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According to analysis conducted by the Environmental Working Group on U.S. water quality tests conducted in 2011, toxic chemicals called trihalomethanes were found to be contaminating virtually all of the tap water samples tested.

Samples were collected and analyzed from more than 200 of the country’s largest municipal water systems serving nearly one-third of the country’s population—more than 100 million Americans—throughout 43 states. Some of the data found that 84 percent of the water systems tested for trihalomethane levels higher than 21 parts per billion—a number two studies conducted in Taiwan found led to much higher risks of stillbirths and bladder cancer.

Trihalomethanes, according to EcoWatch, “are formed when chlorine, added to treated water as a disinfectant, reacts with rotting organic matter such as farm runoff, sewage or dead animals and vegetation. Their concentrations tend to rise when storms increase organic pollution in waters that serve as sources for tap water.” The chemicals have been linked to an increased risk of bladder cancer and miscarriages among other conditions. EcoWatch says scientists think thousands of bladder cancer cases each year can be traced to trihalomethanes in drinking water.

“Many people are likely exposed to far higher concentrations of trihalomethanes than anyone really knows,” said Renee Sharp, a senior scientist at EWG and co-author of the analysis. “For most water systems, trihalomethane contamination fluctuates from month to month, sometimes rising well beyond the legal limit set by the federal Environmental Protection Agency.”

The EWG is urging federal agencies to enforce stricter regulations to minimize the contamination risks.

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Image: Joe Shablotnik