Glyphosate Use and Cancer Incidence in the Agricultural Health Study: An Epidemiologic Perspective

In this issue of the Journal, Gabriela Adreiotti and colleagues report the results of an updated analysis of glyphosate exposure and cancer risk in the Agricultural Health Study (AHS) (1). The AHS, a prospective cohort study of 57 310 licensed pesticide applicators and 32 347 spouses in Iowa and North Carolina, was initiated in the early 1990s, in large part to address possible causes for the higher incidence of lymphohematopoietic and certain other cancers in farmers compared with the general population (2). From an epidemiologic perspective, designing a study to investigate cancer risk factors associated with farming is challenging because of the difficulty in establishing and recruiting a well-defined population, the large number of farming-related exposures that may be associated with cancer, and the need to create and validate quantitative exposure metrics. The AHS study met these challenges by defining and recruiting the study population from applicants for a restricted-use pesticide license in two states, collecting detailed information on the frequency and duration of use of 50 common pesticides, as well as other farming-related and general population exposures, and estimating lifetime days and intensity-weighted lifetime days of exposure for specific pesticides (2). Intensity of exposure was estimated using an algorithm that accounted for reported use of personal protective equipment, method of application, and whether the applicator personally mixed the pesticides (3). Additional studies were done to validate and refine the pesticide exposure intensity metrics, and exposure information was updated in 1999–2005 by computer-aided telephone interviewing (3,4). A method for multiple imputation was developed to assign pesticide use for nonresponders to the follow-up questionnaire (5).

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