Total Trihalomethanes (TTHMs) in Drinking Water

Total trihalomethanes (TTHMs) are a group of four chemical compounds -- chloroform, bromodichloromethane, dibromochloromethane, and bromoform -- that form when chlorine used to disinfect your water reacts with naturally occurring organic matter like decaying leaves, algae, and soil. In other words, the very process that makes your water safe from bacteria and viruses creates these potentially harmful byproducts. TTHMs are among the most common drinking water contaminants in the United States, and they are the leading cause of EPA violations nationwide.

The health concerns around TTHMs are significant. Long-term exposure has been associated with increased risks of bladder cancer, and some studies suggest links to colon and rectal cancers as well. Research has also connected TTHM exposure to liver and kidney problems, central nervous system effects, and adverse reproductive outcomes including miscarriage and low birth weight. Because the MCLG is set at zero, the EPA acknowledges that any exposure carries some theoretical risk, but the current enforceable standard of 80 parts per billion represents a balance between cancer risk and the need for effective water disinfection.

TTHM levels tend to be highest in water systems that use surface water sources (rivers, lakes, reservoirs) and chlorine disinfection. Levels also rise during warmer months when organic matter in source water increases, and they tend to be higher at points farther from the treatment plant where water has had more time to react. If you notice a strong chlorine taste or smell in your tap water, your system likely has higher TTHM formation potential.