Trihalomethanes

What is Trihalomethanes?

Trihalomethanes (THMs) are the best-known disinfection byproducts: chloroform, bromodichloromethane, dibromochloromethane and bromoform. They form when the chlorine used to disinfect water reacts with natural organic matter such as leaf litter and algae in the source water. Almost every chlorinated system has some; levels are higher in surface-water systems and in summer, and they rise the longer water sits in the pipes.

THMs are regulated because long-term exposure is associated with a higher risk of bladder cancer and, less consistently, with reproductive effects. The EPA limit for total trihalomethanes is 80 µg/L as an annual average, and utilities must report their results. Because THMs are volatile, a meaningful share of exposure comes from breathing them in during hot showers, not only from drinking.

On a water filter, "trihalomethanes" or "TTHM" means the filter has been tested to reduce them, a claim certified under NSF/ANSI 53. Activated carbon handles them well; a shower filter with enough carbon contact time can cut the inhaled share. Reverse osmosis alone is less effective for these small volatile molecules, which is why RO systems pair with carbon stages.

Commonly found in: Municipal tap water, Well water, Water filter claims, Lab-tested products

Sources: Stage 1 and Stage 2 Disinfectants and Disinfection Byproducts Rules · National Primary Drinking Water Regulations · Guidelines for drinking-water quality, 4th edition