Objectives: This small-area ecologic study in Yorkshire, northern England, examines the hypothesis that exposure to higher levels of nitrate in drinking water increases the risk of stomach, esophageal, or brain cancer in adults.
Methods: Nitrate levels over the period 1990-95 and numbers of incident cancers from 1975-94 were available for 148 water supply zones, geographically defined areas each supplying water of homogeneous chemical composition to an average population of around 20,000.
Results: No relationship was found between nitrate concentrations and the incidence of stomach or esophageal cancers. The incidence of cancer of the brain and central nervous system was found to be higher in areas with higher nitrate levels, with a relative risk of 1.18 (95 percent confidence interval = 1.08-1.30) in the quartile of the population with the highest average levels (mean 29.8 mg/l) compared with the lowest quartile (mean 2.4 mg/l). The increase in risk remained statistically significant (P < 0.01) after allowing for other covariates and for extra-Poisson variation in a regression model.
Conclusions: This study does not support the hypothesis of an increased risk of stomach or esophageal cancer associated with higher nitrate levels in drinking water. The observed relationship with brain cancer requires confirmation in other studies, including those involving data on individuals.