Objectives: Accurate assessment of exposure is a key factor in occupational epidemiology but it can be problematic, particularly in retrospective studies where the exposures of interest may be many decades removed from the relevant health outcomes. Many studies have traditionally relied on crude surrogates of exposure based on job title only. In particular, studies of agricultural pesticide exposure and cancer have often relied on the assumption that farm-related job title is an adequate surrogate for pesticide exposure.
Methods: This analysis was based on data collected for a large community-based case-control study of prostate cancer in Western Australia undertaken in 2000-2001. Using a multivariate regression model, we compared expert-assessed likelihood of pesticide exposure based on detailed, individual-specific questionnaire and Job Specific Module interview information with reported farm-related job titles as a surrogate for pesticide exposure.
Results: The majority (68.8%) of jobs with likely pesticide exposure were farm jobs, however 78.3% of farm jobs were assessed as having no likelihood of pesticide exposure. Likely pesticide exposure was more frequent among jobs on farms where crops were raised than jobs on animal farms. Likely pesticide exposure was also more frequent among jobs commenced in more recent decades and jobs of longer duration. Our results suggest that very little misclassification would have resulted from the inverse assumption that all non-farming jobs are not pesticide exposed since only a very small fraction of non-agricultural jobs had likely pesticide exposure.
Conclusions: Classification of all farm jobs as pesticide exposed is likely to substantially over-estimate the number of individuals exposed. Our results also suggest that researchers should pay special attention to farm type, length of service and historical period of employment when assessing the likelihood of pesticide exposure in farming jobs.
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