Total Trihalomethanes (TTHMs)

Category: disinfection byproduct

Written by WaterFilterMatch Editorial TeamApril 2026

EPA MCL

0.08 mg/L

Status

EPA Regulated

NSF Standard

NSF/ANSI 53

Health Effects

Liver, kidney, and central nervous system problems. Increased risk of bladder and colorectal cancer with long-term exposure. Some trihalomethanes are probable human carcinogens.

Source: EPA National Primary Drinking Water Regulations

Where It Comes From

Formed when chlorine used for disinfection reacts with naturally occurring organic matter (such as decaying leaves) in source water. The four TTHMs are chloroform, bromodichloromethane, dibromochloromethane, and bromoform.

Where It's Commonly Found

All chlorinated water systems produce some TTHMs. Higher levels in systems using surface water with high organic content, during warm months, and in water that travels long distances through the distribution system.

Found in virtually all chlorinated water systems. Thousands of systems have exceeded the EPA MCL. EWG health guideline of 0.15 ppb is far stricter than the EPA MCL of 80 ppb.