EWG: A Novel Method for Assessing Combined Risk of Multiple Tap Water Pollutants

Array of Contaminants in California Water Could Cause 15,000 Cases of Cancer

WASHINGTON – The array of toxic pollutants in California drinking water could cause more than 15,000 cases of cancer, according to a peer-reviewed EWG study that is the first ever to assess the cumulative risk from all contaminants in the state’s public water systems.

In a paper published today in the journal Environmental Health, EWG scientists used a novel analytical method that calculated the combined health impacts of carcinogens and other toxic contaminants in 2,737 community water systems in California.

“This cumulative approach is common in assessing the health impacts of exposure to air pollutants but has never before been applied to drinking water contaminants,” said Tasha Stoiber, Ph.D., a senior scientist at EWG and the lead author. “Right now, policymakers set health limits one chemical at a time. This doesn’t match reality. Multiple contaminants are often detected in drinking water across the U.S.”

This lifetime cumulative cancer risk estimate for California should be considered conservative because mixtures of contaminants may be even more toxic than the sum of individual chemicals.

“This could and should be a big deal,” said Olga Naidenko, Ph.D., EWG’s senior science advisor. “We need to prioritize the treatment of our tap water. This novel approach to risk assessment offers a significant improvement over the current model and, if adopted, will be a huge step toward improving public health. It will help communities and policymakers evaluate the best options to treat drinking water.”

Water systems in California with the highest risk serve smaller communities with fewer than 1,000 people. In these communities, exposure to arsenic is the biggest factor in increased cancer risk. These communities are in need of improved infrastructure and resources to provide safe drinking water to their residents.

This assessment is based on water quality reports published by the California State Water Resources Control Board and data published by the EPA’s Unregulated Contaminant Monitoring Rule.


The Environmental Working Group is a nonprofit, non-partisan organization that empowers people to live healthier lives in a healthier environment. Through research, advocacy and unique education tools, EWG drives consumer choice and civic action.

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