 25% at the
       end of the study. At termination, red blood cell AChE was depressed by 53% and
       plasma ChE by 91%, but brain AChE remained unchanged (Dav85). Groups of
       CD-1 mice (n=12/sex/group) were fed chlorpyrifos (purity: 94%) at levels
       equivalent to approximately 1, 10, 40, 90, or 200 mg/kg bw, for 13 weeks. Dose-
       related increases in mortality rate and in the frequency of ocular opacities were
       observed at the 2 highest dose levels. At termination, plasma ChE activity was
       significantly depressed at all doses in both males and females. Red blood cell
       AChE activity was significantly inhibited in females at 10 mg/kg bw/day and
       above, but remained unaffected in males at any dose level. Brain AChE activity
       was depressed at 10 mg/kg bw/day and above in females and at 40 mg/kg bw in
       males. The absolute and relative organ weights remained unaffected, except for
       an increase in the relative liver weight. Microscopic examination showed
       abnormalities in the adrenal glands (lipogenic pigmentation) at 40 mg/kg bw/day
       and above, and in the eyes (keratitis) at the top dose. The NOAEL was 1 mg/kg
       bw/day, based on inhibition of brain and red blood cell AChE (Cro87).
           In a recent 6-week study, beagle dogs (n=4/sex/group) received chlorpyrifos
       (purity: 97.6%) via the diet at doses equivalent to 0.0, 0.5, 1.0, or 2.0 mg/kg
       bw/day for males, and 0.0, 0.5, 1.1, or 1.9 mg/kg bw/day for females. No
       mortality or clinical signs of intoxication were observed in any of the groups.
       There were no statistically significant differences in mean body weight and body
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<pre>       weight gain, mean food consumption, macroscopic examination of organs, or
       microscopic examination of the adrenal glands between the treatment and control
       groups. At 18 hours after the beginning of feeding, combined male and female
       mean red blood cell AChE activities were statistically significantly inhibited by
       11 and 10% in the mid- and high-dose animals, respectively. Progressive
       inhibition of red blood cell AChE activity was apparent after 1 week of exposure,
       and a steady state inhibition was reached after 3 weeks of exposure. Terminal
       mean red blood cell AChE activities, measured immediately after necropsy, were
       depressed by 53, 71, and 85% (males), and by 38, 64, and 80% (females) for
       low-, mid-, and high-dose animals, respectively, with respect to mean baseline
       and control values. Brain AChE activity, measured immediately after necropsy,
       was 97, 90, and 92% (males), and 104, 101, and 96% (females) of controls for
       the low-, mid-, and high-dose groups. The change in the mid-dose males was
       statistically significant. However, the inhibition was not dose related, and
       according to the committee, the mid-dose finding was due to normal variability
       rather than a treatment-related effect. When male and female data were
       combined, brain AChE activity in the low-, mid-, and high-dose groups were
       inhibited by 0, 4, and 6%, respectively. Because the AChE was inhibited in both
       the high-dose males and females, the committee considered the change as a
       treatment-related effect. No statistically significant inhibitions were found in
       male or female left atrium, diaphragm, quadriceps, or nodose ganglion AChE
       activities in any of the treatment groups (Mar01, Ste01). According to the
       committee, the NOAEL for brain and peripheral tissue AChE inhibition was 1.0
       mg/kg bw/day. No NOAEL could be set for red blood cell AChE (LOAEL: 0.5
       mg/kg bw/day). The NOAEL for red blood cell AChE after 1-day exposure was
       set at 0.5 mg/kg bw/day.
           In another study, beagle dogs (n=4/sex/group) were given chlorpyrifos
       (purity: 96%) at doses of 0, 0.01, 0.22, or 5 mg/kg bw/day by capsule, for 13
       weeks. No clinical signs of toxicity, haematological, clinical chemical,
       ophthalmoscopic, gross, or microscopic abnormalities were observed. Dose-
       related inhibitions were observed in plasma ChE activity at 0.01 mg/kg bw/day
       and above, in red blood cell AChE at 0.22 mg/kg bw/day and above, and in brain
       AChE (by 46%) at 5 mg/kg bw/day. The NOAELs for red blood cell AChE and
       brain AChE were 0.01 and 0.22 mg/kg bw, respectively (Har89).
           Beagle dogs (n=3/sex/group) received diets containing chlorpyrifos (purity:
       97%) at levels equivalent to 0, 0.01, 0.03, 0.1, 1.0, or 3.0 mg/kg bw/day for 1
       year. The same doses were given to 4 beagles/sex/group for 2 years. No
       treatment-related mortality, signs of toxicity, or abnormalities in food
067-23 Chlorpyrifos
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<pre>       consumption, body weights, haematological or clinical chemical parameters
       were observed. No differences in absolute and relative organ weights were found
       between controls and treated dogs, except for an increase in relative liver weight
       at the top dose. Gross and microscopic examination of the tissues revealed no
       treatment-related alterations. Progressive inhibition of red blood cell AChE
       activity was apparent after 1 week of exposure, and a steady state inhibition was
       reached after 30 days of treatment in both studies. Statistically significant red
       blood cell AChE depressions were found for the groups of dogs receiving doses
       of 1.0 and 3.0 mg/kg bw/day, with values ranging from 61 to 75% in males and
       60 to 70% in females, respectively. Brain AChE activity was inhibited at 3
       mg/kg bw/day only (21% in males and 19% in females), but the difference with
       the control group was not statistically significant. Plasma ChE activity was
       significantly decreased at 0.1 mg/kg bw/day and above in both males (49-75%)
       and females (35-64%) at all assay times. The NOAELs for inhibition of red
       blood cell and brain AChE were 0.1 and 1.0 mg/kg bw, respectively (Mat01,
       McC74).
            Groups of rhesus monkeys (n=1-2/sex/group) received chlorpyrifos (purity:
       not given) at doses of 0, 0.08, 0.4, or 2 mg/kg bw/day by stomach tube, for 6
       months. No treatment-related changes in body weight gain, food consumption,
       haematological and clinical chemical parameters, or gross and microscopic
       examination were observed. At week 24, plasma ChE and red blood cell AChE
       activities were significantly inhibited at 0.4 mg/kg bw/day (by 61% and 27%,
       respectively) and at 2 mg/kg bw/day (by 55% and 38%, respectively). At
       termination, brain AChE activity was not significantly inhibited at any dose level
       (Cou71, FAO00).
