How to Determine if an Additional 10 Safety Factor Is Needed for Chemicals: A Case Study with Chlorpyrifos

Transcription

1 TOXICOLOGICAL SCIENCES 48, (1999) Copyright 1999 by the Society of Toxicology How to Determine if an Additional 10 Safety Factor Is Needed for Chemicals: A Case Study with Chlorpyrifos James E. Gibson, 1 William L. Chen, and Robert K. D. Peterson Dow AgroSciences, LLC, 9330 Zionsville Road, Indianapolis, Indiana Received September 25, 1998; accepted November 25, 1998 THE FOOD QUALITY PROTECTION ACT (FQPA) AND ITS ASSOCIATED 10 SAFETY FACTOR In August of 1996, the U.S. Congress passed new pesticide food safety legislation, the Food Quality Protection Act (FQPA). The FQPA states that in the case of threshold effects for purposes of clause (ii) (I) (reasonable certainty that no harm will result), an additional tenfold margin of safety for the pesticide chemical residue and other sources of exposure shall be applied for infants and children, to take into account potential pre- and post-natal toxicity and completeness of the data with respect to exposure and toxicity to infants and children. Notwithstanding such requirements for an additional margin of safety, the Administration may use a different margin of safety for the pesticide chemical residue only if, on the basis of reliable data, such margin will be safe for infants and children (EPA, 1996). The legislative requirements of FQPA are based on the recommendations in the National Academy of Sciences (NAS) report (NRC, 1993), Pesticides in the Diets of Infants and Children, which concluded that infants and children may have significantly different exposures and/or responses to pesticides than adults. Therefore, it was believed that an additional uncertainty factor may, in some cases, be needed to account for the incomplete data. HOW TO DETERMINE THE NEED FOR THE FQPA 10 SAFETY FACTOR How should the risk of chemicals to infants and children be assessed? In reviewing the EPA s procedure for applying the additional uncertainty factor (FQPA factor) to protect infants and children, the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) Science Advisory Panel (SAP), in the meeting of March, 1998 (SAP, 1998), recommended a 5-step procedure: (1) hazard identification; (2) dose-response assessment; (3) exposure assessment; (4) risk characterization; and (5) 1 To whom correspondence should be addressed at Dow AgroSciences, LLC, 9330 Zionsville Rd., Indianapolis, IN Fax: (317) jgibson@dowagro.com. decision whether to apply the FQPA factor as the final step in the process. The first four steps in this process mirror the risk-assessment procedure outlined by the National Academy of Sciences in 1983 (NRC, 1983), and, we believe, is the most effective way to reach a decision in step 5. In cases where data gaps exist, the EPA should use its data call-in process to try to provide the needed information (EPA, 1997). Hazard is the inherent ability to do harm to living organisms. Under the current hazard-identification methodology, animal toxicological studies are required to include the highest dose that is sufficient to elicit a significant toxic effect; this becomes known as the maximum tolerated dose (MTD). The MTD is usually unrealistically high and never encountered in real life. Therefore, it is important to conduct an exposure assessment that will consider the route, extent, frequency, and duration of human exposure. Furthermore, it is essential that available pharmacokinetic, metabolic, and mechanistic data are utilized to assess the relevance to humans of effects observed in animal toxicity studies. This integrated toxicity and exposure assessment should be based on sound science, taking into account the weight of the scientific evidence from all relevant data sources. A CASE STUDY WITH CHLORPYRIFOS Using the SAP procedure, Dow AgroSciences scientists and independent experts have critically reviewed the toxicology and exposure database on chlorpyrifos, with respect to the adequacy of the existing safety factor in protecting infants and children. The 5-step process should end with an appropriate decision having been made regarding the need for an FQPA 10 safety factor. Hazard Identification and Dose Response Assessment Neurotoxicity: Organophosphate-Induced Delayed Neuropathy (OPIDN) and Neurophysiological Effects Epidemiology. An epidemiology panel composed of international health experts 2 concluded, after examining the rele- 2 James W. Albers, Department of Neurology, University of Michigan; Philip Cole, University of Alabama at Birmingham; Ray Greenberg, Chairman, 117

