EWG's Tap Water Database — 2021 UPDATE

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Disinfection Byproducts

April 2020


Drinking-water disinfection is essential. It saves people from dying of 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.

People are exposed to disinfection byproducts through contact with chemically disinfected water – drinking it, eating food prepared with it, and bathing or swimming in it.

It is critical that water be 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 the developing fetus.

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. Hundreds of other disinfection byproducts form in drinking water and may harm human health.

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.

The Environmental Protection Agency has set national legal limits of 80 parts per billion, or ppb, for total trihalomethanes, and 60 ppb for the five haloacetic acids. The EPA standards were negotiated based on the technical feasibility and cost of water treatment and did not consider the long-term toxicity of these contaminants.

Federal legal limits and health-based guidelines for disinfection byproducts

Contaminant1 Federal legal limit2 Health-based guideline3
Group of four trihalomethanes (THM4/TTHM), including: 80 ppb 0.15 ppb
     Chloroform N/A 0.4 ppb
     Bromoform N/A 0.5 ppb
     Bromodichloromethane N/A 0.06 ppb
     Dibromochloromethane N/A 0.1 ppb
Group of five haloacetic acids (HAA5), including: 60 ppb 0.1 ppb
     Monochloroacetic acid N/A 53 ppb
     Dichloroacetic acid N/A 0.2 ppb
     Trichloroacetic acid N/A 0.1 ppb
     Monobromoacetic acid N/A 25 ppb
     Dibromoacetic acid N/A 0.04 ppb
Chlorite 1,000 ppb 50 ppb
Chlorate N/A 210 ppb
Bromate 10 ppb 0.1 ppb
N-Nitrosodimethylamine N/A 0.003 ppb

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

2. If no number is listed in the "federal legal limit" column, the EPA has not set a Maximum Contaminant Level for the chemical, so it is marked not available (N/A).

3. Health-based guidelines for chloroform, bromoform, bromodichloromethane and dibromochloromethane represent one-in-a-million lifetime cancer risk levels proposed by the California Office of Environmental Health Hazard Assessment, or OEHHA, in 2018. Health-based guidelines for monochloracetic acid, monobromoacetic acid, dichloroacetic and trichloracetic acids represent public health goals proposed by OEHHA in 2020. Health-based guidelines for chlorite, bromate and N-nitrosodimethylamine are public health goals established by the state of California. The health-based guideline for chlorate is a reference concentration developed by the EPA. The health-based guideline for the THM4/TTHM and HAA5 groups and for dibromoacetic acid were defined in peer-reviewed scientific studies by EWG and correspond to a one-in-a-million lifetime cancer risk level.

Studies find that drinking tap water with disinfection byproducts increases the risk of bladder cancer. Disinfection byproducts also increase the risk of problems during pregnancy, including miscarriage, cardiovascular defects, neural tube defects and low birth weight.

The EPA classified bromodichloromethane and bromoform, two of the four regulated trihalomethanes, as "likely to be carcinogenic to humans." In 2018, the National Toxicology Program classified six haloacetic acids as "reasonably anticipated to be human carcinogens." The list includes dichloroacetic acid, dibromoacetic acid, chlorobromoacetic acid, bromodichloroacetic acid, chlorodibromoacetic acid and tribromoacetic acid.

In March 2020, EWG scientists published a peer-reviewed study that offered the first side-by-side comparison of cancer risk from drinking water disinfection byproducts. The analysis was based on data derived from animal and human studies. The study also presented the first analysis of data on unregulated haloacetic acids that had become available from the latest, fourth round of the EPA-mandated unregulated contaminant monitoring program.

A toxicological assessment indicated that haloacetic acids, and in particular brominated haloacetic acids, are more carcinogenic and are associated with a greater number of attributable cancer cases than trihalomethanes. Epidemiological data suggest that the lifetime cancer risk from disinfection byproducts for the U.S. population served by community water systems is approximately 3.0 × 10−3 (with lower and upper confidence intervals of 2.1 × 10−4 – 5.7 × 10−3), or a lifetime cancer risk of three cases per thousand people.

