Nitrate in U.S. Tap Water May Cause More Than 12,500 Cancers a Year: Methodology
Full methodology and sources are available in the EWG study published in the journal Environmental Research.
Estimated range of nitrate-attributable cancer cases
The estimated range of nitrate-attributable cancer cases was calculated using the following equation, based on a methodology published in 2010 by scientists from the Netherlands:
Nitrate attributable cancer cases = population exposed X increased risk in the exposed population X baseline national incidence
The nitrate concentration cutoffs and increased risks in the exposed population were reported in different epidemiological studies demonstrating increased cancer risk associated with nitrate in drinking water. We used five scenarios described in three papers for colorectal cancer, two scenarios from one paper for ovarian cancer, two scenarios from one paper for thyroid cancer, and one scenario from one paper each for kidney and bladder cancer.
The scenarios that yielded the highest number of cancer cases within each cancer type were summed to determine the maximum total number of cancer cases. Similarly, the minimum number of cancer cases is the sum of the estimated cancer cases from each scenario per cancer type that yielded the lowest number of cancer cases.
Nitrate concentrations in community water systems were collected from 2010 to 2017 and then averaged for each system. The population exposed was based on the number of people served by community water systems with average nitrate concentrations above the nitrate concentration cutoffs for a given scenario. To estimate the number of people exposed to nitrate through private wells, we used nitrate concentrations in non-community water systems and the smallest community water systems as a proxy for nitrate levels in private wells. The percentage of the state population on private wells was estimated using U.S. Geological Survey data.
The national cancer incidence values were obtained from the Centers for Disease Control and Prevention. Baseline incidence was calculated using the following equation:
Baseline incidence = total cancer cases / ((exposed population X risk in exposed population) + (unexposed population X risk in unexposed population))
In this analysis, the relative cancer risk in the population that does not have nitrate in drinking water, defined as “unexposed population,” is equivalent to 1.
Estimated nitrate-attributable state cancer risk
The frequency of state-specific, nitrate-attributable cancer cases was determined by dividing the maximum number of estimated nitrate-attributable cancer cases in each state by the total state population. This frequency is represented as the number of estimated nitrate-attributable cancer cases per 100,000 people.
Population-weighted average nitrate concentrations
This study relies on a comprehensive contaminant occurrence dataset for community water systems in the U.S. presented in the EWG’s national Tap Water Database. Methodology for the entire database is available here. The present study includes data from 2010 to 2017. For the populated-weighted state averages, average nitrate concentrations were calculated for each community water system in each state. These water-system-level average nitrate concentrations were weighted based on the population served by each community water system. We used those concentrations to calculate the overall state population-weighted nitrate average.