An alarming new study has found that drinking tap water in California over a lifetime could significantly increase your risk of developing cancer, and may have already contributed to more than 15,500 cases.

The state’s drinking water is contaminated with chemicals like arsenic, hexavalent chromium and radioactive elements such as uranium and radium, according to researchers at the Environmental Working Group (EWG) who published their findings in the journal Environmental Health on Tuesday.

The team tested 2,737 different public water systems across California as part of their research, which served 98 percent of the state’s population. The researchers then calculated the cancer risk by establishing the average contaminant levels between 2011 to 2015, and adding them together to create a lifetime risk.

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These water systems are regulated by the federal Safe Drinking Water Act and overseen by state regulators – which are supposed to test for both regulated and unregulated contaminants. But as the EWG notes: “Legal doesn’t always mean safe.” The California Environmental Protection Agency told CNN they would not be commenting on the study’s findings.

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The study’s lead author, Tasha Stobier, pointed out that this research is specific to California, and a risk assessment should be carried out in other communities. She added that simple steps like adding a water filter could greatly reduce exposure to the contaminates.

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