            Male Long-Evans rats (n=7-8/group) were given chlorpyrifos (purity: not
       given) by subcutaneous injection at doses of 0, 15, 30, and 60 mg/kg bw in
       peanut oil, for 20 weeks. Injections were given at weekly intervals during weeks
       1-5, at 14-day intervals during weeks 6-10, and then weekly dosing was
       reinstated for weeks 11 to 20. Whole blood ChE activity was depressed by 60%
       (at 15 mg/kg bw) to 90% (at 60 mg/kg bw) after 5 weeks. At 60 mg/kg bw,
       tremors were observed as well as working memory impairment and slowing on
       motor activity. Reducing the chlorpyrifos injection frequency relieved the
       inhibition of whole blood ChE activity to 50%-75% of control and cholinergic
       signs, and behaviour returned to normal levels. Reinstatement of weekly
       injections for 10 weeks inhibited whole blood ChE by 75% to 90%. Tremor was
       not observed, but motor slowing and working memory impairment persisted
       throughout the dosing period in all treated groups. The authors conclude that
067-24 Health-based Reassessment of Administrative Occupational Exposure Limits
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<pre>           repeated chlorpyrifos injections lead to persistent impairment of cognitive and
           motor function, while tolerance to the muscarinic effects of chlorpyrifos was
           observed (Bus94).
                Adult white Leghorn hens (n=15) were given chlorpyrifos (purity: 100%)
           orally for 20 days at a dose of 10 mg/kg bw/day, followed by a 4-week
           observation period. Body weights were decreased by 25% and some birds
           showed clinical signs of toxicity during week 1 of treatment. During days 4-20,
           brain AChE activity was depressed by 58-70%, but returned to normal values
           after the 4-week observation period. NTE activity in brain and lymphocytes
           throughout the study was 82-99% and 85-128% of that of controls, respectively.
           The 18% inhibition of brain NTE on days 10 and 20 was statistically significant.
           No behaviour signs of OPIDN were observed (Ric93).
                The results of subacute and subchronic toxicity studies in rats, mice, dogs,
           monkeys, and chickens are summarised in Table 2.
Table 2 Summary of subacute and subchronic toxicity studies for chlorpyrifos.
exposure    species               dose levels              exposure     critical effecta NOAEL        reference
route       (strain; number; sex)                          duration
inhalation  rat                   0.34 mg/m3               5 days       BAChE; RAChE     >0.34 mg/m3  New88a
            (F344; females; n= ?) (nose only)
            rat                   0.172 mg/m3              2 weeks      BAChE; RAChE     >0.172 mg/m3 Lan86
            (F344; female;        (nose only)
            n= ?)
            rat                   0, 0.014, 0.072          2 weeks      BAChE; RAChE     >0.072 mg/m3 Str87
            (F344; ?)             mg/m3 whole body)
            rat                   0, 10, 94, 388           2 weeks      BAChE; RAChE     LOAEL: 10    Ken88
            (Wistar; ?)           mg/m3 (whole body)                                     mg/m3
            rat                   0, 0.07, 0.14, 0.28      13 weeks     BAChE; RAChE     >0.28 mg/m3  New88b
            (F344;                mg/m3 (nose only)
            n=15/sex/group)
            rat                   0, 0.072, 0.143, 0.287   13 weeks     BAChE; RAChE     >0.287 mg/m3 Cor89
            (F344;                mg/m3 (nose only)
            n=10/sex/group)
dermal      rat                   0, 1, 10, 100, 500       4 days       RAChE            1 mg/kg      Cal88,
            (F344; ?)             mg/kg bw/d                                                          Cal89
            rat                   0, 0.1, 0.5 or 5         21 days      BAChE; RAChE     5 mg/kg      Cal88,
            (F344; ?)              mg/kg bw/d                                                         Cal89
067-25     Chlorpyrifos
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<pre>oral    rat              0, 0.03, 0.15, 0.75      6 months BAChE                 >0.75 mg/kg    Cou71
        (Sprague-Dawley; mg/kg bw/d                        RAChE                 0.15 mg/kg
        n=20/sex/group)
        rat              0, 0.025, 0.5, 10        13 weeks red blood cell count; LOEL: 10 mg/kg Cro85
        (Sprague-Dawley; mg/kg bw/d                        haemoglobin level
        n=20/sex/group)
        rat              0, 0.1, 1, 5, 15         13 weeks BAChE                 1 mg/kg        Sza88
        (F344; ?)        mg/kg bw/d                        RAChE                 0.1 mg/kg
        rat              0, 1, 3, 10              4 weeks  BAChE                 1 mg/kg        Mat96
        (Long-Evans;     mg/kg bw/d                        RAChE                 LOAEL:1 mg/kg
        n=10/sex/group)                                    NTE                   >10 mg/kg
                                                           cognitive function    3 mg/kg
        rat              0, 0.1, 1, 5, 15         13 weeks neurotoxicity         >15 mg/kg      Sha93
        (F344;           mg/kg bw/d                        (FOB,
        n=10/sex/group)                                    neuropathology)
        mouse            0 or 2.7 /3.4 (M/Fd)     4 weeks  BAChE                 >2.7/3.4 mg/kg Dav85
        (CD-1;           mg/kg bw/d                                              LOAEL: 2.7/3.4
        n=40/sex/group)                                    RAChE                 mg/kg
        mouse            0, 1, 10, 40, 90, 200    13 weeks BAChE; RAChE          1 mg/kg        Cro87
        (CD-1;           mg/kg bw/d
        n=12/sex/group)
        dog              0, 0.5, 1.0, 2.0         6 weeks  BAChE                 1.0 mg/kg      Mar01
        (beagle;         mg/kg bw/d                        RAChE                 LOAEL:0.5
        n=4/sex/group)                                                           mg/kg
                                                           PTAChE                1.0 mg/kg
        dog              0, 0.01, 0.22, 5         13 weeks BAChE                 0.22 mg/kg     Har89
        (beagle;          mg/kg bw/d                       RAChE                 0.01 mg/kg
        n=4/sex/group)
        dog              0, 0.01, 0.03, 0.1, 1, 3 1 year   BAChE                 1 mg/kg        McC74
        (beagle;         mg/kg bw/d                        RAChE                 0.1 mg/kg
        n=3/sex/group)
        dog              0, 0.01, 0.03, 0.1, 1, 3 2 years  BAChE                 1 mg/kg        McC74
        (beagle;         mg/kg bw/d                        RAChE                 0.1 mg/kg
        n=4/sex/group)
        monkey           0, 0.08, 0.4, 2          6 months BAChE                 >2 mg/kg       Cou71
        (rhesus;          mg/kg bw/d                       RAChE                 0.08 mg/kg
        n=1-2/sex/group)
        chicken          10 mg/kg bw/d            20 days  lymphocyte NTE,       LOAEL: 10      Ric93
        (Leghorn;                                          brain NTE, BAChE      mg/kg
        n=15)
067-26 Health-based Reassessment of Administrative Occupational Exposure Limits
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<pre>subcutaneous rat                 0, 15, 30, 60        20 weeks  cognitive function LOAEL: 15  Bus94
             (Long-Evans; male;   mg/kg bw/d                                       mg/kg
             n=7-8/group)
a
  BAChE=brain AchE; RAChE= red blood cell AChE; PTAChE= peripheral tissues AChE.
