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Residential Use of Pesticide Products Containing Chlorpyrifos, Diazinon, and Malathion
Colleen Peter, David L. MacIntosh
Department of Environmental Health Science, College of Agricultural and Environmental Sciences, University of Georgia, Athens, GA
Jorge H. Atiles
Department of Housing and Consumer Economics, College of Family and Consumer Sciences, University of Georgia, Athens, GA
Jane A. Hoppin
Epidemiology Section, National Institute of Environmental Health Sciences, Research Triangle Park, NC
Acknowledgments:
This research was supported by the United States Department of Agriculture Hatch Project Number GEO00843; the University of Georgia, College of Agricultural and Environmental Sciences, Undergraduate Research Initiative; and the Georgia Gerontology Consortium Seed Grant Program.
ABSTRACT
The U.S. Environmental Protection Agency estimates that 1 to 2 million kilograms of chlorpyrifos, diazinon, and malathion, as active ingredients in pesticide products are applied to homes and gardens of the U.S. each year. A questionnaire was developed, tested, and distributed to a convenience sample of 2399 residents of Georgia, a state in the Southeastern United States to obtain data on the proportion of users in the population, frequency of use and locations of use for products containing those organophosphorous insecticides. Eighteen percent of the surveys distributed were completed and returned in postage-paid envelopes. Approximately half of the respondents reported using products with chlorpyrifos (56%) or diazinon (49%) as the active ingredient, while 10% reported use of products containing malathion. Users of chlorpyrifos and diazinon were more frequently male and 25 years old than otherwise, while users of malathion were more frequently female. Use of chlorpyrifos and diazinon was significantly positively associated with annual household income. The proportion of households that reported use of chlorpyrifos, diazinon, and malathion did not differ between residences with and without children. Use patterns were consistent among the three active ingredients: used mostly outdoors; frequency of use was most often less than once per month; and the application was intended to treat observed pests. The information on the proportion and characteristics of individuals and households that reported residential use of chlorpyrifos, diazinon, and malathion can be used as inputs to quantitative exposure and risk models used in regulatory settings. In addition, this work provides guidance on the design of simple, reliable, and inexpensive tools for assessment of non-dietary residential exposure to pesticides.
INTRODUCTION
Approximately 8 million kilograms of insecticides are applied in homes and gardens of the United States every year (Aspelin and Grube, 1999). Passed into law in 1996, the U.S. Food Quality Protection Act (FQPA) requires a more comprehensive assessment than ever before of pesticide exposure, dose, and effects (Goldman, 1998). In particular, the FQPA requires pesticide risk assessments to consider exposure to potentially sensitive subgroups of the population, coincident dietary and non-dietary (i.e., aggregate or total) exposure, and contemporaneous multi-chemical (i.e., cumulative) exposure. The U.S. Environmental Protection Agency (EPA) utilizes exposure models to evaluate the potential health risks of non-dietary insecticide exposure (EPA, 1997), yet little information is available to assess the magnitude, duration, and frequency of actual non-dietary insecticide exposure in residential settings. This information would be valuable for evaluating the relationship between personal exposure and possible health effects through epidemiology or through quantitative risk assessment.
Some work has been completed on non-dietary exposure following residential application of organophosphate insecticides (Byrne et al., 1998; Zartarian et al., 2000), however little information is available on residential organophosphate use patterns, especially for specific chemicals. We are aware of only one study that has investigated the residential use of modern insecticides at the active ingredient level, including members of the organophosphate, pyrethroid, and carbamate chemical classes (Adgate et al., 2000). The investigation performed in Minnesota found that products containing selected organophosphates, chlorpyrifos and diazinon, were used in the last year by 17% and 11%, respectively, of the residences studied. We are not aware of similar pesticide use data at the active ingredient level from other regions of the United States.
In this paper, we report the results of a pilot study of residential organophosphate insecticide use in the area of Athens, Georgia, USA. The objectives of this study were to: (1) describe residential use of three organophosphate insecticides, chlorpyrifos, diazinon, and malathion in a convenience sample of households; (2) examine the data for demographic determinants of insecticide use patterns, and (3) generate information that could aid the design of future exposure-effect studies for organophosphate insecticides.
