An Exposure-Dependent Reduction of Lung Cancer Risk in Dairy Farmers: A Nested Case-Referent Study

Abstract

Consistent with findings in other similarly exposed populations, lung cancer mortality was significantly reduced in 1,561 dairy farmers but not in 722 crop/ orchard farmers included in a cohort of Italian small farmers. Unlike previous investigations of this phenomenon, this study controlled for smoking. From the above cohort of 2,283 male farmers, 38 lung cancer cases and 353 referents were selected. The sources (and information collected) were clinical records (diagnosis, smoking habits), a mail questionnaire (smoking habits), and the historical files of self-employed small farmers in the Province of Padova (length of farm work, land area of the farm and number of dairy cattle). The risk of lung cancer was estimated by the odds ratio (OR) with the 95% confidence interval (CI) calculated using logistic regression models. In dairy farmers, the lung cancer OR decreased by 19% per head of cattle (OR = 0.81; CI = 0.68–0.97), when the influence of age and smoking habits was controlled. No relationship was found between lung cancer risk and size of farm or length of work, either in dairy farmers or in crop/orchard farmers. The increasing number of dairy cows, which probably increases the daily hours of exposure in cowsheds, is an indirect indicator of the suggested true protection factor: airborne biologic agents (delivered from bacteria proliferating on organic dusts), particularly endotoxin, which has been shown to be a potent stimulator of endogenous antineoplastic mediators.