            In summary, inhibition of cholinesterases occurs at dose levels that are lower
            than those causing other toxic effects. In all species, plasma ChE and red blood
            cell AChE are more sensitive for inhibition by chlorpyrifos than brain AChE.
            Monkeys and dogs seem to be more sensitive and mice less sensitive for effects
            on cholinesterases than rats.
            Chronic toxicity and carcinogenicity
            Groups of rats (Sherman; n=25/sex/group) were fed chlorpyrifos (purity: 97%) at
            doses equivalent to 0, 0.01, 0.03, 0.1, 1.0, or 3.0 mg/kg bw/day, for 2 years. No
            treatment-related mortality, signs of toxicity, or changes in food consumption,
            body weights, or haematological or clinical chemical parameters were observed.
            No differences were found in absolute and relative organ weights or upon gross
            and microscopic examination of tissues when control and treated rats were
            compared. The numbers and types of tumours were similar for treated and
            control rats. No further details were provided. Plasma ChE and red blood cell
            AChE activities were significantly decreased at 1.0 and 3.0 mg/kg bw/day in
            both sexes, with inhibitions ranging from 53 to 66% for plasma ChE and 65 to
            86% for red blood cell AChE. At termination, brain AChE activity was
            significantly inhibited at 3.0 mg/kg bw/day (44% in males and 43% in females).
            At 1.0 mg/kg bw, brain AChE was inhibited by 7% in females and 10% in males,
            but these effects were not considered to be of biological significance. The
            NOAELs for inhibition of red blood cell and brain AChE were 0.1 and 1.0
            mg/kg bw, respectively (McC74). This study was criticised and found to be
            inadequate for assessing long-term effects and carcinogenicity, as there were
            shortcomings in data collection and in reporting of gross and microscopic
            examination (FAO00).
                 In a later conducted 2-year study, in compliance with GLP standards, Fischer
            rats (n=60/sex/group) received diets containing chlorpyrifos (purity: 98.5%) at
            concentrations equivalent to 0, 0.05, 0.1, 1, or 10 mg/kg bw/day. At 10 mg/kg
            bw/day, body weight gain was reduced relative to controls in the absence of
            reduced food consumption, and an increase in the weight of adrenal glands was
            seen, characterised microscopically by fatty vacuolation. Red blood cell count,
067-27      Chlorpyrifos
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<pre>       haemoglobin, total protein, and globulin concentrations were decreased, and
       platelet counts increased. Effects in males were more pronounced than in
       females. There was no increase in the incidence of any type of tumour. At 24
       months, plasma ChE, red blood cell AChE, and brain AChE activities were
       inhibited by approximately 82%, 25%, and 57% respectively. At 1
       mg/kg bw/day, the only effects attributable to treatment were inhibition in
       plasma ChE (by 60%) and in red blood cell AChE (by 15%). The latter was not
       statistically significant. Brain AChE activity remained unaffected at this level.
       The NOAEL for inhibition of red blood cell AChE was 0.1 mg/kg bw/day, and
       for inhibition of brain AChE 1 mg/kg bw (You88).
            In a third 2-year study, conducted according to accepted test guidelines and
       GLP, Fischer rats (n=60/sex/group) were fed diets containing chlorpyrifos
       (purity: 96%) at concentrations equivalent to 0.012, 0.3, or 6 mg/kg bw/day.
       Mortality rates and the incidences of clinical signs or palpable masses were not
       affected by the treatment. Body weights were slightly reduced in the high-dose
       group. A variety of non-neoplastic and neoplastic lesions were recorded, but the
       incidence was within the normal range and did not show statistically significant
       dose-response relationships. At 6 mg/kg bw/day, plasma ChE was inhibited by
       95% and 96%, red blood cell AChE by 34 and 18%, and brain AChE by 58% and
       61%, in males and females, respectively, at termination of the study. At 0.3
       mg/kg bw, plasma ChE activity was still significantly reduced (by 36%), but red
       blood cell AChE inhibition (by 17% in males and 11% in females) was not
       statistically significantly different. Brain AChE was not affected at this level.
       The NOAEL for inhibition of brain AChE was 0.3 mg/kg bw and of red blood
       cell AChE 0.012 mg/kg bw (Cro88).
            CD-1 mice (n=56/sex/group) were fed doses of chlorpyrifos (purity: 99.6%)
       equivalent to 0, 0.075, 0.75, or 2.25 mg/kg bw/day, for 105 weeks. No unusual
       behavioural changes were observed. Absolute and relative organ weights varied
       unrelatedly to dose. The relative liver weights for both male and female mice
       were significantly decreased at the low and mid dose, but not at the high dose.
       Microscopic examination revealed a significant difference between control and
       mid-dose males for the incidence of hyperplastic nodules of the liver. There was
       also a significant increase in the incidence of spindle cell hyperplasia of the
       adrenal gland for male mice at the low- and mid-dose levels, and in female mice
       at the low dose only. Lesions observed in the lung included a significant increase
       of alveologenic adenomas in mid-dose male mice only. The histological changes
       identified appear to be spontaneous in nature and without a demonstrated dose
       response. Plasma ChE and red blood cell AChE activities, measured in animals
067-28 Health-based Reassessment of Administrative Occupational Exposure Limits
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<pre>             in the high-dose group at week 1 of the study, were inhibited by 90% and 50%,
             respectively. No other ChE measurements were conducted in any group at any
             time point. The NOAEL for systemic effects was 2.25 mg/kg
             bw/day (War80). Because of an inadequate data collection on ChE activities, the
             committee cannot establish NOAELs/LOAELs for ChE inhibition.
                   In another carcinogenicity study in mice, conducted according to accepted
             test guidelines and GLP, CD-1 mice (n=64/sex/group) were fed chlorpyrifos
             (purity: 96%) at concentrations equivalent to 0, 0.9, 8.8, or 45 mg/kg bw in males
             and 0, 0.9, 9.8, or 48 mg/kg bw in females, for 80-82 weeks. At the high dose,
             clinical signs (ocular opacities, lachrymation) and reductions in body weight
             were observed. Microscopic examination revealed increased incidences of
             keratitis and hepatocyte fatty vacuolation. In addition, an increased incidence of
             lung nodules was seen, which were characterised as bronchial-alveolar adenomas
             or carcinomas by microscopic examination. However, the increase of these
             lesions was not statistically significant. No statistically significant changes in the
             incidence of other neoplasms in treated animals, compared with controls, were
             found. In the male and female animals of the high-dose group, plasma ChE, red
             blood cell AChE, and brain AChE activities were inhibited by ca. 98%, 23% (not
             statistically significant), and 80-85%, respectively, at week 42, and 98%, ca.
