Contaminants in California tap water could result in over 15,000 additional cancer cases, study says

A new study finds that drinking tap water in California over the course of a lifetime could increase the of cancer.

Researchers from the environmental advocacy group Environmental Working Group estimated that the contaminants found in public water systems in California could contribute to about 15,500 cancer cases there over the course of a lifetime. These contaminants include chemicals such as arsenic, hexavalent chromium and radioactive elements such as uranium and radium. The study was published Tuesday in the journal Environmental Health.

"We need to look at contaminants as a group -- not just one at a time. It's more important to analyze co-occurring contaminants to understand the real world exposure," said lead author Tasha Stoiber, a senior scientist with EWG. She explained that cancer risks are typically determined at the individual element level.

Stoiber and her colleagues evaluated 2,737 different public water systems across the state of California by assessing the level of reported contaminants in the systems. These water systems are the main providers of drinking water in California, serving 98% of the state's population. They are regulated by the federal Safe Drinking Water Act and overseen by state regulators.

Water systems test for a host of both regulated and unregulated contaminants to maintain a safe drinking water supply. The study's authors calculated cancer risk by looking at the reported contaminant levels from 2011 to 2015. assessing the yearly averages of all the reported contaminants and adding them together to determine cumulative risk.

Nearly 500 high-risk water systems

The authors divided the water systems into four different categories of risk. The highest category estimated an additional cancer risk of more than 1 in 1,000 people being diagnosed with cancer over the course of a lifetime from drinking tap water. Researchers did not identify which water systems were considered high risk.

Regardless of the role of drinking water, according to the American Cancer Society, nearly40% of all Americans will be diagnosed with cancer during their lifetime.

Nearly 500 of the community water systems evaluated fit the highest risk category. The researchers estimated that drinking water from these systems over a lifetime would result in 4,860 cancer diagnosis.

When asked about the report, the California Environmental Protection Agency said it would not comment on the study at this time.

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