2 118 GIBSON, CHEN, AND PETERSON vant data, that chlorpyrifos had not been shown to be a concern for public health (Albers et al., 1999). The panel examined available scientific evidence on a variety of neurological, behavioral, and immunological disorders, multiple complaints (often called multiple chemical sensitivities), and birth defects. After extensive review, the panel was not persuaded that exposure to chlorpyrifos-containing products caused any of these conditions in humans. The panel s report was submitted to the EPA on October 20, Reference dose (RfD). In April, 1997, a panel of independent experts 3 was convened to review and evaluate all relevant animal and human studies, with respect to the determination of RfDs for chlorpyrifos (Clegg and Van Gemert, 1999a,b). The panel s report was submitted to the EPA on December 11, The RfD panel reviewed a toxicity database comprising the following: biochemical data; acute-toxicity data in a number of species; short-term studies in rats, dogs and monkeys; longterm studies in mice and rats; 3-rat-multigeneration reproduction studies; developmental studies in mice and rats; studies on neuropathy target esterase inhibition; cognitive studies; chlorpyrifos-induced delayed neuropathy studies on hens, rats, cats and humans; and toxicity studies using human volunteers. In addition, several case reports were reviewed on organophosphate-induced delayed neuropathy (OPIDN) and the sensory neuropathy potential of chlorpyrifos. No evidence of OPIDN was found when chlorpyrifos was tested in hens in an acute study at twice the LD50 (100 mg/kg body weight [bw]), or in a subchronic hen study at 10 mg/kg bw/day. The RfD Panel concluded that OPIDN was only observed after lethal doses. Neurotoxicity studies. Chlorpyrifos has been evaluated for neurotoxic effects in rats, using single-dose, as well as 13-week repeated dose, EPA FIFRA test guidelines and good laboratory practice (GLP) conditions. The studies were published in the scientific literature (Mattsson et al., 1996) and included a functional observational battery, automated motor-activity testing, and comprehensive neurohistopathology testing of perfused tissues. Administration of up to 100 mg/kg bw by gavage in the single-dose study, and up to 15 mg/kg bw/day in diet for 13 weeks in the repeated-dose study were evaluated. These dosages provided an opportunity to characterize chlorpyrifos over-exposure in rats the effects of which included cholinergic Medical University of South Carolina; Jack S. Mandell, University of Minnesota; Richard R. Monson, Harvard School of Public Health; John H. Ross, California Department of Pesticide Regulation; Wayne R. Snodgrass, Texas Poison Center; and Anne Spurgeon, University of Birmingham, U.K. 3 James W. Albers, Department of Neurology, University of Michigan; David J. Clegg, former Head of Pesticide Toxicology, Health Canada; Philip S. Guzelian, Department of Medicine, University of Colorado; Marcello Lotti, Istituto di Medicina del Lavoro, Universita di Padova; Rudy J. Richardson, Toxicology Program, University of Michigan; and Mike Watson, former Head of Toxicology, Pesticide Safety Directorate, U.K. overstimulation. Single gavage doses of chlorpyrifos caused a range of clinical signs characteristic of cholinergic overstimulation, but effects were minimal in the 13-week study and there was no evidence of accumulation of toxicity during treatment. Motor activity was decreased in the high dose rats at week four, but was not significantly different from controls in subsequent weeks. The NOEL for the single-dose study was 10 mg/kg bw (LOEL was 50 mg/kg bw with minimal cholinergic effects) and the NOEL for the 13-week study was 1 mg/kg bw/day (LOEL was 5 mg/kg bw/day with minimal cholinergic effects). Comprehensive neuropathological examination revealed no treatment-related lesions in either study. Susceptibility of Infants and Children All mammalian chlorpyrifos-toxicology studies required to support EPA registration, including developmental and reproductive studies conducted according to FIFRA guidelines and under GLP have been submitted and judged by the agency to be acceptable. Chlorpyrifos did not affect