METHODS
Survey Instrument
Development of the survey began with research of common products used in and around homes that contained chlorpyrifos, diazinon, or malathion as the active ingredient. These three active ingredients were selected for study because of their reported annual retail sales volume of at least 1 million kg for use in homes and gardens (A.L. Aspelin and A.H. Grube, 1999). Sixteen such products were identified by visiting major retailers in the Athens area that were considered to be the most likely points of pesticide purchases for local residents (details of the retailer survey are available from the authors).
The survey instrument was formatted to fit on one sheet of 11" x 14" paper, with printing on both sides and folded into lengthwise quadrants. Large black fonts, lines, arrows and images were used to make the questionnaire easy to read and follow. It begins with a title page "What do you use to kill bugs at home?" and then opens to a brief letter that explains the purpose of the survey. The instrument includes fourteen questions, three of which have multiple subsets of questions. For consistency, all possible answers have a corresponding check box, which denotes a "yes" or "no" answer. Some questions ask respondents to check all boxes that apply.
The questionnaire focuses on use of the organophosphate insecticides chlorpyrifos, diazinon, and malathion. The question format was identical for each of the insecticides and included the sixteen actual products names identified from the retail visits, as opposed to the common chemical name, to improve recall by respondents (e.g., Ortho Bug-Be-Gone instead of diazinon, see Table 1). In addition, a question was included for each insecticide that asked whether any product was used that contained an active ingredient identified as chlorpyrifos or Dursban; diazinon; or malathion. This general category is intended to account for products that contain chlorpyrifos, diazinon, or malathion, but are not listed by name in the questionnaire. If any of the products containing a certain insecticide are used, then the respondent is instructed to answer three subsets of questions on use practices. Subset 1 asks about the location of insecticide application, specifically outdoors and/or indoors. To obtain additional insight into the possible exposure routes, indoor users were asked to indicate specific indoor locations among the following: kitchen, bedroom, bathroom, and living room. Subset B inquires about the rationale for use of the insecticide. The choices for this subset are: a) apply only when insect pests are seen, b) application only when insect pests are not present (i.e., preventive application), or c) application for both reasons. Finally, subset 3 asks how frequently pesticides are used. Respondents can check daily, weekly, monthly, or less than monthly.
The remaining questions of the survey concern demographic characteristics of the respondents. Information acquired includes age of respondent, number of children in household, annual household income, race, and educational background. Other questions ask specifically about pesticide use around children because the young may be more susceptible to these chemicals (Landrigan et al., 1999). There are also queries about reading safety instructions on pesticide labels, opening doors or windows when products are applied, and if a commercial pesticide service is used.
Two pilot studies were conducted to test the questionnaire and to provide information needed to improve the questionnaire design. The first pilot group included 50 parents that utilize a child development laboratory on the University of Georgia campus and from whom we received comments on survey content and layout. Based on the responses, several minor changes were made to the survey including rewording of some questions; inclusion of a check box for each possible answer to improve uniformity; inclusion of contact information for the investigators; and a question about use of commercial pesticide services.
Reports of results from previous community-based surveys indicate that response rates are often lower from low-income populations compared to medium income and high income populations (Armstrong et al., 1992). The second pilot study was designed to evaluate the ability of a nominal incentive to increase response rates from residents of low-income areas. Fifty residences in a relatively low-income Athens neighborhood and 45 residences in a relatively high-income neighborhood received surveys with 50% containing a dollar bill and 50% without the nominal incentive. The return rate from households that received a questionnaire that included the incentive was approximately two times greater than the return rate for residences with questionnaires that did not contain the dollar. However, there was no difference in response rates between the two neighborhoods when stratified by dollar/no dollar status. Although the incentive did seem to improve overall response rates, we did not have the resources required to include the incentive in the full study.
Population
The survey was distributed to a convenience sample of 2,399 residents in northeast Georgia during May 2000. This location and time period was chosen because of its proximity to the investigators and evidence from previous studies that insecticide use is greater in the southeast in the spring and summer than in other regions of the country and seasons of the year (Whitemore et al., 1994; Whitemore et al., 1992). The convenience sample included three subpopulations defined by geographic location and means of contact.
The first subpopulation consisted of 839 residences within the Urban Services District (USD) of Athens, Georgia, the core of the city, and included twelve census tracts and 40,145 residents. Five hundred residences were identified at random from a list of billing addresses for water service obtained from the local municipality and to whom questionnaires were distributed by mail. An additional 290 surveys were distributed directly to homes in neighborhoods selected on the basis of racial majority and median household income indicated by census data (ITOS, 2000). The remaining 50 surveys for the USD subpopulation consisted of the 50 distributed at the preschool facility described earlier. The second population consisted of all 1,438 employees of the Unified Government of Athens-Clarke County (ACC) to whom questionnaires were distributed via their department mailboxes. The third population consisted of 60 members of an environmental advocacy group in Rome, Georgia, to whom questionnaires were distributed in person at a meeting of the organization.
