Trihalomethanes are harmful disinfection by-products generally formed when disinfectants, such as chlorine, interact with naturally occurring organic or inorganic compounds in the water. The Total Trihalomethanes consists of four chlorination by-products: Trichloromethane (chloroform), Bromodichloromethane, Dibromochloromethane, and Tribromomethane (bromoform).
Trihalomethanes are referred to as Total Trihalomethanes in the realm of water quality because this group of disinfection by-products is monitored and regulated as a sum of each chemical. This is to say, when added together, the sum of chloroform, bromodichloromethane, dibromochloromethane, and bromoform cannot exceed 0.080 mg/L or 80 ppb.
| Chemicals | Maximum Contaminant Level Goal (mg/L or ppb) | Total Trihalomethane (as an annual average) |
| Bromodichloromethane | Zero (0.6 ppb) | 0.00 mg/L or 80 ppb |
| Bromoform | Zero (5 ppb) | |
| Bromochloromethane | 0.06 mg/L or 60 ppb | |
| Chloroform | 0.07 mg/L or 70 ppb |
Michigan water utilities that rely on chlorination as a primary disinfectant adhere to EPA standards on TTHMs, which are directly addressed in Stage 2 of the Disinfectants and Disinfection Byproducts Rules.