             30%, and ca. 85%, respectively, at week 78. In the other dose groups, there was a
             considerable intra-group variation. In the mid-dose group, reductions were
             observed in plasma ChE activities in males and females at week 42 and 78 (ca.
             95% inhibition; p<0.001), in red blood cell AChE activities in females at week
             42 (41% inhibition; p<0.01) and in males at week 78 (29% inhibition; p<0.05),
             and in brain AChE in females at week 42 (46% inhibition; p<0.05) and in males
             at week 42 (43% inhibition; not significant) and week 78 (47% inhibition; not
             significant). In the low-dose group, only plasma ChE activities were significantly
             inhibited (45-50%). The NOAEL for red blood cell and brain AChE was 0.9
             mg/kg bw/day (Gur91). The results of chronic toxicity/carcinogenicity studies in
             rats, mice are summarised in Table 3.
Table 3 Summary of chronic oral toxicity/carcinogenicity studies for chlorpyrifos.
species                       dose levels                    exposure        critical effecta NOAEL        reference
(strain ; number; sex)        (mg/kg bw/d)                   duration                         (mg/kg bw/d)
rat                           0, 0.01, 0.03, 0.1, 1, 3       2 years         BAChE            1            McC74
(Sherman; n=5/sex/group)                                                     RAChE            0.1
rat                           0, 0.012, 0.3, 6               2 years         BAChE            0.3          Cro88
(Fischer; n=60/sex/group)                                                    RAChE            0.012
067-29       Chlorpyrifos
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<pre>rat                          0, 0.05, 0.1, 1, 10            2 years  BAChE            1          Yon88
(Fischer; n=60/sex/group)                                            RAChE            0.1
mouse                        0, 0.075, 0.75 or 2.25         2 years  systemic effects 2.25       Wa80
(CD-1; n=56/sex/group)
mouse                        0, 0.9, 8.8/9.8, 45/48 (males/ 78 weeks BAChE; RAChE     0.9        Gur91
(CD-1; n=64/sex/group)       females)
a
     BAChE= brain blood cell AChE; RAChE= red blood cell AchE.
             Mutagenicity and genotoxicity
             Mutagenicity assays comprise tests for the detection of gene mutations in
             bacteria, yeast, and mammalian cells (in vitro) and cytogenicity (in vitro and in
             vivo) and other genotoxicity assays (in vitro).
             •   In vitro tests:
                 • Gene mutation assays. Tests for reverse mutations in 5 strains of S.
                     typhimurium were negative at concentrations up to 30,000 µg/plate both
                     with and without metabolic activation by a rat liver microsomal S9
                     preparation (Gol95, Poo77, Sim77). Negative results were also found in
                     E. coli WP2 (concentration not specified) (Poo77). Chlorpyrifos (purity:
                     97%) did not induce gene mutations in the hprt forward gene mutation
                     assay in cultured Chinese hamster ovary cells (CHO) when tested at
                     concentrations of 17.5 µg/mL in the absence or presence of S9 activation
                     (Gol95). However, a farm-grade formulation containing 20% chlorpyrifos
                     (no further specification) induced sex-linked recessive lethal mutations in
                     D. melanogaster (Pat92).
                 • Cytogenicity assays. A test for the induction of sister-chromatid
                     exchanges (SCE) in cultured human lymphocytes showed unequivocal
                     results at doses of 2 and 20 µg/mL, in the absence of metabolic activation
                     (Sob82). Negative SCE results were seen in cultured CHO cells at
                     concentrations up to 100 µg/mL without metabolic activation. At this
                     concentration, the frequency of chromosome aberrations also remained
                     unaffected (Mus84). In another study, an increase in chromosome
                     aberration rate was observed in chlorpyrifos-treated cultured mouse
                     spleen cells at 4 µg/mL, without metabolic activation (Ame92). However,
                     no increased frequency in chromosome aberrations was observed in rat
                     lymphocytes at concentrations up to 167 µg/mL, the highest attainable
                     level, with and without metabolic activation (Gol95).
067-30       Health-based Reassessment of Administrative Occupational Exposure Limits
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<pre>           • Other genotoxicity assays. Chlorpyrifos did not induce mitotic
              recombination in S. cerevisae (Poo77). The chemical did not increase
              unscheduled DNA synthesis in primary rat hepatocytes at doses up to the
              highest attainable level of 35 µg/mL (Gol95).
       • In vivo tests:
       CD-1 mice did not show an increased incidence of micronuclei in polychromatic
       erythrocytes when treated with oral doses of chlorpyrifos (purity: 97%) of 0, 7,
       22, or 70 mg/kg bw or of 90 mg/kg bw, corresponding to 0.8 LD50 in a
       subsequent test (Gol95). In an earlier study, the test was positive in bone marrow
       cells of mice receiving 2 intraperitoneal injections of 45 mg/kg bw of
       chlorpyrifos (purity: not given) over a period of one week (Ame82). A positive
       result was also reported in bone marrow of mice treated with a single
       intraperitoneal dose of 30.5 mg/kg bw, corresponding to 0.5 LD50 (Ben89).
       Alkylation of DNA has been reported in mice following intraperitoneal
       administration of chlorpyrifos (Mos83). A statistically significant, dose-related
       increase in mean comet tail length indicating DNA damage was seen in
       peripheral blood leukocytes 24 hours after treating male Swiss mice (n=5/group)
       with single (gavage) oral doses ranging from 0.28 to 8.96 mg/kg bw. Mean tail
       lengths were gradually decreased in all dose groups, but still statistically
       significantly increased at doses > 56 mg/kg bw at 48 hours post-treatment and at
       4.48 and 8.96 mg/kg bw at 72 hours post-treatment. At 96 hours post-treatment,
       they had returned to control levels indicating repair of damaged DNA (Rah02).
       In conclusion, chlorpyrofos induced clastogenic and DNA-damaging effects in
       some of the tests. This may be due to the test system used (e.g., route of
       administration) and the type of test material used (e.g., the purity of the
       compound).
       Reproduction toxicity
       In a 3-generation study, Sprague-Dawley rats (10 males and 20 females/group)
       were given chlorpyrifos (purity: not given) in the diet equivalent to 0, 0.03, 0.1,
       or 0.3 mg/kg bw/day for the first generation, and 0, 0.1, 0.3, or 1 mg/kg bw /day
       for the second and third generations. F0 animals were fed test diets from 60 days
       prior to mating and offspring were fed test diets from weaning. F2b parent
       animals were continued on test diet throughout the breeding period, except that
       females were placed on normal diet during organogenesis. No clinical signs of
067-31 Chlorpyrifos
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<pre>       toxicity were observed in the parents or the offspring (2 litters per generation).
