Association between gastric cancer mortality and nitrate content of drinking water: ecological study on small area inequalities.

The carcinogenic feature of N-nitroso compounds has been well established. Similarly, the transformation of ingested nitrate to N-nitroso compounds in the stomach has been thoroughly documented, nevertheless nitrates' carcinogenic effect has not been proved convincingly in human. The present study was aimed to investigate a population of small villages provided by drinking water with high and widely variable nitrate content (72 mg/l median, 290.7 mg/l 95-percentile concentration). Empirical Bayes estimates for settlement-specific age-, sex-, and year-standardised mortality ratios of gastric cancer (GC) were related to the settlement level average nitrate concentrations in drinking water controlling for confounding effects of smoking, ethnicity and education. The log-transformed average nitrate concentration showed significant positive association with stomach cancer mortality in linear regression analysis (p = 0.014). The settlements were aggregated according to the nitrate concentration into 10-percentile groups and the standardised mortality ratios (SMRs) were calculated. Those groups with higher than 88 mg/l average nitrate concentration showed substantial risk elevation and the log-transformed exposure variables proved to be significant predictors of mortality (p = 0.032) at this level of aggregation also. The association seemed to be fairly strong (r2 = 0.46). Although this investigation constituting an ecological study has certain limitations, it supports the hypothesis that the high level of nitrate in drinking water is involved in the development of GC.