fetal development or reproduction at maternally non-toxic dosages, as noted in the Hazard Identification Assessment Review Committee (HIARC) report (EPA, 1998). Relevant developmental toxicity studies in the scientific literature, where dams were dosed, demonstrate that the fetus and neonate are less sensitive to chlorpyrifos than the adult. (Barone et al., 1997; Chandra et al., 1995; Chandra and Pope, 1996; Phillips et al., 1997; Tang et al., 1997). Other published studies (Campbell et al., 1997; Moser and Padilla, 1998; Pope and Charkraborti, 1992; Pope et al., 1991; Song et al., 1997; Whitney et al., 1995) in which chlorpyrifos was administered as a single dose directly to neonates are not appropriate to assess the relative sensitivity of young animals compared to adults, because: (1) The routes of exposure used in animals were often inappropriate as potential routes of exposure in human infants and children. Most of the animal data were generated with subcutaneous or interperitoneal injections, which are not encountered with human subjects. (2) Very high MTD of chlorpyrifos (defined as the dose producing no more than 10% mortality; 18 to 45 mg/kg bw) were administered to juveniles, and these doses were much larger than a neonate or infant could absorb via an appropriate route of exposure. (3) Other data indicate that, at lower levels of exposure, differences in sensitivity are minimal or non-existent. For example, Pope and Liu (1997) have shown that juveniles are not more sensitive than adults to neurochemical alterations following repeated lower-level exposures. The FIFRA Science Advisory Panel (SAP) report of March, 1998, in discussing the FQPA 10 safety factor, also raised concerns over these acute high dose studies (SAP, 1998):

3 DETERMINING THE 10 SAFETY FACTOR 119 Some data are presently available... much of this existing information was generated in acute treatment experiments, frequently at very high exposure levels. Such data may not be appropriate to extrapolate to low-dose situations (e.g., organophosphates), where much, if not all, of the age-related differences may be attributable to differences in the magnitude of activity of detoxification enzymes. In such cases, differences in toxicity between adults and juveniles would be substantially greater at high doses where detoxification mechanisms are saturated than at low doses where they are not. Developmental neurotoxicity study. In May 1998, Dow AgroSciences submitted a developmental neurotoxicity study to the EPA, which was conducted according to FIFRA guidelines and under GLP (Hoberman, 1998; Maurissen, 1998). The study was designed in concert with EPA to ensure that it addressed the EPA s concerns. A series of experiments were performed where pregnant female rats (dams) were given chlorpyrifos by gavage daily, starting from gestation day 6 through lactation day 10 at dosages of 0, 0.3, 1, and 5 mg/kg bw. This represented a repeated low dose exposure lasting 25 days, wherein the fetuses were exposed to the compound through the mother, and postnatally through the maternal milk. Developmental and cognitive measurements, along with neurochemical measurements (AChE activity in plasma, red blood cells, heart and brain tissues), were conducted in both the dams and pups. Chlorpyrifos had no effect on cognitive functions at any of the doses and did not cause any developmental effects on the offspring with the two lower doses, showing that it was not a selective developmental neurotoxicant in the rat. Only at the highest dose, which was clearly maternally toxic, were transient effects seen in the pups that were suggestive of delayed development, and these were attributed to maternal neglect. The NOEL for maternal and pup toxicity was 1 mg/kg/day. Furthermore, AChE measurements showed that AChE activity in heart and brain tissues of neonates was 10- to 47-fold less inhibited than in adults. There was no unique sensitivity of the neonates to chlorpyrifos, as compared to the mother, when dose levels and route of exposure were appropriate. Therefore, empirical evidence suggests that chlorpyrifos is not a selective developmental neurotoxicant. Conservatism of the Current RfDs The RfD values for chlorpyrifos currently used by EPA are conservative. The NOEL for human plasma butyrylcholinesterase inhibition endpoint, which is the basis for the EPA RfD values, is a marker of