Farming, pesticide exposure and breast cancer in North Carolina

We investigated the role of farming and pesticide exposure among women in the Carolina Breast Cancer Study (CBCS), a population-based, case-control study of breast cancer in North Carolina. Between May 1993 and May 1996, the CBCS enrolled 862 cases and 790 controls, of whom approximately 50% were African-American and 50% were under age 50. A supplemental questionnaire was developed to obtain complete histories of farm residence and farm work, and administered by telephone. The reliability of farming-related exposure data was assessed by repeating an abbreviated interview among 10% of participants. For 12 categorical items, proportions in exact agreement ranged from 0.62 to 0.97. Kappa coefficients ranged from 0.21 to 0.84. Intraclass correlation coefficients for continuous variables ranged from 0.51 to 0.58. In general, cases gave less liable information than controls. Reliability also varied by age, education, and time between interviews. Investigation of the association between farming duration and breast cancer revealed decreasing odds ratios (ORs) with increasing duration of living or working farms (relative to non-farmers, adjusted ORs = 1.18, 0.79, 0.73, 0.57, P for trend = 0.007, for 1--10, 11--17, 18--23, and >23 years). ORs for duration of farming and pesticide exposure did not increase when exposure was assessed within calendar time periods of DDT use (1945--1973, 1950--1966), or at ages 9--16. Women who washed laundry for pesticide users for eleven years or more showed positive associations, even after adjusting for duration of farming and breast cancer risk factors, OR = 2.01 [95% confidence interval (CI) 1.19--3.40] for 11--20 years and OR = 1.69 (95% CI 0.99--2.88) for over 20 years. Among women who lived or worked on farms in North Carolina (327 cases, 362 controls), a positive association was observed for presence in fields during or shortly after pesticide application, OR = 1.76 (95% CI 1.12--2.77). We conclude that residence or work on farms does not increase the risk of breast cancer, and may be associated with reduced risk, relative to women who have never farmed. However, our results suggest positive but weak associations among a small subgroup of women with potential high exposure to pesticides.