Trihalomethanes

Of the identified DBPs, the two classes of greatest concern are trihalomethanes (THMs) and haloacetic acids (HAAs) — typically tasteless and odorless, but suspected human carcinogens. The quantity formed depends on several factors, including the amount and type of organic material present, its composition and structure, temperature, pH, chlorine dosage, contact time, and bromide concentration — meaning the optimal DBP-minimization strategy generally requires bench and pilot testing along with some trial and error.

THM formation from water chlorination was first identified in the 1970s, and THMs are considered carcinogenic environmental pollutants. Studies of populations drinking chlorinated water — where chloroform is the predominant THM — have consistently shown small increases in rectal, colon, and bladder cancer incidence, with the strongest evidence for bladder cancer. Among people who take hot showers and baths, inhalation and dermal absorption during bathing account for greater THM exposure than drinking water itself.

Another approach to THM contamination is switching from chlorine to chloramine (chlorine plus ammonia) disinfection, which is less aggressive toward organic precursors — though also less effective as a bactericide, viricide, and taste/odor oxidizer, so care is needed when converting a chlorination system to chloramination.