exposure, not an adverse effect. Hence, EPA RfD values already include a very conservative intraspecies safety factor which is greater than the standard 10-fold safety factor. The EPA Federal Insecticide, Fungicide and Rodenticide Act Scientific Advisory Panel (FIFRA SAP), in June of 1997 (SAP, 1997), also concluded that blood cholinesterases (plasma and RBC) are not involved in cholinergic transmission, and as such, have no toxicological significance to organophosphate toxicity. The panel stated that measured inhibition of cholinesterase activities in any of the blood fractions are best regarded as an imperfect mirror of enzyme inhibition in the true target tissues: brain, neuromuscular junctions, autonomic ganglia, and autonomic synapses. When, or if, direct measurements at the probable target sites become available, data from the blood might be under-weighted or even ignored. Other regulatory bodies, e.g., the California Department of Pesticide Regulation (DPR), the governments of Canada and European Union (EU) member countries, and the World Health Organization (WHO), base exposure standards on human red blood cell (RBC) acetylcholinesterase (Canada, EU Countries, and WHO) or brain acetylcholinesterase in the rat (California DPR). Therefore, basing the RfD on RBC AChE inhibition would contribute to global harmonization in the regulation of cholinesterase inhibitors, and still allow for a significant safety factor. Comprehensive studies of the inhibition of AChE activities in blood, brain, and peripheral tissues after treatment with chlorpyrifos, at relevant doses in adult and neonatal rats, were conducted by several groups (Chen et al., 1999; Nostrandt et al., 1997). The significance of these data in selecting the toxicological endpoint, the setting of RfD (ADI), and the need for an additional 10 FQPA safety factor, was assessed. Conclusions from this evaluation are as follows: (1) There are adequate data on the effects of chlorpyrifos on blood, brain, and peripheral tissue AChE in both adult and neonatal rats to establish a conservative RfD that will protect the health of human adults and children. (2) RBC AChE is inhibited 12 to118-fold more than brain or peripheral tissues AChE in adult rats given a single oral dose of chlorpyrifos, demonstrating that basing the RfD on inhibition of RBC AChE introduces a conservative safety factor for setting exposure guidelines. (3) Furthermore, AChE activities in brain and peripheral tissues of neonates are about 20- to 47-fold less inhibited than in adult tissues when chlorpyrifos is given to the mother. There is no unique sensitivity of neonates to chlorpyrifos when dose levels and routes of exposure are administered in an appropriate manner. Evidence indicates that chlorpyrifos is not a selective developmental neurotoxicant. Human RBC AChE level is an appropriate and conservative surrogate enzyme for establishing the chlorpyrifos RfD. This RfD for chlorpyrifos, based upon RBC AChE inhibition, would be protective of infants and children. Exposure Assessment and Risk Characterization To characterize risk properly, considerations of exposure are critical. Further, exposure scenarios specific to infants, children, and other potentially sensitive subpopulations need to be assessed. Results from national market-basket survey, dietary risk analyses, residential exposure, and biomonitoring studies demonstrate that exposures of infants and children to chlorpyr-

4 120 GIBSON, CHEN, AND PETERSON TABLE 1 Aggregate (Total Dietary and Residential) Exposure to Chlorpyrifos as Determined by Monte Carlo Probabilistic Analysis and Biomonitoring Population Aggregate exposure ( g/kg bw/day) Children (0 to 6 years old)* 1.2 Monte Carlo analysis at 99.5 percentile Children Pesticide Exposure Study (CPES) Biomonitoring data** Children biomonitoring data at 100 percentile 1.4 Adults* Monte Carlo analysis at 99.5 percentile 0.46 General population biomonitoring data (CDC)*** Adults biomonitoring data at 99 percentile 0.82 Adults biomonitoring data at 100 percentile 1.7 * Shurdut et al., 1998 ** Quackenboss et al., 1998 *** Hill et al., 1995 ifos are well below established regulatory guidelines (Bolles and Cordes, 1997; Byrne et al. 1998; Dixon-White and Tomerlin, 1995; Hill et al., 1995; Quackenboss et al., 1998; Shurdut et al., 1998; Tomerlin et al., 1995; Vaccaro et al. 1987, 1991, 1993, 1996a,b; Wright and Jackson, 