The authors examined the association between pesticide use and breast cancer incidence
among farmers' wives in a large prospective cohort study in Iowa and North Carolina.
Participants were 30,454 women with no history of breast cancer prior to cohort enrollment
in 1993-1997. Information on pesticide use and other information was obtained by self-administered
questionnaire at enrollment from the women and their husbands. Through 2000, 309 incident
breast cancer cases were identified via population-based cancer registries. Rate ratios
were calculated for individual pesticides using Poisson regression, controlling for
confounding factors. Breast cancer standardized incidence ratios were 0.87 (95% confidence
interval: 0.74, 1.02) for women who reported ever applying pesticides and 1.05 (95%
confidence interval: 0.89, 1.24) for women who reported never applying pesticides.
There was some evidence of increased risk associated with use of 2,4,5-trichloro-phenoxypropionic
acid (2,4,5-TP) and possibly use of dieldrin, captan, and 2,4,5-trichlorophenoxyacetic
acid (2,4,5-TP), but small numbers of cases among those who had personally used the
pesticides precluded firm conclusions. The authors found no clear association of breast
cancer risk with farm size or washing of clothes worn during pesticide application,
but risk was modestly elevated among women whose homes were closest to areas of pesticide
application. Further follow-up of this cohort should help clarify the relation between
pesticide exposure and breast cancer risk.