       Parental body weights and food consumption were not affected. The fertility,
       gestation, and lactation indices were comparable between treated groups and the
       controls (20 males and 40 females). However, the viability index was decreased
       at 1 mg/kg bw/day. According to the authors, this effect was not clearly treatment
       related. Red blood cell AChE activity, measured in F2 animals only, was
       significantly reduced at 0.3 mg/kg bw/day and above. No abnormalities were
       observed in haematological and clinical chemical parameters in these animals.
       No treatment-related gross or microscopic abnormalities were recorded in F3a
       pups, and no treatment-related developmental effects were observed in F3b
       animals. Based on inhibition of red blood cell AChE, the parental NOAEL was
       0.1 mg/kg bw; the NOAEL for reproduction toxicity was 1 mg/kg bw/day (the
       highest dose tested) (Tho71). The committee considered the NOAEL for
       reproduction toxicity of 1 mg/kg bw/day questionable, because of the equivocal
       result of the viability index.
            To supplement the finding of the equivocal decrease in viability index at 1
       mg/kg bw/day in the aforementioned study, a subsequent 2-generation study was
       performed in which Sprague-Dawley rats (n=30/sex/group) were given
       chlorpyrifos (purity: 96.6-99%) in the diet at doses of 0, 0.5, 0.8, or 1.2 mg/kg
       bw/d for 135 days, and then bred to produce F1 litters. On lactation day 21, 30
       pups of each sex were randomly selected, dosed in the same way as the F0
       animals for 120 days, and then bred to produce the F2 litters. The F2 pups were
       weaned on lactation day 21. The F0 and F1 matings were in a 1:1 ration, with
       pairing for 5 days, a 7-day rest, and a pairing with a different male for a further 5
       days. No clinical signs were observed in the parental animals of either
       generation. In the F0 animals, body weight and food intake were not affected. In
       the high-dose group, there were no effects on mean fertility index or length of
       gestation, but in the 2 lowest dose groups, reductions were seen in both
       endpoints. Litter sizes, survival indices during lactation, and pup weights were
       all comparable at all doses. In the F1 animals, decreased body weights were seen
       in the male animals of the high-dose group on days 160-182 after cohabitation.
       Female body weights were slightly, sporadically increased when compared with
       controls but were not affected during lactation. The mean fertility index in the
       treated groups exceeded the control values; litter size, survival index during
       lactation, and pup weight were comparable among all groups. Data on
       cholinesterase activities were not reported (Die83).
            In a third 2-generation study, groups of male and female rats (CrL:COBS CD
       (SD) BR; F0: n=28/sex/group; F1: n=24/sex/group) were given technical grade
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<pre>       chlorpyrifos (purity: 95.8%) in their diets at doses equivalent to 0.1-0.2, 0.5-0.9,
       and 2.5-4.6 mg/kg bw /day in F0 animals and to 0.1-0.3, 0.7-1.6, and 3.3-8.1
       mg/kg bw/day in F1 animals. These doses were selected from a preliminary one-
       generation study in which a dietary dose of chlorpyrifos of 50 mg/kg feed (50
       ppm: i.e., the high dose in the definite 2-generation study) induced inhibition of
       plasma ChE activity of 86% in adult females and of about 60% in weanling
       males and females, of red blood cell AChE of 70% in adult females and of about
       50% in weanlings, and of brain ChE of 38% in adult females when compared
       with those in controls). However, in the 2-generation study, cholinesterase
       activity was not measured. Under the conditions of this study, no adverse
       treatment-related effects on any of the endpoints investigated were observed at
       any dose in any of the groups (Jam88).
           Finally, in a fourth 2-generation study, Sprague-Dawley rats (n=30/sex/
       group) received chlorpyrifos (purity: 98%) in the diets at concentrations
       equivalent to 0, 0.1, 1.0, or 5.0 mg/kg bw/day. Exposure to the treated diets was
       continuous throughout the duration of the study. After 10 weeks on test diets, F0
       parental rats were mated to produce F1 litters. After 12 weeks of treatment
       following weaning, F1 adults were bred to produce the F2 litters. At 5.0 mg/kg
       bw/day, body weights and body weight gain were slightly reduced in F1 and F2
       parents. At termination, some 19-21 weeks after starting the treatment, F1 and F2
       parental effects included decreased plasma ChE and red blood cell AChE
       activities at 1.0 mg/kg bw/day, and decreased plasma ChE, red blood cell AChE,
       and brain AChE activities at 5.0 mg/kg bw/day. Microscopic examination
       revealed alterations of the adrenal zona fasciculata at 5.0 mg/kg bw/day,
       characterised as slight vacuolation consistent with fatty changes. No effects were
       observed on histological examination of reproductive tissues, or on fertility,
       length of gestation, time to mating, or litter size in either generation at any dose
       level. No neonatal effects were observed at 0.1 and 1.0 mg/kg bw/day in the F1
       and F2 litters. Parental toxicity at the high dose was accompanied by decreased
       pup body weight and increased pup mortality in the F1 litters only. The NOAEL
       for parental effects was 0.1 mg/kg bw/day, based on inhibition of red blood cell
       AChE. The NOAEL for reproduction toxicity was 1 mg/kg bw/day (Bre96).
       Chlorpyrifos was also evaluated for developmental toxicity in rats, mice, and
       rabbits and for developmental neurotoxicity in rats.
           In a rat study, groups of pregnant Fischer 344 rats (n=31-33/group) were
       given chlorpyrifos (purity: 96.6%) by gavage in corn oil at doses equivalent to 0,
       0.1, 3, or 15 mg/kg bw/day, on days 6-15 of gestation. Blood samples for
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<pre>       cholinesterase determinations were collected on day 15, and fetuses were
       surgically removed and maternal animals sacrificed on day 21. Maternal effects
       at the highest dose were cholinergic signs (tremors and excessive salivation),
       decreased body weights and body weight gain and inhibition of plasma ChE (by
       97%) and red blood cell AChE (by 79%). The only maternal effects observed at 3
       mg/kg bw/day were depressions of plasma ChE (by 89%) and red blood cell
       AChE (by 74%). No effects on absolute and relative liver weights were noted.