Data Analysis
First, participants were categorized as a user or non-user of each insecticide. Next, the frequency of users was tabulated for each of several demographic characteristics hypothesized to be associated with insecticide use or indicative of susceptibility to risk from insecticide exposure. Demographic factors studied at the level of individual respondents were gender, age, ethnicity, and level of education. Household-level demographics evaluated were annual income, type of residence, and use of a commercial pesticide or exterminator service. Age of respondent was categorized as 18-24 years, 25-35, 36-64, and greater than 65. Ethnicity was summarized as African-American, Caucasian, and other (including Latin American). Three levels of education were considered: less than a 4-year college degree, a 4-year college degree, and a graduate degree. Residences were described as a single family home, apartment, or other. Annual household income was categorized as less than $20,000; $20,000-$49,999; $50,000 to $74,999; and greater than $75,000. Age of the respondent and age of any children in the home were indicated on the questionnaire. This information was utilized to generate data describing the percentage of respondents with children in the home and respondents of a reproductive age (18-40 years).
To evaluate associations between use of each insecticide and the various demographic characteristics, first we cross-tabulated user/non-user status and the levels of each characteristic. Next, we used the chi-square procedure (SAS, 1998) to test for independence between use of each insecticide and each demographic characteristic. For users of each insecticide, we summarized responses to the questions on location, frequency, and rationale for application of chlorpyrifos, diazinon, or malathion. In some cases, users of the chemicals did not answer these questions and their responses were coded as "No Reply". The chi-square procedure also was used to test for significant associations between demographic characteristics and location, frequency, and rationale for application. This data analysis plan yielded 108 chi-square tests and raised the potential for Type I statistical errors resulting from a multiple comparisons (Kleinbaum et al., 1988). To minimize the potential impact of multiple comparisons on the interpretation of statistical tests, we used a significance level of 0.005, rather than the conventional 0.05, as the threshold for rejection of a null hypothesis (e.g., independence between user/non-user status and gender).
RESULTS
Of the 2,399 questionnaires distributed, 424 responses were received for an overall return rate of 17.7%. Considering the three subpopulations, 1,438 questionnaires were distributed to the ACC, 839 to USD, and 60 to NGA. The NGA population returned the highest percentage of questionnaires (82%) and accounted for 12 percent of the total number of completed surveys. The ACC population had the lowest return rate (13%), but represented 47% of total returned questionnaires. Residents from the USD returned 21% of the total number of questionnaires.
Respondents were approximately evenly split between women (49%) and men (48%), while 3% of respondents did not reply to the gender question. The education of respondents was approximately evenly distributed over the three levels; i.e., 31% had not attained a college degree and 34% held a graduate degree. Half were between the ages of 36-64 years, with the lowest percentages in the youngest (6%) and oldest (8%) age groups. Eighty percent of the returned questionnaires were completed by Caucasians, and 13% from African-Americans. Most respondents (37%) reported an annual household income of $20,000 to $49,999 dollars. The lowest income bracket was least represented (8%) according to return rates.
The number and relative frequency of users of chlorpyrifos, diazinon, and malathion for the whole population and by demographic characteristic are shown in Table 2. Approximately half of all respondents reported use of chlorpyrifos and diazinon, while 10% reported use of malathion. Nearly 33% of respondents indicated use of two of these insecticides (not shown in Table 2). The combination of chlorpyrifos and diazinon was the pair reported most frequently (31% of respondents). Eight percent of the population reported use of all three insecticides.
Demographics of Use
Many demographic factors were associated with pesticide use (Table 2). Males reported significantly greater use of chlorpyrifos (p=0.0003) and diazinon (p=0.0011) than women, but the use of malathion was independent of gender (p=0.0528). Household income was associated significantly with use of chlorpyrifos (p=0.0005) and diazinon (p=0.0026). The general trend was greater insecticide use with greater income. Malathion use exhibited this trend as well, yet the association was not significant (p=0.2485), perhaps because of the small number of users. Nominal use of chlorpyrifos and diazinon was greater in households with children, than in households without children, but the differences were not statistically significant. Ethnicity, age category, educational level, reproductive age status, type of residence, and household use of a commercial pesticide service, were not associated with reported use of any of the three insecticides.