Chlorite and chlorate

Chlorite and chlorate are two structurally similar disinfection byproducts that form when chlorine dioxide, also known as chlorine gas, or hypochlorite, the active ingredient in bleach, is used as a disinfectant.

In 2009, California published a public health goal of 50 ppb for chlorite in drinking water. OEHHA determined that the most sensitive effects of chlorite exposure can be detected by monitoring red blood cell damage in laboratory animals. State scientists also reported that chlorite affects sperm and thyroid function and causes stomach ulcers. It may alter the 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 damages red blood cells and can affect thyroid function. Chlorate harms the thyroid by decreasing iodide uptake, which leads to enlargement of the thyroid, a condition known as a goiter.

There are no enforceable federal or state limits for chlorate in drinking water.

In 2002, California state scientists proposed an advisory value, called an action level, for chlorate at 200 ppb. Between 2013 and 2015, the EPA’s Unregulated Contaminant Monitoring program found that more than 15 percent of nationwide 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 water is treated by the infusion of ozone gas. Studies performed 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, which is a hundredfold lower than the federally allowable level.

What about disinfection byproducts for which there are no federal standards?

To comply with federal standards, many water utilities have moved from chlorine to alternatives such as chloramine, chlorine dioxide and ozone, which reduce TTHM and HAA5 formation. But studies show that these alternative disinfectants can form other byproducts that can harm human health.

It is unclear which type of disinfection is safest. Meanwhile, the most effective solution is keeping plant and animal waste out of water in the first place, lessening the need for disinfectants that form harmful byproducts.

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

A 2015 study 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 by Stephen Duirk and colleagues reported that iodine in water can cause the formation of iodinated byproducts, some of the most toxic disinfection byproducts.

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 one of the most common nitrosamines, called N-nitrosodimethylamine, is 0.003 ppb – 266 times lower than the state’s draft public health goal for the total trihalomethanes group.

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

Water utilities should work with nearby farms to reduce the fertilizer, 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. Simple filtration methods, such as countertop carbon filters, can decrease the levels of common disinfection byproducts in water, such as trihalomethanes and haloacetic acids.


California Office of Environmental Health Hazard Assesssment. 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. First Public Review Draft, Trihalomethanes in Drinking Water: Chloroform, Bromoform, Bromodichloromethane, Dibromochloromethane. 2018. Available at

Colman J. 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.

Evans S et al. Analysis of Cumulative Cancer Risk Associated with Disinfection Byproducts in United States Drinking Water. Int. J. Environ. Res. Public Health, 2020, 17(6): 2149.

Diana M et al. Disinfection byproducts potentially responsible for the association between chlorinated drinking water and bladder cancer: A review. Water Research, 2019, 162:492–504.

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

Kaufman JA et al. Associations Between Disinfection By-Product Exposures and Craniofacial Birth Defects. Journal of Occupational and Environmental Medicine, 2018, 60(2): 109-119.

Kogevinas M. 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.

Li XF and Mitch WA. Drinking Water Disinfection Byproducts (DBPs) and Human Health Effects: Multidisciplinary Challenges and Opportunities. Environmental Science & Technology, 2018, 52(4): 1681–1689.

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

NTP, RoC Review of Haloacetic Acids Found as Water Disinfection By-Products. 2018. Available at

Pressman JG 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.

Regli S 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.

Rivera-Núñez Z and Wright JM. Exposure to Disinfectant By-products and the Risk of Stillbirth in Massachusetts. Occupational and Environmental Medicine, 2018, 75(10): 742–751.

Rivera-Núñez Z and Wright JM. Association of brominated trihalomethane and haloacetic acid exposure with fetal growth and preterm delivery in Massachusetts. Journal of Occupational and Environmental Medicine, 2013, 55(10): 1125–34.

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

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

Wright JM et al. Disinfection By-Product Exposures and the Risk of Specific Cardiac Birth Defects. Environmental Health Perspectives, 2017, 125(2):269–277.