Ryan W. Miller | USA TODAY

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Drinking California tap water could increase the number of cancer cases statewide by more than 15,000 over the course of a lifetime, a new study says.

Looking at how drinking water pollutants interact to affect human health, rather than looking at each one separately, researchers analyzed data in more than 2,700 California community water systems from 2011 to 2015.

The estimated 15,449 added cancer cases over the course of 70 years means 221 annual cancer cases from drinking-water pollution, researchers said in an article published Tuesday in the peer-reviewed journal Environmental Health.

The researchers were a team of scientists from the Environmental Working Group, a nonprofit advocacy group.

"Drinking water rarely contains only one contaminant, yet regulators currently assess the health hazards of tap water pollutants one by one. This ignores the combined effects of multiple pollutants, which is how people ingest them in the real world," the group said in a statement.

The group looked at levels of carcinogens and toxic contaminants in the drinking water and found small to midsize communities faced some of the greatest cancer risks.

In 495 water systems affecting more than 3 million people, there was a cancer risk greater than 1 additional case per 1,000 people. In 1,177 systems, affecting more than 28 million people, there was a risk of 1 per 1,000 to 1 per 10,000 people.

The group reported that arsenic contributed to an additional 7,251 estimated cancer cases. Other potentially harmful chemicals included various disinfectant byproducts and hexavalent chromium, also known as chromium-6 or the "Erin Brockovich" chemical.

Kelly Reynolds, director of the Environment, Exposure Science and Risk Assessment Center at the University of Arizona, who was not involved with the study, told CNN "there's no safe level" of arsenic in drinking water.

The researchers said pollutants below legal limits contributed to more than 85% of the estimated cancer risk, highlighting the need for lower limits for these pollutants.

More research is needed in understanding how pollutants' interactions affect humans, Tasha Stoiber, one of the study's authors, told CNN. Because little is known about these interactions, the study could overestimate or underestimate the risks.

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