1975; Wright and Leidy, 1978). Pursuant to the FQPA, a state-of-the-art probabilistic model was developed to determine potential aggregate (total dietary and residential) exposures from all use patterns (Shurdut et al., 1998). Using this model, estimated aggregate exposure was low for infants and children, 1.2 g/kg /day, and was well below the acute and chronic RfD values for chlorpyrifos of 10 g/kg and 3 g/kg/day, respectively (based on the NOEL of human plasma cholinesterase inhibition and applying at 10 uncertainty factor of intraspecies variation). Comparison of these results to actual measurements of the primary metabolite of chlorpyrifos, 3,5,6-trichloropyridinol, made by the Centers for Disease Control (CDC) in the U.S. population suggests that the robust exposure database for chlorpyrifos probably characterizes the range of potential exposures to residents (Hill et al., 1995). Preliminary results from the Child Pesticide Exposure Study (CPES) also suggest that the highest chlorpyrifos exposures to children, as estimated from urinary TCP analysis, are less than 1.4 g/kg/day for chlorpyrifos-equivalent exposures (Quackenboss et al., 1998). Consequently, through this independent validation using biomonitoring results from the U.S. population (the children s sub-population was intentionally selected to represent a highly exposed population), exposures have been characterized with a high degree of certainty (Table 1). Decision on the Need for Additional Safety Factor Based on the SAP decision-making process, and based on a weight-of-the-evidence evaluation and a reasonable certainty of no harm, we believe no additional uncertainty factor is needed for chlorpyrifos to protect infants and children. The bases for this conclusion are as follows: (1) comprehensive and complete database for chlorpyrifos with respect to toxicity and exposure. (2) Organophosphate-induced delayed neurotoxicity (OPIDN) was only observed after lethal doses. No evidence of OPIDN was found when chlorpyrifos was tested in hens in an acute study at twice the LD50 (100 mg/kg bw), or in a subchronic study at 10 mg/kg bw/day. (3) FIFRA guideline reproductive and developmental toxicity studies showed no potential pre- or post-natal effects of concern. (4) The recently completed developmental neurotoxicity study demonstrated that chlorpyrifos is not a selective developmental neurotoxicant. (5) Many published studies in which chlorpyrifos was administered directly to neonates are not appropriate to assess the relative sensitivity of infants and children relative to adults, because the route was inappropriate and much larger doses of chlorpyrifos were administered than a neonate or infant could absorb via an appropriate route of exposure. (6) Results from the national market-basket survey, dietary risk analyses, and residential exposure/biomonitoring studies demonstrated that total exposure of infants and children to chlorpyrifos is low ( 1.4 g/kg/day) and well below established guidelines. (7) The RfD values for chlorpyrifos used by EPA are conservative. The human plasma cholinesterase inhibition endpoint, on which EPA RfD values are based, is a marker of exposure, not an adverse effect. Hence, EPA RfD values actually include more than a 10-fold safety factor. CONCLUDING REMARKS Risk assessment approaches are crucial to making informed regulatory and policy decisions about chemicals such as pesticides. Decisions must be firmly based on scientific weight of evidence with respect to toxicity and exposure, and, especially, sound science. Because the SAP s 5-step process is based on a firmly established risk-assessment paradigm, its use leads to an objective determination of the need for, or against, an additional 10 safety factor. This approach takes all of the available information for a chemical into account rather than focusing on only selected consideration of identified potential hazards. Additionally, consideration of all of the available information for a particular chemical allows for the identification of inadequate, or missing, data. Using chlorpyrifos as a case study, we have demonstrated how scientific evidence of toxicity and exposure can, and should, inform the FQPA safety-factor decision process. When the SAP s scientific weight-of-evidence approach is applied to

