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Southern Nevada Water Authority

Trihalomethanes (THMs)

Trihalomethanes (THMs) are disinfection byproducts created when chlorine is used to treat water containing natural organic matter.

Although some studies have indicated an association between elevated levels of THMs and adverse health effects among pregnant women, no causal relationship has been established. The Southern Nevada Water Authority (SNWA) encourages the Environmental Protection Agency (EPA) to continue health effects research so it can provide water agencies the information necessary to minimize health risks associated with THMs.

The EPA has set the limit of total THMs in treated water at 0.080 milligrams per liter. Southern Nevada's municipal water supply meets that health-based standard.

Several factors affect THM formation, including the amount of chlorine used during treatment, the concentration of total organic carbons in the source water, temperature, pH and retention time in the distribution system. The Water Authority proactively takes measures to manage the formation of THMs during the treatment process.

While science has not established a causal relationship between THMs and adverse health effects associated with pregnancy, SNWA advises Southern Nevada residents—particularly pregnant women—to call the EPA's Safe Drinking Water Hotline at (800) 426-4791 about any concerns related to THMs.

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