What are TTHMs?

Total trihalomethanes (TTHM) are a group of disinfection byproducts that form when water disinfectants such as chlorine or ozone react with other naturally occurring chemicals in the water. They are colorless and will evaporate out of the water into the air. There are four significant TTHM (chloroform, bromoform, bromodichloromethane, and dibromochloromethane) potentially found in disinfected drinking water and their combined concentration is referred to as Total THM (TTHM).

Levels of TTHM generally increase in the summer months due to the warmer temperatures but can also be affected by seasonal changes in source water quality or by changing amounts of disinfection added. Water systems often can experience temporary increases in TTHM due to short-term increases in disinfectants which can occur when there is a water main break, when water systems are under repair, or when there is a potential microbial (example: bacteria) problem or threat. All water systems that disinfect the water with chlorine or ozone are required by federal and state law to sample for TTHM on a regular basis (quarterly, or once every three months).