Location, Rationale, and Frequency of Use
Chlorpyrifos, diazinon, and malathion were applied exclusively outdoors by the majority of the study population and exclusively indoors by 10-14% of the population (Table 3). The application location of chlorpyrifos (p=0.0007) and diazinon (p=0.0026) differed significantly among residence types although the small number of users in apartment residences limits confidence in this finding (Table 4). For chlorpyrifos, Caucasians reported the greatest frequency of outdoor only use and African-Americans reported the greatest frequency of both outdoor and indoor use (Table 4), but the differences (p=0.0230) were not significant at the 0.005 confidence level. The application location for chlorpyrifos, diazinon, and malathion was not significantly associated with any of the other demographic characteristics.
The majority of chlorpyrifos, diazinon, and malathion users applied the insecticide less than once a month and less than 20% reported use on a weekly or daily basis (Table 3). Application frequency of chlorpyrifos differed significantly among age groups (p=0.0029) and annual household income (p=0.0007), but frequency of diazinon and malathion application did not vary by any demographic factor (Table 5). The age group association with frequency of chlorpyrifos application is due to the relatively high application frequency reported by the 8 individuals that did not report their age. In chi-square procedures done without the "No Reply" age group there was not an association (p=0.2821) between application frequency and age. Chlorpyrifos users in households with income between $50,000 and $74,999 reported less frequent application of this insecticide than chlorpyrifos users in other income levels, while households with annual income <$20,000 reported the most frequent application. The small number of users in the lowest income group limits our confidence in this finding. Application frequency of chlorpyrifos, diazinon, and malathion was not associated with any of the other demographic characteristics.
The majority of insecticide users apply chlorpyrifos, diazinon, and malathion only when pests are seen (Table 3). Fewer than 12% of users of each insecticide apply the chemical only as part of a prevention effort, i.e., when pests are not seen. Nearly 30% of malathion users apply the chemical both in response to viewing pests and in a preventive manner. The rationale for applying diazinon was significantly (p=0.0006) associated with ethnicity (Table 5). Caucasians mostly frequently reported diazinon use when pests are seen, while African-Americans reported diazinon use with approximately equal frequency both when seeing pests and for prevention. The rationale for application of chlorpyrifos, diazinon, and malathion was not associated with any of the other demographic characteristics.
DISCUSSION
Exposure to organophosphate insecticides and members of other chemical classes of pesticides is associated with a range of neurological effects in animals and has been implicated as a potential contributor to some neurological conditions in humans (Rice and Barone, 2000; Ritz and Yu, 2000). There is a need for improved understanding of risks of neurological disease posed by pesticide exposure to vulnerable populations because of their unique exposure potential (e.g., children) and their differential susceptibility (e.g., children and older adults) (Adams et al., 2000; Hubal et al., 2000; Weiss, 2000). Efforts are under way to improve information on human exposure to neurotoxic pesticides and the potential for consequent frank and subtle effect (ILSI, 1998; MacIntosh et al., 1999a; MacIntosh et al., 1999b; Shurdut et al., 1998; Wilkinson et al., 2000; V.G. Zartarian et al., 2000). Methods for characterizing potential exposure to residential use insecticides such as the questionnaire described here, may be useful for assessing risks of neurological outcomes in large populations.
Epidemiological studies can be used to assess the health risks of exposure to organophosphate insecticides. Simple, reliable, and low cost methods are desirable for assessing personal exposure to pesticides in epidemiological investigations. The questionnaire described in this paper is appealing because it accounts for user/non-user status as well as the location and frequency of insecticide application within the residential environment. However, additional information on the ability of the questionnaire to discriminate among levels of actual insecticide exposure is needed before it is recommended for use in an epidemiological investigation. A household inventory of pesticide products and measurements of chlorpyrifos, diazinon, and malathion in residences (e.g., indoor air or settled dust) could be standards to which questionnaire responses are compared. We have begun a study with this basic design to evaluate the discriminatory power of the questionnaire. If demonstrated to be sufficiently discriminating, analysts could assess non-dietary residential exposure to chlorpyrifos, diazinon, and malathion for members of a selected population using a protocol that required only limited collection of new data from the study participants; i.e., completion of the questionnaire.