5 DETERMINING THE 10 SAFETY FACTOR 121 chlorpyrifos, it is evident that the additional 10 safety factor for children is not needed. REFERENCES Albers, J. W., Cole, P., Greenberg, R. S., Mandel, J. S., Monson, R. R., Ross, J. H., Snodgrass, W. R., Spurgeon, A., and Van Gemert, M. (1999). Analysis of chlorpyrifos exposure and human health: expert panel report. J. Toxicol. Environ. Health (in press). Albers, J. W., Clegg, D. J., Guzelian, P. S., Lotto, M., Richardson, R. J., Van Gemert, M., and Watson, M. (1997b). Chlorpyrifos: Report of the Reference Dose Data Review Panel. Washington D.C. Submitted to EPA on December 11, Barone, S., Jr., Lau, C., Moser, V. C., Phillips, P. M., McDaniel, K. L., Hunter, D. L., Marshall, R., Kodavanti, P., Derr-Yellin, E., Padilla, S., and. Lassiter, T. L., (1997). Developmental effects of gestational exposure to chlorpyrifos. The Toxicologist 36, 256 (1301). Bolles, H. G., and Cordes, R. C. (1997). Tier III acute dietary risk assessment for chlorpyrifos using chlorpyrifos national food survey data. GH-C Research report of Dow AgroSciences, MRID Byrne, S., Shurdut, B. A., and Saunders, D. G. (1998). Potential chlorpyrifos exposure to residents following crack and crevice treatment. Environ. Health Perspect. 106, Campbell, C. G., Seidler, F. J., and Slotkin, T. A. (1997). Chlorpyrifos interferes with cell development in rat brain regions. Brain Res. Bulletin 43, Chanda, S. M., Harp, P., Liu, J., and Pope, C. N. (1995). Comparative developmental and maternal neurotoxicity following acute gestational exposure to chlorpyrifos in rats. J. Toxicol. Environ. Health. 44, Chanda, S. M., and Pope, C. N. (1996). Neurochemical and neurobehavioral effects of repeated gestational exposure to chlorpyrifos in maternal and developing rats. Pharmacol. Biochem. Behav. 53, Chen, W. L., Sheets, J. J., Nolan, R. J., and Mattsson, J. L. (1999). Human red blood cell acetylcholinesterase inhibition as the appropriate and conservative surrogate enzyme for establishing chlorpyrifos reference dose. Reg. Tox. Pharm. 29, Clegg, D. J., and Van Gemert, M. (1999a). Determination of the reference dose for chlorpyrifos: proceedings of an expert panel. J. Toxicol. Environ. Health (in press). Clegg, D. J., and Van Gemert, M. (1999b). Expert panel report of human studies on chlorpyrifos and/or other organophosphate exposures. J. Toxicol. Environ. Health (in press). Dixon-White, H. E., and Tomerlin, J. R. (1995). Chlorpyrifos national food survey. GH-C Research report of Dow AgroSciences. EPA MRID EPA (U.S. Environmental Protection Agency) (1996). Is an additional uncertainty factor necessary and appropriate to assess pre- and post-natal developmental and reproductive effects in infants and children exposed to pesticides through chronic dietary exposure? EPA (U.S. Environmental Protection Agency) (1997). FIFRA Sec 3 [c] (2) (B) EPA document 730l EPA (U.S. Environmental Protection Agency) (1998). U.S. EPA Hazard Identification Assessment Review Committee (HIARC) Report. Hill, R. H., Head S. L., Baker S., Gregg M., Shealy D. B., Bailey S. L., Williams C. C., Sampson E. J., and Needham, L. L. (1995). Pesticide residues in urine of adults living in the United States: Reference range concentrations. Environ. Res. 71, Hoberman, A. M. (1998). Developmental neurotoxicity study of chlorpyrifos administered orally via gavage to Crl:CD BR VAF/Plus pregnant rats. Unpublished Report of The Dow Chemical Co. Lab Report No MRID Mattsson, J. L., Wilmer, J. W., Shankar, M. R., Berdasco. N. M., Chrisman, J. W., Maurissen, J. P., and Bond, D. M. (1996). Single dose and 13-week repeated-dose neurotoxicity screening studies of chlorpyrios insecticide. Food Chem. Toxicol. 34, Maurissen, J. (1998). Chlorpyrifos: Assessment for developmental neurotoxicity in rats (report of an OP, assessed using proposed EPA test guidelines). Pesticides and susceptible populations: Who are at risk and when. Conference Proceedings, Little Rock, AR (in press). Moser, V. C., and Padilla, S. (1998). Age- and gender-related differences in the time-course of behavioral and biochemical effects produced by oral chlorpyrifos in rats. Toxicol. Appl. Pharmacol. 149, Nostrandt, A. C., Padilla, S. and Moser, V. C. (1997). The relationship of oral chlorpyrifos effects on behavior, cholinesterase inhibition, and muscarinic receptor density in rat. Pharmacol. Biochem. Behav. 