Disinfection Byproducts

What Are Trihalomethanes and Haloacetic Acids?

Trihalomethanes (chloroform, bromoform, bromodichloromethane, and chlorodibromomethane) and Haloacetic Acids (monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, monobromoacetic acid, and dibromoacetic acid) are commonly found in drinking water that has been chlorinated or chloraminated. Trihalomethanes (THMs) and Haloacetic Acids (HAAs) form when chlorine reacts with organic matter in the water. These disinfection byproducts are found mainly in water that originally came from surface sources, such as rivers and lakes. THM and HAA levels are typically low in groundwater (produced by wells).

Why Is Drinking Water Chlorinated/Chloraminated?

Drinking water is often chlorinated or chloraminated to kill microorganisms that could cause serious illnesses. Overall, chlorination of drinking water has benefited the public health enormously. There are other methods of disinfecting public drinking water, but they are often more expensive, and the potential health effects of using these other methods are generally less well understood. Additionally, some alternative disinfectants do not remain effective as the water is transported from the source to the tap through the distribution system.

Is There a Regulatory Standard for These Chemicals in Drinking Water?

THMs and HAAs have been associated with increased cancer risk, at least in animals, and the EPA has for many years regulated the amount of THMs and HAAs allowable in drinking water. Drinking water utilities that use chlorination or chloramination are required by law to sample water throughout their distribution system, average the total THM and HAA measurements, and report the results to the Department of Health. The Maximum Contaminant Level (MCL) permissible in drinking water by Washington and federal law is currently an average of 80 parts per billion (ppb) of THM and 60 ppb of HAA over four consecutive quarters at every location that is sampled. In other words, a utility has not violated the standard unless the running average at any single location sampled, or the system-wide average over the past year is more than 80 ppb or 60 ppb.

There are currently no standards for the individual trihalomethanes or haloacetic acids.