The results of quantitative risk analysis serve as inputs to regulatory decisions made about registration of pesticides regulated under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) and the FQPA (Oliver et al., 2000). Population-based exposure and risk models as well as scenario-based models are components of these risk analyses (EPA, 1997; EPA, 2000). Both model types require information on the number or proportion of pesticide users and characteristics of use, such as the location and frequency of application, preferably for specific active ingredients. In this paper, we present residential use data for 3 organophosphate insecticides that are not currently available in the peer-reviewed literature. Data for the Southeast United States are of particular interest because of existing evidence for elevated pesticide exposure in this region (Whitemore et al., 1994).
Some aspects of this pilot study, however, limit the utility of this information for use in quantitative risk assessment. We surveyed a convenience sample of individuals rather than a stratified random probability sample. Thus, there is potential for self-selection bias and over reporting or under reporting by selected demographic groups. For example, data from non-Caucasian individuals, low-income households, and residents of multi-family residential units (e.g., apartments) in the Athens, Georgia area are under represented in this data set. As described earlier, the convenience sample of individuals was composed of three distinct sub-populations. It is possible that observed associations between insecticide use and demographic characteristics could represent socioeconomic or lifestyle differences (i.e., multi-demographic attributes) among the three sub-populations instead of single-demographic determinants of user status and use practices. For example, the majority of respondents in the ACC sub-population were male (59%) while females constituted the majority (57%-61%) in the other two sub-populations. This difference could in part explain the gender difference observed in use of chlorpyrifos and diazinon, although it is unlikely to be a major influence as males reported more use of these insecticides than females in all three sub-populations. The survey was conducted in the spring season for this region of the United States and questionnaire responses may not well represent the percentage of users and use practices in other parts of a year. The majority of respondents reported applying products containing chlorpyrifos, diazinon, or malathion with frequency less than one time per month. The questionnaire did not allow respondents to specify application frequencies with intervals greater than one month, thus limiting use of these data for longitudinal models of exposure and risk.
Nevertheless, this study contains data on the proportion of users and use patterns of retail products in the Southeast United States that contain three high-volume organophosphate active ingredients heretofore not reported. Participants in this study reported more use of chlorpyrifos and diazinon than individuals elsewhere. Adgate et al. (2000), in a study of Minnesota residents, found that 17% used chlorpyrifos and 11% used diazinon in the last year. In contrast, approximately half of the respondents in the present study reported using products with chlorpyrifos or diazinon as the active ingredient. This discrepancy is most likely due to differences in the intensity of pest pressures in Minnesota and Georgia. Also in the present study, users of chlorpyrifos and diazinon were more frequently male and 25 years old than otherwise, while users of malathion were more frequently female. Use of chlorpyrifos and diazinon was significantly positively associated with annual household income. The proportion of households that reported use of chlorpyrifos, diazinon, and malathion did not significantly differ between residences with and without children. Use patterns were consistent among the three active ingredients: used mostly outdoors; frequency of use was generally less than once a month; and the application was intended to treat observed pests. In some cases, there were significant interactions between use patterns and selected demographic characteristics. We caution against strict reliance on the interaction findings because of limitations associated with the convenience sample of individuals noted earlier.
In conclusion, this investigation provides preliminary information on the proportion and characteristics of individuals and households with potential for exposure to chlorpyrifos, diazinon, and malathion from residential insecticide application; and guidance on the design of future work intended to develop simple, reliable, and inexpensive means of assessing non-dietary residential exposure to pesticides.
LITERATURE CITED
Adams, J., Barone, S., Jr., LaMantia, A., Philen, R., Rice, D.C., Spear, L. and Susser, E. (2000). Workshop to identify critical windows of exposure for children's health: neurobehavioral work group summary. Environmental Health Perspectives 108 Supplement 3, 535-44.
Adgate, J.L., Kukowski, A., Stroebel, C., Shubat, P.J., Morrell, S., Quackenboss, J.J., Whitmore, R.W. and Sexton, K. (2000). Pesticide storage and use patterns in Minnesota households with children. Journal of Exposure Analysis and Environmental Epidemiology 10, 159-67.
Armstrong, B., White, E. and Saracci, R. (1992). Principals of Exposure Measurement in Epidemiology. Oxford University Press, Oxford.
Aspelin, A.L. and Grube, A.H. (1999). Pesticide industry sales and usage: 1996 and 1997 market estimates, 733-R-99-001. (November 1999). Office of Prevention, Pesticides and Toxic Substances, U.S. Environmental Protection Agency, Washington, DC.
