Total Trihalomethanes (aka TTHMs) are byproducts often formed when disinfecting water for drinking. When chlorine or bromine are used in the disinfection process, they can react with bits of organic matter in the water being treated. This reaction produces chloroform, dibromochloromethane, bromodichloromethane, and other chemicals with long names that make up the trihalomethane family.
In large enough amounts, TTHMs can be harmful to humans. People are more likely exposed to TTHMs by inhaling water vapor in the shower, though the amount is far less than drinking water. Fortunately, the United States Environmental Protection Agency strictly limits the amount of TTHMs in water to 80 parts per billion. At that concentration, risks of TTHMs are negligible.