       No abnormalities were seen in the number of litters, implantation sites, and live
       and resorbed fetuses. Mean fetal body weight and crown-rump length did not
       show changes in any of the treated groups compared to the controls, and no
       treatment-related teratogenicity was observed. The maternal NOAEL was 0.1
       mg/kg bw/day and the developmental NOAEL at least 15 mg/kg bw (Bre95,
       Oue83). In another study, groups of CD Sprague-Dawley rats (n=32/group)
       received chlorpyrifos (purity: 96%) in maize oil at oral (gavage) doses equivalent
       to 0, 0.5, 2.5, or 15 mg/kg bw/day, on days 6-15 of gestation. Blood samples
       were collected on day 15 and necropsy took place on day 20. Maternal effects
       observed at 15 mg/kg bw/day were cholinergic signs (tremors), decreases in food
       consumption, body weight gain, and plasma ChE activity. Red blood cell or brain
       AChE activities were not determined. At 15 mg/kg bw/day, the mean number of
       live male fetuses was decreased and post-implantation loss was statistically
       significantly elevated. Mean fetal weight and mean crown-rump length were
       statistically significantly increased. No treatment-related teratological effects
       (visceral organs, skeleton, abdomen) were noted at any dose level. The NOAEL
       for systemic maternal toxicity and for developmental toxicity was 2.5 mg/kg bw/
       day (Rub87a).
            Groups of pregnant CF-1 mice (n= 40-47/group) were given 0, 1, 10, or 25
       mg/kg bw/day of chlorpyrifos (purity: 96.8%) by gavage, on days 6 through 15
       of gestation. Maternal blood samples were collected on gestational days 6, 10,
       and 15, and animals were sacrificed at day 18. Maternal effects included
       mortality, cholinergic signs (tremors, excessive salivation), decreased body
       weight gain, and decreased plasma ChE (by 98%) and red blood cell AChE (by
       57%) activity at 25 mg/kg bw/day, and cholinergic signs and decreased plasma
       ChE (by 96%) and red blood cell AChE (by 43%) activity at 10 mg/kg bw/day.
       No effects were observed on number of pregnant animals with litters, number of
       implantation sites per dam, and number of fetuses per litter. Fetal body weight
       and crown-rump length were reduced at 25 mg/kg bw and fetal tissue
       homogenate ChE was significantly reduced at 10 and 25 mg/kg bw. A
       statistically significant increased incidence of delayed ossification was seen in
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<pre>       fetuses at 25 mg/kg bw and an increased incidence of exencephaly was observed
       at the lowest dose, but not at 10 or 25 mg/kg bw/day. The study was repeated
       with doses of 0, 0.1, 1, or 10 mg/kg bw/day of chlorpyrifos on groups of 35-41
       pregnant mice per dose level. No evidence of a teratogenic response was found.
       However, plasma ChE and red blood cell AChE activities were significantly
       decreased among maternal mice given 1 and 10 mg/kg bw/day, but not at the
       lowest dose. Fetal tissue homogenate ChE was only inhibited at 10 mg/kg
       bw/day. Based on inhibition of cholinesterases, the maternal NOAEL was 0.1
       mg/kg bw/day and the developmental NOAEL 1 mg/kg bw/day (Dea80).
            In a developmental toxicity study in rabbits, groups of New Zealand white
       rabbits (n=14/group) were given chlorpyrifos (purity: 96.1%) in maize oil by
       gavage at doses of 0, 1, 9, 81, or 140 mg/kg bw/day, on days 7-19 of gestation.
       Blood samples were collected after at least 10 days of dosing, and the does were
       sacrificed on day 29. Maternal effects noted at 140 mg/kg bw/day included
       reduced food consumption, body weight loss, and a 72% reduction of plasma
       ChE activity. The inhibition of plasma ChE was dose related and was still 56% at
       the lowest dose level. Brain or red blood cell AChE activity was not determined.
       Statistically significant increases in post-implantation loss were observed at 9
       and 140 mg/kg bw/day, but not at 81 mg/kg bw/day. Slightly decreased fetal
       weights and crown-rump lengths, and an increased incidence of fetuses with
       unossified fifth sternebra and/or xiphisternum were observed at the highest dose
       level. The maternal NOAEL for systemic toxicity was 81 mg/kg bw and the
       maternal LOEL 1 mg/kg bw, based on inhibition of plasma ChE. The
       developmental NOAEL was 81 mg/kg bw/day (Rub87b).
            Several studies have been reported on developmental neurotoxicity in rats.
       Pregnant Sprague-Dawley rats (n=5/group) were injected subcutaneously with
       either peanut oil or chlorpyrifos (purity: 98%) at 200 mg/kg bw as a single dose
       on gestation day 12, and then sacrificed on either day 16 or day 20 of gestation or
       on post-natal day 3, for measurement of maternal and developmental toxicity.
       Maternal effects noted were decreased body weights on day 3 after treatment,
       inhibition of brain AChE (82-88%) at all 3 time points after dosing, and
       reduction of brain muscarinic receptor binding (30-32%) on day 20 of gestation
       or post-natal day 3. At days 16 and 20 of gestation, chlorpyrifos did not alter
       fetal body and brain weights, but fetal brain AChE activity was inhibited by 42-
       44%. At post-natal day 3, inhibition of pup brain AChE was still 30%. Pup brain
       muscarinic receptor binding was inhibited by 16 and 11%, respectively, on day
       20 of gestation or on post-natal day 3. No parameters for teratogenicity were
       studied (Cha95). In a follow-up study, rats were given subcutaneous doses of
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<pre>       6.25, 12.5, or 25 mg/kg bw/day, on days 12-19 of gestation, and then sacrificed
       on day 20 of gestation or on post-natal day 3. No maternal toxicity was seen at
       any dose, but brain AChE activity on day 20 of gestation was inhibited by 75%
       and 90% at 6.25 and 25 mg/kg bw/day, respectively. At 25 mg/kg bw/day, pup
       weight on post-natal day 1 was significantly reduced. Fetal brain AChE activity
       on day 20 of gestation was inhibited by 40% and 60% at 6.25 and 25 mg/kg bw/
       day, respectively. Muscarinic receptor binding was reduced in a dose-related
       manner in both maternal and fetal brain. These neurochemical effects were more
       severe in dams than in the developing fetuses and were more severe following
       repeated exposure compared to the equivalent acute dose (Cha96).
