EWG's Tap Water Database


Disinfection Byproducts

July 2017


Drinking water disinfection is essential and saves lives from microbial diseases such as dysentery and cholera. But when chlorine and other disinfectants react with plant and animal waste in drinking water supplies, they form harmful contaminants, known collectively as disinfection byproducts.

It is critical that water is free of pathogens, but every measure must also be taken to decrease the amount of disinfection byproducts in finished drinking water served at the tap. These unintended chemical pollutants increase the risk of cancer and may damage developing fetuses. The EPA estimates that more than 250 million Americans ingest disinfection byproducts from tap water. People are also exposed to disinfection byproducts when bathing or using swimming pools.

Federal water standards set maximum allowable levels for some disinfection byproducts:

The federally regulated disinfection byproducts are just a small subset of a larger group of toxic contaminants that form during water disinfection. Several hundred other disinfection byproducts have also been identified in drinking water.

Trihalomethanes and haloacetic acids

Among disinfection byproducts in public water supplies, trihalomethanes and haloacetic acids are the largest group by weight, according to a 2016 report by the National Toxicology Program. Human epidemiological studies find that drinking tap water with disinfection byproducts increases the risk of developing bladder cancer. In animal studies, all trihalomethanes and multiple haloacetic acids cause liver, kidney and intestinal tumors.

Disinfection byproducts also increase the risk of problems during pregnancy, including miscarriage, cardiovascular defects, neural tube defects and low birth weight. Developmental toxicity of disinfection byproducts has been a subject of intense debate, with research groups disagreeing on data interpretation. To avoid any potential risk, EWG recommends drinking filtered tap water, especially during pregnancy.

The EPA has set national legal limits of 80 parts per billion, or ppb, for TTHM and 60 ppb for the five haloacetic acids. The standards were negotiated based on the technical feasibility of disinfection byproduct removal and the cost of water treatment, and did not consider long-term toxicity of these contaminants, as reported in a 1995 publication co-authored by Alan Robertson, formerly of the American Water Works Association, and other water industry specialists who participated in the negotiations.

In 2010, the California Office of Environmental Health Hazard Assessment estimated that a one-in-a-million cancer risk for the total trihalomethanes group occurs at a concentration of 0.8 ppb, or 100 times lower than the federal legal limit.

No health-based limit is currently available for the five haloacetic acids the federal government regulates. Health-based limits have been developed by the EPA for two of the most common haloacetic acids, di-and tri-chloroacetic acids. The one-in-a-million cancer risk for these two contaminants correspond to water concentrations of 0.7 and 0.5 ppb, respectively. In 2016, the National Toxicology Program launched an evaluation of carcinogenic activity of the haloacetic acid group, making an important step toward protecting Americans from these contaminants.

Existing federal standards and health-based limits for disinfection byproducts

Contaminant* Federal legal limit** Health-based limit Type of health-based limit Public agency
Total trihalomethanes (TTHM) group, including 80 ppb 0.8 ppb Draft public health goal California Office of Environmental Health Hazard Assessment (OEHHA)
Chloroform N/A 1 ppb One-in-a-million cancer risk level California OEHHA
Bromoform N/A 5 ppb One-in-a-million cancer risk California OEHHA
Bromodichloromethane N/A 0.4 ppb One-in-a-million cancer risk California OEHHA
Dibromochloromethane N/A 0.7 ppb One-in-a-million cancer risk California OEHHA
Five haloacetic acids (HAA5) group, including 60 ppb N/A N/A U.S. EPA
Monochloroacetic acid N/A N/A N/A N/A
Dichloroacetic acid N/A 0.7 ppb One-in-a-million cancer risk U.S. EPA
Trichloroacetic acid N/A 0.5 ppb One-in-a-million cancer risk U.S. EPA
Monobromoacetic acid N/A N/A N/A N/A
Dibromoacetic acid N/A N/A N/A N/A
Chlorite 1000 ppb 50 ppb Public health goal California OEHHA
Chlorate N/A 210 ppb Reference concentration U.S. EPA
Bromate 10 ppb 0.1 ppb Public health goal California OEHHA
N-Nitrosodimethylamine N/A 0.003 ppb Public health goal California OEHHA

* Click on a contaminant above to see its nationwide testing results.

** Where no number is listed in the “federal legal limit” column, the EPA has not set a Maximum Contaminant Level for the chemical.

Chlorite and chlorate

Chlorite and chlorate are two structurally similar disinfection byproducts that form when chlorine dioxide or hypochlorite are used as disinfectants.

In 2009, California published a public health goal of 50 ppb for chlorite in drinking water. The state Office of Environmental Health Hazard Assessment determined that the most sensitive effects of chlorite exposure can be detected by monitoring red blood cell damage caused by oxidative stress in laboratory animals. State scientists also reported that chlorite affects sperm and thyroid function, and causes stomach ulcers. It may alter behavior and neurodevelopment of rat pups exposed in the womb, and may act as a weak carcinogen. The federal legal limit of 1,000 ppb allows 20 times as much chlorite in water as California’s public health goal.

