Background The Agricultural Health Study (AHS) is a prospective study of licensed pesticide applicators and their spouses in Iowa and North Carolina. We evaluate the impact of occupational pesticide exposure misclassification on relative risks using data from the cohort and the AHS Pesticide Exposure Study (AHS/PES).
Methods We assessed the impact of exposure misclassification on relative risks using the range of correlation coefficients observed between measured post-application urinary levels of 2,4-dichlorophenoxyacetic acid (2,4-D) and a chlorpyrifos metabolite and exposure estimates based on an algorithm from 83 AHS pesticide applications.
Results Correlations between urinary levels of 2,4-D and a chlorpyrifos metabolite and algorithm estimated intensity scores were about 0.4 for 2,4-D (n=64), 0.8 for liquid chlorpyrifos (n=4) and 0.6 for granular chlorpyrifos (n=12). Correlations of urinary levels with kilograms of active ingredient used, duration of application, or number of acres treated were lower and ranged from −0.36 to 0.19. These findings indicate that a priori expert-derived algorithm scores were more closely related to measured urinary levels than individual exposure determinants evaluated here. Estimates of potential bias in relative risks based on the correlations from the AHS/PES indicate that non-differential misclassification of exposure using the algorithm would bias estimates towards the null, but less than that from individual exposure determinants.
Conclusions Although correlations between algorithm scores and urinary levels were quite good (ie, correlations between 0.4 and 0.8), exposure misclassification would still bias relative risk estimates in the AHS towards the null and diminish study power.
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The findings and conclusions in this report are those of the author(s) and do not necessarily represent the views of the National Cancer Institute, National Institute of Environmental Health Sciences, U.S. Environmental Protection Agency or National Institute for Occupational Safety and Health.
Funding This research was partially supported by the Intramural Research Program of the NIH (Division of Cancer Epidemiology and Genetics, National Cancer Institute (Z01CP010119) and the National Institute of Environmental Health Sciences (Z01-ES049030-1)). This work has been funded in part by the U.S. Environmental Protection Agency under Contracts 68-D99-011 and 68-D99-012, and through Interagency Agreement DW-75-93912801-0.
Competing interests None.
Ethics approval This study was approved by the National Institutes of Health Special Studies Institutional Review Board, protocol number OH93-NC-N013, and also by Institutional Review Boards at the University of Iowa, Westat, Inc., RTI International and Battelle, Inc.
Provenance and peer review Not commissioned; externally peer reviewed.
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