           In a rat development neurotoxicity study, pregnant Sprague-Dawley rats
       (n=24-25/ group) were given chlorpyrifos (purity: 99.8%) in corn oil at oral
       (gavage) doses of 0, 0.3, 1, or 5 mg/kg bw/day, from gestation day 6 through
       lactation day 10. F1 generation litters (n=8-20/sex/group) were observed up to
       post-natal day 91. An additional 5 pregnant rats per dose level received
       chlorpyrifos on gestation day 6-20, and were used to obtain brain, plasma, and
       red blood cell samples for cholinesterase determinations. Effects in dams given
       doses of chlorpyrifos of 5 mg/kg bw/day were cholinergic toxicity (muscle
       fasciculations, hyperpnoea, and hyperreactivity), a treatment-related reduced
       body weight gain toward the end of gestation and the beginning of lactation (not
       statistically significant), reduced food consumption at the beginning of lactation,
       and marked inhibition of brain AChE and plasma ChE (approximately 90%), and
       of red blood cell AChE (100%) on gestation day 20. At 1.0 mg/kg bw/day, brain
       and red blood cell AChE of the dams were depressed by 18% and 84%,
       respectively, and plasma ChE by 69%. At the low dose, brain AChE activity was
       not affected, while red blood cell AChE and plasma ChE were still depressed by
       41% and 43%, respectively. Developmental parameters in dams, i.e., duration of
       gestation, number of implantations per dam, and mean litter size on post-natal
       day 0 were comparable across the groups. A statistically significant increase in
       pup mortality on post-natal day 0 and post-natal days 1-4, lower pup body
       weights on post-natal days 0 and 4, and a decreased viability index were seen at 5
       mg/kg bw/day. By post-natal day 65, the weights of high-dose female offspring
       were comparable with controls, while the weights in male offspring remained
       lower. At 5 mg/kg bw/day, increased pinna detachment, preputial separation, and
       vaginal opening were also observed. Post-natal day-11 brain measurements
       revealed decreased absolute brain weights, increased relative brain weights, and
       decreased brain layer thickness (e.g., of the parietal cortex, and the
       hippocampus) in high-dose male and female offspring. The differences in brain
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<pre>       layer thickness disappeared after correction for brain weight. In post-natal day-
       65 rats, no brain effects were observed, except a decrease in the parietal cortex of
       adult females in the mid- and high-dose groups, compared with controls.
       However, after correction for brain weight, the high-dose female adult group was
       not statistically significantly different from the control group anymore. Cognitive
       functions (learning, short-term memory, and habituation in 2 different tasks) in
       the pups were not affected by treatment. There were no gross or microscopic
       lesions of the nervous system. The authors conclude that no evidence of specific
       toxicity of chlorpyrifos for the developing nervous system was identified and
       that the effects seen in high-dose offspring were a consequence of maternal
       toxicity, rather than a direct effect on the pups. The maternal LOAEL was <0.3
       mg/kg bw, based on inhibition of red blood cell AChE and plasma ChE. The
       offspring NOAEL was 1 mg/kg bw/day (Mau00). In another study, it was
       demonstrated that fetal exposure from high-dose dams (5 mg/kg bw/day) caused
       a 57% inhibition of fetal brain AChE on gestation day 20. On post-natal day 1,
       pup brain AChE was inhibited by 35%, but no inhibition of brain AChE was seen
       anymore in pups on post-natal day 5. In dams, brain AChE activities on lactation
       day 1 and lactation day 5 were inhibited by 85% and 78%, respectively. No
       inhibition of brain or red blood cell AChE, or plasma ChE occurred in fetuses or
       pups from dams given 0.3 or 1.0 mg/kg bw/day. Neonatal mortality from high-
       dose dams between post-natal days 1 and 5 did not correlate with inhibition of
       neonatal brain AChE (Mat00). According to the committee, these data justify the
       above offspring NOAEL of 1 mg/kg bw/day.
7      Existing guidelines
       The current administrative occupational exposure limit (MAC) for chlorpyrifos
       in the Netherlands is 0.2 mg/m3, 8-hour TWA, with a skin notation.
           Existing occupational exposure limits for chlorpyrifos in some European
       countries and in the USA are summarised in the annex.
8      Assessment of health hazard
       The health hazard assessment of chlorpyrifos is based to a large extent on
       toxicology reviews issued by the Health Effect Division of the United States EPA
       for reregistration eligibility and by the FAO/WHO Joint Meeting on Pesticide
       Residues for recommendation of an acceptable daily intake (ADI). The
       toxicology profile in these reviews is obtained mainly from unpublished reports
067-37 Chlorpyrifos
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<pre>       of toxicology studies conducted for registration purposes by the chemical
       companies manufacturing or marketing the product.
           Workers can be exposed to chlorpyrifos through inhalation of vapour or
       aerosols or by direct skin contact with a formulation of the compound. The
       committee did not find data on the percentage of uptake of the compound
       through the lungs in humans. In rats exposed to chlorpyrifos by nose-only, 72%
       of the inhaled dose was absorbed. The dermal absorption of chlorpyrifos was
       about 2% of the applied dose (in 120 hours); in mice about 74% (in 8 hours). The
       extent of absorption following oral intake is at least 70% in humans and
       essentially 100% in the rat. Following absorption, the compound is metabolised
       into breakdown products (e.g., DEP, DEPT, 3,5,6 TCP), which are mainly
       excreted in the urine (>70% in humans and rats). There is no evidence of
       accumulation of the compound in any of the tissues.
           Case studies in humans show that following ingestion of high doses of
       chlorpyrifos, nerve injury may develop, consistent with organophosphorus-
       induced delayed polyneuropathy (OPIDN). In a recent acute oral toxicity study
       with chlorpyrifos in human volunteers, the NOAEL for inhibition of red blood
       cell AChE activity was 1 mg/kg bw. In a short-term oral human volunteer study,
       ingestion of 0.014 mg/kg bw/day chlorpyrifos for 27 days did not produce
       statistically significant inhibition of either red blood cell AChE or plasma ChE.
       Red blood cell AChE activity was not inhibited following oral intake of 0.1
       mg/kg bw/day for 9 days. At this dose, the mean plasma ChE was inhibited by
       66%, indicating that in humans, plasma ChE is much more sensitive for
       inhibition by chlorpyrifos than red blood cell AChE. Results from 3 cohort
       studies on chlorpyrifos workers from the same manufacturing plant did not show
       neurobehavioural effects or changes in central and peripheral nervous system
       function. Mean plasma ChE inhibitions of 5-20% were found at airborne
       chlorpyrifos levels between 0.01 and 0.2 mg/m3, 8-hour TWA, but the committee
       considers that part of the observed plasma ChE inhibition might be due to dermal
       absorption. No changes were found in red blood cell AChE of workers who
       excreted 1-year average 3,5,6 TCP concentrations in overnight urine samples up
       to 1.5 mg/g creatinine, corresponding with an absorbed chlorpyrifos dose of
       approximately 0.043 mg/kg bw/day.
           In experimental animals, the compound is slightly irritating to the eyes and
       the skin, but has no sensitising potential. Based on the results of acute lethal
       toxicity studies in test animals, the committee considers the compound as toxic
       after inhalation and oral exposure, but unlikely to present a hazard by dermal
       contact. There was considerable variation in susceptibility between and within
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<pre>       species. Young rats were more susceptible to adverse effects compared to the
       adult animal after an acute exposure to chlorpyrifos. The earliest neurotoxic
       effect was inhibition of brain AChE activity. In rats, chlorpyrifos did not cause
       either neurological changes consistent with OPIDN or inhibition of neuropathy
       target esterase (NTE). However, in chicken, inhibition of NTE and mild delayed
       neuropathy was observed in a number of studies.
