Statistical techniques applied to environmental health sciences

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We have recently guided the data analysis for several projects, primarily involving data from the Agricultural Health Study (AHS), a cohort of licensed pesticide applicators (mostly farmers) from Iowa and North Carolina. In the first project, we sought to assess whether injury mortality was associated with pesticide-use practices among male farmer applicators in the AHS. From 1993 to 2008, we observed 338 injury fatalities over 727,543 person-years of follow-up. We used Cox proportional hazards modeling to study the association of fatal injury with use of specific pesticides, personal protective equipment, specific types of farm equipment, and other factors. Exposure information was collected at enrollment, from 1 to 15 years preceding death. Fatal injury increased with the number of days per year that a farmer applied pesticides and decreased with use of chemical-resistant gloves. Certain herbicides were associated with fatal injury. We regard these associations as deserving of further study, especially with better data on the temporal relationship of exposure and death, and we emphasize that these results must be interpreted with caution. Organophosphorous insecticides (OPs) are among the most commonly used insecticides in the US. In the second project, we characterized the use of OPs among farmer pesticide applicators in the AHS. For this effort, we assessed lifetime use of OPs among 701 farmer applicators who were recruited for a neurological testing study with augmented informaton about OP use. This sample was randomly selected in complex sampling design that oversampled highly exposed individuals (based on the usual AHS data) and restricted to individuals residing within 150 miles of four clinical sites. We used empirical sampling weights so that our estimates reflect the OP experience of all 3863 male applicators eligible for the neurological study. Malathion and chlorpyriphos were the most commonly used OPs. OP use declined over the 15 year course of the AHS. Another project is probing a possible relationship between pesticide exposure and depression among male private applicators in the AHS. We used inverse probability weighting to account for exclusions due to missing data and polytomous logistic regression to estimate odds ratios. Ever use of fumigants as a class and several individual pesticides were associated with depression. Using data from the AHS, we have also studied whether farming exposures contribute to exacerbation of asthma. We noted inverse association of asthma with several farming activities and with ever use of two herbicides but no positive associations (except in a subset of cases with asthma and allergy). These observations are consistent with the possibility that asthma cases prone to exacerbation tend to avoid exposures that trigger symptoms. A last project using data from the AHS examined the possible relationship between diabetes and ever use of specific pesticides before enrollment. Using age as the time scale and diagnosis with diabetes as the event of interest, we fit Cox proportional hazards models to estimate hazard ratios. Ever use of five pesticides (three of them organophosphates) were associated with diabetes among farmers wives who reported ever mixing or applying pesticides.