What You Need to Know About Disinfection Byproducts in Drinking Water

Drinking water drawn from surface water sources, such as Lake Champlain, must be disinfected to kill bacteria, viruses and other organisms that can cause serious sickness and death.

The Environmental Protection Agency (EPA) requires that public water systems use chlorine at specified levels for “primary” disinfection at the water treatment facility. “Secondary” disinfection is also required to treat any type of contamination that could happen after the water leaves the treatment plant. The EPA approves three disinfectants for use as a secondary disinfectant: chlorine, chlorine dioxide and monochloramine.


Monochloramine is a chemical that is made by combining chlorine with ammonia. Although it is a weaker disinfectant than chlorine, monochloramine is more stable, remains effective in the water system over longer distances for a longer period of time, and forms lower levels of regulated disinfection byproducts than chlorine.

Monochloramine has been used in the U.S. as a secondary disinfectant since the 1930s. While the EPA does not know the absolute number of people who are using water treated with monochloramine, it is estimated that the number exceeds the 68 million who were identified in a 1998 survey. In New England, more than 3 million people in 135 communities are served by monochloraminated water.

Review of scientific literature on monochloramine

The Health Department reviewed the history, science and potential health risks associated with the use of monochloramine, and conducted an extensive review of all scientific literature on the subject.

The Health Department determined that the use of monochloramine as a secondary disinfectant will not likely result in negative health effects. The Health Department believes that the use of monochloramine will lower the concentration of regulated and possibly unregulated disinfection byproducts in drinking water. This reduction may contribute to fewer negative health effects compared to drinking water treated with free chlorine as a secondary disinfectant.

All available research at this time still supports the EPA conclusion in its 1994 review, Drinking Water Criteria Document for Chloramines, that: “In humans, health effects do not appear to be associated with levels of residual monochloramines typically found in drinking water.” However, there still remain gaps in the data regarding the health effects of all water disinfectants.

Read the Health Department’s Public Health Review of Monochloramine, published on July 25, 2012, and updated on October 19, 2012.

Disinfection Byproducts

Most public water systems use a disinfectant to kill viruses and bacteria that can cause sickness — such as gastrointestinal disorders or diarrhea. Chlorine is the most commonly used disinfectant, sometimes used in combination with other disinfectants — such as ozone, chloramine, chlorine dioxide and ultraviolet light. 

Disinfection byproducts (DBPs) are a family of chemicals formed when disinfectants react with naturally occurring organic matter and other substances in the source water. The levels of DBPs depend upon the nature of the source water, the type of treatment to remove particles and organic matter, and the type and concentration of disinfectant.

The risk of illness from DBPs is much lower than the risk of illness from drinking most surface water and some groundwater sources that have not been disinfected. The major health risks from DBPs result from long-term exposures.

Surface water sources such as reservoirs and streams are more likely to have higher DBP levels than disinfected groundwater sources. If you get your drinking water from a private drinking water well, DBPs are unlikely to be present in the water.

EPA rules require water systems to improve treatment methods to minimize the formation of DBPs. The aim is to have a treatment method that uses enough disinfectant to protect consumers from waterborne disease while at the same time producing as few DBPs as possible.

Haloacetic Acids (HAA5)

Haloacetic acids (HAA5) are in drinking water when naturally occurring organic material in the water reacts with chlorine or chloramine used to disinfect the water. Consuming drinking water containing total haloacetic acids in excess of the maximum contaminant level (MCL) over many years may increase the risk of cancer. The current MCL for HAA5 is 60 micrograms per liter (µg/L) or 60 parts per billion (ppb).

Total Trihalomethanes (TTHM)

Total Trihalomethanes (TTHM) are in drinking water when naturally occurring organic material in the water reacts with chlorine or chloramine used to disinfect the water. Consuming drinking water containing TTHM in excess of the maximum contaminant level (MCL) over many years may lead to an increased risk for liver, kidney, and central nervous system problems, as well as an increased risk for cancer. The current MCL for TTHM is 80 micrograms per liter (µg/L) or 80 parts per billion (ppb).

Learn more about testing for DBPs

Last Updated: