Historical Pesticide Exposure in California Using Pesticide Use Reports and Land-Use Surveys: An Assessment of Misclassification Error and Bias

Article excerpt

We used California's Pesticide Use Report (PUR) and land-use survey data to conduct a simulation study evaluating the potential consequences of misclassifying residential exposure from proximity to agricultural pesticide application in health effect studies. We developed a geographic model linking the PUR with crop location data from land-use surveys to assess the impact of exposure misclassification from simpler exposure models based solely on PUR or land-use data. We simulated the random selection of population controls recruited into a hypothetical case-control study within an agricultural region. Using residential parcel data, we derived annual exposure prevalences, sensitivity, and specificity for five pesticides and relied on the PUR plus land-use model as the "gold standard." Based on these estimates, we calculated the attenuation of prespecified true odds ratios (ORs), assuming nondifferential exposure misclassification. True ORs were severely attenuated a) when residential exposure status was based on a larger geographic area yielding higher sensitivity but low specificity for exposure, in contrast to relying on a smaller area and increasing specificity; b) for less frequently applied pesticides; and c) with increasing mobility of residents among the study population. Considerable effect estimate attenuation also occurred when we used residential distance to crops as a proxy for pesticide exposure. Finally, exposure classifications based on annual instead of seasonal summaries of PUR resulted in highly attenuated ORs, especially during seasons when applications of specific pesticides were unlikely to occur. These results underscore the importance of increasing the spatiotemporal resolution of pesticide exposure models to minimize misclassification. Key words: agriculture, bias, California, epidemiology, exposure assessment, geographic information systems, land use, misclassification, pesticide use, residential exposure. Environ Health Perspect 111:1582-1589 (2003). doi: 10.1289/ehp.6118 available via http://dx.doi.org/[Online 20 May 2003]


Agricultural pesticides are the largest group of poisonous substances intentionally disseminated throughout the environment for the purpose of combating animal pests and diseases that devastate crops. Many are known to be acutely toxic to nontargeted organisms, including humans (Ecobichon and Joy 1994). Most epidemiologic studies investigating acute or chronic health effects from human pesticide exposure have been conducted in heavily exposed occupational groups such as pesticide applicators or manufacturers (Zahm et al. 1997). The number of workers occupationally exposed to a specific pesticide formulation, however, is often relatively small or not representative of certain susceptible populations (e.g., pregnant women), thus hampering investigations of less common chronic diseases suspected to be caused by some pesticides, including specific cancers (Zahm et al. 1997), Parkinson's disease (Engel et al. 2001), and birth defects (Shaw et al. 1999). Furthermore, it may not be appropriate to use results from studies of acute exposures to predict chronic health effects, especially when low-level and long-term exposures are more widespread in the general population.

Residential proximity to agricultural pesticide applications may be an important source of ambient environmental exposure in rural communities throughout the United States. Pesticides applied from the air or ground have been observed to drift from their intended treatment sites, with measurable concentrations detected in the air and in plants and animals several hundred meters away (Chester and Ward 1984; Currier et al. 1982; Frost and Ware 1970; MacCollom et al. 1986; Woods et al. 2001). Herbicides transported downwind can cause unintended damage to crops (Byass and Lake 1977), and acute pesticide poisonings have been observed in communities downwind from agricultural fields after applications (Ames et al. …