58, NRC (National Research Council). (1983). Risk Assessment in the Federal Government: Managing the Process. National Academy Press, Washington, DC. NRC (National Research Council) (1993). Pesticides in the diets of infants and children. National Academy Press, Washington, D.C. Phillips, P. M., McDaniel, K. L., Lassiter, T. L., Barone, S., Jr., and Moser, V. C. (1997). Behavioral effects of gestational exposure to chlorpyrifos in rats. The Toxicologist 36, 256 (1300). Pope, C. N., Chakraborti, T. K., Chapman, M. L., Farrar, J. D., and Arthun, D. (1991). Comparison of in vivo cholinesterase inhibition in neonatal and adult rats by three organophosphate insecticides. Toxicology 68, Pope, C. N., and Charkraborti, T. K. (1992). Dose-related inhibition of brain and plasma cholinesterase in neonatal and adult rats following sublethal organophosphate exposures. Toxicology 73, Pope, C. N. and Liu, J. (1997). Age-related differences in sensitivity to organophorphorus pesticides. Environ. Toxicol. Pharmacol. 4, Quackenboss, J.J., Pellizari, E., Freeman, N., Head, S., Whitmore, R., Zelan, H. and Stroebel, C. (1998). Use of Screening Questionnaires to Identify Exposed and Sensitive Population Groups in the Region V NHEXAS Children s Pesticide Study. Presented at the Tenth Conference of the International Society of Environmental Epidemiology and Eight Conference of the International Society of Exposure Analysis, Boston. SAP (Science Advisory Panel) (1997). Report of the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), p. 24. Science Advisory Panel Meeting June 3 4, 1997, Washington, DC. SAP (Science Advisory Panel) (1998). Report of the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) Science Advisory Panel (SAP) Meeting March 24 25, 1998, Washington, DC. Shurdut, B. A., Barraj, L., and Francis, M. (1998). Evaluation of pesticide aggregate exposures under the Food Quality Protection Act: A case study using chlorpyrifos. Reg. Tox. Pharm. 28, Song, X., Seidler, F. J., Saleh, J. L.,. Zhang, J, Padilla, S., and Slotkin, T. A. (1997). Cellular mechanisms for developmental toxicity of chlorpyrifos: Targeting the adenylyl cyclase signaling cascade. Toxicol. Appl. Pharmacol. 145, Tang, J., Chambers, J. E., Carr, R. L., and Ho, L. L. (1997). The effect of chlorpyrifos exposure on esterase activities of neonatal rats. The Toxicologist 36, 346 (1760). Tomerlin, J. R., Kidwell, J. L., Barraj, L. M., and Petersen, B. J. (1995). Chronic and acute dietary exposure analysis for chlorpyrifos. GH-C Research report, Dow AgroSciences, MRID Vaccaro, J. R., Beard, K. K., Maxey, S. W., Murphy, P. G., Myers, C. R., Nolan, R. J., Ormand, J. R., and Timchalk, C. (1996a). Chlorpyrifos: Exposure to adults and children upon reentry to domestic lawns following treatment with a chlorpyrifos-based granular insecticide. Unpublished re-

6 122 GIBSON, CHEN, AND PETERSON search report of the Dow Chemical Company HEH (134). MRID Vaccaro, J. R., Bohl, R., Skowronski, B., and Morabito, P. (1987). Airborne chlorpyrifos concentrations measured during and following applications of Dursban* TC Insecticide to residential dwellings. Research report of the Dow Chemical Company GH-P MRID Vaccaro, J. R., Hugo, J. M., Nolan, R. J., and McNett, D. A. (1993). Chlorpyrifos: Exposure to adults and children upon reentry to domestic lawns, following treatment with a chlorpyrifos-based mixture. Research report of the Dow Chemical Company DECO-HEH (121). MRID Vaccaro, J. R., Nolan, R. J., Hugo, J. M., Pillepich, J. L., Murphy, P. G., and Bartels, M. J. (1991). Evaluation of dislodgeable residues and absorbed doses of chlorpyrifos to crawling infants following indoor broadcast applications of chlorpyrifos-based emulsifiable concentrate. DECO-HEH (95). Research report of the Dow Chemical Company, MRID Vaccaro, J.R., Nolan, R. J., Murphy, P. G., and Berbrich, D. B. (1996b). The use of unique study design to estimate exposure to adults and children to surface and airborne chemicals. American Society of Testing Materials, ASTM STP 1287, Whitney, K. D., Seidler, F. J., and Slotkin, T. A. (1995). Developmental neurotoxicity of chlorpyrifos: Cellular mechanisms. Toxicol. Appl. Pharmacol. 134, Wright, C. G., and Jackson, M. D. (1975). Insecticide residues in non-target areas of rooms after two methods of crack and crevice application. Bull. Environ. Contam. Toxicol. 13, Wright, C. G., and Leidy, R. B. (1978). Chlorpyrifos residues in air after application to crevices in rooms. Bull. Environ. Contam. Toxicol. 19,