           Systemic effects in short-term or long-term toxicity studies in rats or mice
       revealed ocular opacity, keratitis, fatty vacuolisation of hepatocytes and adrenals,
       and reduction in haemoglobin levels and erythrocyte count at high oral doses. At
       high subcutaneous doses, reduction in motor activity and impairment of
       cognitive function were observed. However, the committee considers the
       relevance of these findings unclear, because of the route of administration and
       the relatively high dose levels.
           In studies with rats, mice, dogs, and monkeys, inhibition of plasma ChE, and
       of red blood cell AChE or brain AChE has been demonstrated. In all species
       examined, brain AChE has shown to be less sensitive for inhibition by
       chlorpyrifos than red blood cell AChE, which in turn is less sensitive for
       inhibition than plasma ChE. The inhalation NOAEL for inhibition of brain and
       red blood cell AChE was >0.28 mg/m3 for rats (13-week study). The oral
       NOAEL for inhibition of brain and red blood cell AChE was >2 and 0.08 mg/kg
       bw/day, respectively, for monkeys (6-month study), 1 and 0.1 mg/kg bw/day,
       respectively, for dogs and rats (2-year studies), and 0.9 mg/kg bw/day for mice
       (78-week study).
           Chlorpyrifos did not cause gene mutations in in vitro tests, but conflicting
       results were obtained in in vitro and in vivo tests for clastogenic effects.
       According to the committee, the test systems or the type of test material used
       may cause this discrepancy. Carcinogenicity studies in rats and mice did not
       show treatment-related increased incidences of neoplastic lesions. In
       reproduction toxicity studies in rats, treatment-related effects of chlorpyrifos
       mainly consisted of cholinergic signs, reduction in body weights, and of
       inhibition of cholinesterase activity in parental or maternal animals. Fetotoxicity
       was mainly characterised by reduced pup viability, occurring at doses above
       those causing parental or maternal toxicity. The lowest oral NOAEL for
       reproduction toxicity in the rat was 1 mg/kg bw/day. Developmental NOAELs
       ranged from 1 mg/kg bw/day in a mouse study, to 81 mg/kg bw/day in a rabbit
       study. There was no evidence of treatment-related malformations in any of the
       studies. A developmental neurotoxicity study in the rat did not show treatment-
       related abnormalities in the brain of offspring at the highest dose tested (5 mg/kg
067-39 Chlorpyrifos
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<pre>       bw/day). Comparative cholinesterase inhibition data for the dams and the pups
       showed that pups were much less sensitive for inhibition of brain or red blood
       cell AChE than the dams.
            Based on the above data, the committee concludes that the mechanism of
       toxicity of chlorpyrifos in mammals is through inhibition of AChE activity in
       nerve tissue, occurring at dose levels that are lower than those that cause other
       toxic effects. Therefore, the committee identifies inhibition of brain AChE as the
       critical effect. In human beings, for obvious reasons, brain AChE cannot be
       measured. Instead, red blood cell AChE, being the same molecular target for
       inhibition by organophosphorus pesticides as brain AChE, is used as a surrogate
       for brain AChE in assessing the human health risk of exposure to chlorpyrifos
       (Jey94). Studies in rats, mice, dogs, and monkeys showed that red blood cell
       AChE is approximately 10 times more sensitive for inhibition by chlorpyrifos
       than brain AChE, and it may be assumed that this also is the case in humans.
       Chlorpyrifos did not cause inhibition of red blood cell AChE in any person
       involved in the short- or long-term human studies. The highest absorbed
       chlorpyrifos dose in a plant worker was calculated to be approximately 0.04
       mg/kg bw/day. Based on a 70-kg worker and assuming 100% pulmonary
       absorption in the absence of dermal uptake, the tentative NOAEL for inhibition
       of red blood cell AChE is greater than 0.3 mg/m3/day.
       Because of the absence of a NOAEL for red blood cell AChE in humans, the
       committee takes the 2-year rat study as a starting point in deriving a HBROEL.
       The NOAEL for inhibition of brain AChE was 1 mg/kg bw/day. Since workers
       are exposed for 5 days a week, this NOAEL from a continuous study (i.e., 7
       days/week) is adjusted by multiplying with a factor of 7/5 resulting in a no-
       adverse-effect level (NAEL) of 1.4 mg/kg bw. For the extrapolation to a
       HBROEL, a factor of 4 for allometric scaling from rats to humans, based on
       caloric demand, and an overall factor of 9, covering the absence of a NOAEL
       and inter-and intraspecies variation, are applied, resulting in a NAEL for humans
       of 0.04 mg/kg bw/day. Assuming a 70-kg worker inhales 10 m3 of air during an
       8-hour working day and a retention of 100%, and applying the preferred value
       approach, a HBROEL of 0.2 mg/m3 is recommended for chlorpyrifos.
       The committee recommends a health-based occupational exposure limit for
       chlorpyrifos of 0.2 mg/m3, as an 8-hour time-weighted average (TWA). Because
       chlorpyrifos can be absorbed through the skin in significant amounts, the
       committee recommends a skin notation.
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Bur98  Burns C, Cartmill JB, Powers BS, et al. Update of the morbidity experience of employees potentially
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<pre>              Annex
Occupational exposure limits for chlorpyrifos in various countries.
country                                occupational           time-weighted      type of                 notea         referenceb
- organisation                         exposure limit         average            exposure limit
                                       ppm       mg/m3
the Netherlands
- Ministry of Social Affairs and       -         0.2          8h                 administrative          S             SZW03
Employment
Germany
- AGS                                  -         0.2          8h                                         S             TRG00
- DFG MAK-Kommission                   -         -                                                                     DFG02
Great Britain
- HSE                                  -         0.2          8h                 OES                     S             HSE02
                                                 0.6          15 min
Sweden                                 -         -                                                                     Arb02
Denmark                                -         0.2          8h                                         S             Swe00
USA
- ACGIH                                -         0.1c         8h                 TLV                     S, A4d        ACG03b
- OSHA                                 -         -                                                       S             ACG03a
- NIOSH                                -         0.2          10 h               REL                     S             ACG03a
                                       -         0.6          15 min
European Union
- SCOEL                                -         -                                                                     EC03
a
     S = skin notation, which means that skin absorption may contribute considerably to body burden; sens = substance can
     cause sensitisation.
b
     Reference to the most recent official publication of occupational exposure limits.
c
     Measured as inhalable fraction of vapour and aerosol.
d
     Classified in carcinogen category A4, i.e., not classifiable as a human carcinogen: agents which cause concern that they
     could be carcinogenic for humans but which cannot be assessed conclusively because of lack of data. In vitro or animal
     studies do not provide indications of carcinogenicity which are sufficient to classify the agent into one of the other
     categories.
067-51        Chlorpyrifos
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<pre>067-52 Health-based Reassessment of Administrative Occupational Exposure Limits</pre>

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