Like chlorite, chlorate causes oxidative damage to red blood cells and can affect thyroid function. Chlorate harms the thyroid by decreasing iodide uptake, which leads to enlargement of the thyroid, known as goiters. No enforceable federal or state limits for chlorate in drinking water exist. In 2002, California scientists proposed an advisory value, called an action level, for chlorate at 200 ppb. Between 2013 and 2015, the EPA collected data through the Unregulated Contaminant Monitoring program and found that over 15 percent of water tests had chlorate levels above a health-based reference concentration, which federal scientists defined as 210 ppb – nearly equivalent to the California advisory value.


Bromate is a carcinogenic disinfection byproduct that forms when ozonation is used for water treatment. Studies on laboratory animals show that bromate damages DNA and causes cancer in multiple organs. In 2009, California published a public health goal for bromate of 0.1 ppb, 100-fold lower than the federally allowable level. The testing methods currently prescribed by the EPA are not sensitive enough to detect contamination at 0.1 ppb but can only measure bromate values greater than 1 ppb.

What about disinfection byproducts that don’t have federal standards?

In an effort to comply with federal standards, many water utilities have moved from using chlorine to using alternatives – such as chloramine, chlorine dioxide and ozone – which reduce the amount of TTHM and HAA5 formation. But, as reported by academic and government scientists, these alternative disinfectants often form other byproducts that have adverse effects on human health. The question of which type of disinfection is safest for human health remains unsettled. In the meanwhile, the only effective solution is minimizing plant and animal waste in water, which in turn decreases the formation of disinfection byproducts.

The mix of disinfection byproducts in a water system varies depending on several factors: water turbulence caused by major storms, flooding or drought; algal growth triggered by excessive fertilizer; erosion and surface runoff; water acidity; and the presence of bromine. These changes matter because some individual trihalomethanes and haloacetic acids are more toxic than others.

A 2015 publication by Stig Regli and other EPA scientists estimated that increased bromide concentrations in drinking water sources produce a significant increase in bladder cancer risk. A 2011 study led by the EPA’s Stephen Duirk and Susan Richardson reported that iodine in water can result in formation of iodinated byproducts, some of the most toxic disinfection byproducts identified to date.

Nitrogen-containing disinfection byproducts, such as nitrosamines, a class of very potent carcinogens, can form during the use of chloramine for water disinfection. The California public health goal for N-nitrosodimethylamine, one of the most common nitrosamines, is 0.003 ppb – 266 times smaller than the state’s draft public health goal for the total trihalomethanes group.

What should be done to decrease the levels of disinfection byproducts?

Minimizing the formation of disinfection byproducts is essential for public health. Water utilities can do this by working with nearby agricultural businesses to reduce the fertilizer and plant-based nitrogen and animal waste entering drinking water sources. Urban runoff and human waste can be limited with proper storm water and wastewater treatment.

What can I do to remove disinfection byproducts from my tap water?

EWG recommends using a home filtration system to treat disinfection byproducts in your tap water. Fortunately, simple filtration methods such as counter-top carbon filters readily remove disinfection byproducts. Use EWG’s Water Filter Guide to find filters certified to remove trihalomethanes.


California OEHHA, Memorandum: Proposed Action Level for Chlorate. 2002. Available at

California OEHHA, Final Public Health Goal for Bromate in Drinking Water. 2009. Available at

California OEHHA, Draft Public Health Goal for Trihalomethanes in Drinking Water. 2010. Available at

J. Colman, Identification of Developmentally Toxic Drinking Water Disinfection Byproducts and Evaluation of Data Relevant to Mode of Action. Toxicology and Applied Pharmacology, 2011, 254(2):100-126.

S.E. Duirk et al., Formation of Toxic Iodinated Disinfection By-Products from Compounds Used in Medical Imaging. Environmental Science & Technology, 2011, 45(16):6845-6854.

M. Kogevinas, Epidemiological Approaches in the Investigation of Environmental Causes of Cancer: The Case of Dioxins and Water Disinfection By-Products. Environmental Health, 2011, 10 Suppl. 1:S3.

National Toxicology Program, U.S. Department of Health and Human Services, Draft Report on Carcinogens Concept Di- and Tri- Haloacetic Acids Found as Water Disinfection By-Products. 2016. Available at

M. Plewa and E. Wagner, Mammalian Cell Cytotoxicity and Genotoxicity of Disinfection By-Products. Water Research Foundation, 2009.

J.G. Pressman et al., Concentration, Chlorination, and Chemical Analysis of Drinking Water for Disinfection Byproduct Mixtures Health Effects Research: U.S. EPA's Four Lab Study. Environmental Science & Technology, 2010, 44(19):7184-7192.

S. Regli et al., Estimating Potential Increased Bladder Cancer Risk Due to Increased Bromide Concentrations in Sources of Disinfected Drinking Waters. Environmental Science & Technology, 2015, 49(22):13094-13102. Available at

J.A. Roberson et al., The D/DBP Rule: Where Did the Numbers Come From? Journal American Water Works Association, 1995, 87(10):46-57.

C.M. Villanueva et al., Overview of Disinfection By-Products and Associated Health Effects. Current Environmental Health Reports, 2015, 2(1):107-115.