New Jersey Environmental Public Health Tracking Program

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Trihalomethanes in Drinking Water

What Are Trihalomethanes?

Trihalomethanes (THMs) form in drinking water as a by-product of chlorine disinfection, through the reaction of chlorine with naturally-occurring organic chemicals in the water. There are four regulated THMs: chloroform, bromodichloromethane, dibromochloromethane, and bromoform.  By removing organic contaminants from drinking water before chlorine disinfectant is applied, one can minimize the formation of THMs.

People can be exposed to THMs by drinking tap water.  Since THMs are volatile, people can also be exposed by breathing in THMs during showering, bathing, or washing dishes.  THMs may also be absorbed through the skin to some degree.  THMs are absorbed into the bloodstream and are carried throughout the body. 

Surface water (from rivers, lakes and reservoirs) is likely to contain natural organic materials from plants, especially during the warmer months of the year.  As a result, drinking water from surface water supplies is likely to contain THMs after chlorine is added during water treatment. Groundwater, such as well water, does not commonly contain the organic materials necessary to form significant levels of THMs.

How do Trihalomethanes Affect Human Health?

Several scientific studies have linked exposure to THMs in drinking water to an increased risk of bladder and possibly colorectal cancer.  In addition, some studies of THM exposure have found increased risks of adverse developmental outcomes, such as certain birth defects and miscarriages.  However, other studies have not found such associations.   

What is Being Done to Protect Human Health?

The U.S. Environmental Protection Agency has established a drinking water limit for THMs. The Maximum Contaminant Level (MCL) for the sum of the concentrations of the four regulated THMs in drinking water is 80 micrograms per liter (µg/L), averaged over a year.  New Jersey community water systems are required to monitor the formation of THMs in the drinking water every three months and to comply with the MCL.

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