Byrne, S.L., Shurdut, B.A. and Saunders, D.G. (1998). Potential chlorpyrifos exposure to residents following standard crack and crevice treatment [see comments]. Environmental Health Perspectives 106, 725-31.
EPA. (1997). Draft standard operating procedures (SOPs) for residential exposure. http://www.epa.gov/iris.html (December 19, 1997). Office of Pesticide Programs, U.S. Environmental Protection Agency, Washington, DC.
EPA. (2000). FIFRA Scientific Advisory Panel (SAP), vol. 2000. http://www.epa.gov/scipoly/sap/index.htm (November 7, 2000). Office of Science Coordination and Policy, U.S. Environmental Protection Agency.
Goldman, L.R. (1998). Linking research and policy to ensure children's environmental health. Environmental Health Perspectives 106 Supplement 3, 857-62.
Hubal, E.A.C., Sheldon, L.S., Burke, J.M., McCurdy, T.R., Berry, M.R., Rigas, M.L., Zartarian, V.G. and Freeman, N.C.G. (2000). Children's exposure assessment: a review of factors influencing children's exposure and the data available to characterize that exposure. Environmental Health Perspectives 108, 475-486.
ILSI. (1998). Aggregate exposure assessment. . International Life Sciences Institute, Risk Science Institute, Washington, DC.
ITOS. (2000). Georgia 2000 Information System, vol. 2000. http://ga2000.itos.uga.edu/. Information Technology Outreach Services, University of Georgia.
Kleinbaum, D., Kupper, L. and Muller, K. (1988). Applied regression analysis and other multivariable methods. PWS-Kent, Boston.
Landrigan, P.J., Suk, W.A. and Amler, R.W. (1999). Chemical wastes, children's health, and the Superfund Basic Research Program. Environmental Health Perspectives 107, 423-7.
MacIntosh, D.L., Hammerstrom, K. and Ryan, P.B. (1999a). Longitudinal exposure to selected pesticides in drinking water. Human and Ecological Risk Assessment 5, 575-588.
MacIntosh, D.L., Needham, L.L., Hammerstrom, K.A. and Ryan, P.B. (1999b). A longitudinal investigation of selected pesticide metabolites in urine. Journal of Exposure Analysis and Environmental Epidemiology 9, 494-501.
Oliver, G.R., Bolles, H.G. and Shurdut, B.A. (2000). Chlorpyrifos: probabilistic assessment of exposure and risk. Neurotoxicology 21, 203-8.
Rice, D. and Barone, S., Jr. (2000). Critical periods of vulnerability for the developing nervous system: evidence from humans and animal models. Environmental Health Perspectives 108 Supplement 3, 511-33.
Ritz, B. and Yu, F. (2000). Parkinson's disease mortality and pesticide exposure in California 1984-1994. International Journal of Epidemiology 29, 323-9.
SAS. (1998). Statistical Analysis Software. . SAS Institute, Cary, NC.
Shurdut, B.A., Barraj, L. and Francis, M. (1998). Aggregate exposures under the Food Quality Protection Act: An approach using chlorpyrifos. Regulatory Toxicology and Pharmacology 28, 165-77.
Weiss, B. (2000). Vulnerability to pesticide neurotoxicity is a lifetime issue. Neurotoxicology 21, 67-73.
Whitemore, R.W., Immerman, F.W., Camann, D.E., Bond, A.E., Lewis, R.G. and Schaum, J.L. (1994). Non-occupational exposures to pesticides for residents of two U.S. cities. Archives of Environmental Contamination and Toxicology 26, 47-59.
Whitemore, R.W., Kelly, J.E. and Reading, P.L. (1992). National Home and Garden Pesticide Use Survey. Research Triangle Institute, Research Triangle Park.
Wilkinson, C.F., Christoph, G.R., Julien, E., Kelley, J.M., Kronenberg, J., McCarthy, J. and Reiss, R. (2000). Assessing the risks of exposures to multiple chemicals with a common mechanism of toxicity: how to cumulate? Regulatory Toxicology and Pharmacology 31, 30-43.
Zartarian, V.G., Ozkaynak, H., Burke, J.M., Zufall, M.J., Rigas, M.L. and Furtaw, E.J., Jr. (2000). A modeling framework for estimating children's residential exposure and dose to chlorpyrifos via dermal residue contact and nondietary ingestion. Environmental Health Perspectves 108, 505-514.
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