Associations between stomach cancer incidence and drinking water contamination with atrazine and nitrate in Ontario (Canada) agroecosystems, 1987-1991

Background Nitrate and atrazine are two chemicals that are heavily used in certain sectors of agriculture. They are suspected to be associated with the development of certain types of tumours. Methods Existing data were obtained on the incidence of specific types of cancers, contamination of drinking water with atrazine and nitrate, and related agricultural practices for the 40 ecodistricts in the province of Ontario. The data were merged into a georelational database for geographical and statistical analyses. Weighted (by population size) least squares regression analyses were conducted while controlling for confounding socioeconomic and lifestyle factors. Maximum likelihood spatial error models were estimated when least square regression error terms were found to be spatially autocorrelated using the Moran's I statistic. Results Atrazine contamination levels (range 50-649 ng/l, maximum acceptable concentration [MAC] = 60 000 ng/l) were positively associated (P < 0.05) with stomach cancer incidence and negatively associated with colon cancer incidence. Nitrate levels, (range 0-91 mg/l, MAC = 10 mg/l) were negatively associated with stomach cancer incidence. Conclusion The associations found at the ecodistrict level, both positive and negative, if confirmed by Ether studies, raise serious questions about maximum allowable limits for atrazine, as well as possibilities of complex trade-offs among disease outcomes, and interactions of biophysical and social mechanisms which might explain them. Although the negative associations appear to have no direct biological explanations, such counter-intuitive outcomes may occur in complex systems where social and biological variables interact.