Background
N-nitroso compounds formed endogenously after nitrate/nitrite ingestion are animal renal carcinogens. Previous epidemiologic studies of drinking water nitrate did not evaluate other potentially toxic water contaminants, including the suspected renal carcinogen chloroform.

Methods
In a cohort of postmenopausal women in Iowa (1986–2010), we used historical measurements to estimate long-term average concentrations of nitrate-nitrogen (NO₃−N) and disinfection by-products (DBP) in public water supplies. For NO₃–N and the regulated DBP (total trihalomethanes [THM] and the sum of five haloacetic acids [HAA5]), we estimated the number of years of exposure above one-half the current maximum contaminant level (>½-MCL NO₃–N; >5 mg/L). Dietary intakes were assessed via food frequency questionnaire. We estimated hazard ratios (HRs) and 95% confidence intervals (CIs) with Cox models, and evaluated interactions with factors influencing N-nitroso compound formation.

Results
We identified 125 incident kidney cancers among 15,577 women reporting using water from public supplies >10 years. In multivariable models, risk was higher in the 95th percentile of average NO₃–N (HR_p95vsQ1 = 2.3; CI: 1.2, 4.3; P _trend = 0.33) and for any years of exposure >½-MCL; adjustment for total THM did not materially change these associations. There were no independent relationships with total THM, individual THMs chloroform and bromodichloromethane, or with haloacetic acids. Dietary analyses yielded associations with high nitrite intake from processed meats but not nitrate or nitrite overall. We found no interactions.

Conclusions
Relatively high nitrate levels in public water supplies were associated with increased risk of renal cancer. Our results also suggest that nitrite from processed meat is a renal cancer risk factor.