Skip directly to searchSkip directly to the site navigationSkip directly to this page's context menuSkip directly to the page's main content

Important Facts for Drinking Water: Disinfection Byproducts


A community water system (CWS) is a public water system that serves at least 15 service connections used by year-round residents or regularly serves at least 25 year-round residents.

In order to ensure the public's safety with regards to drinking water, community water suppliers treat their water with several products including: chlorine, ozone, chlorine dioxide, ultraviolet light, etc. These products that are used to kill pathogens, however, sometimes react with naturally-occurring organic matter, and create disinfection byproducts. Disinfection byproducts for which the EPA has created standards include: total trihalomethanes (TTHM), haloacetic acids (HAA5), bromate, and chlorite. Although the risk of illness from drinking water that has not been disinfected is much higher than illness from disinfection byproducts, some increased health risks do occur from consumption of disinfection byproducts. Consumption (through inhalation, ingestion, and absorption through the skin) at high levels over a long period of time has been shown to increase the risk of developing bladder cancer. Rectal and colon cancer along with problems involving the liver, kidney, and nervous system have also been shown to have a correlation with those who have been exposed to disinfection byproducts over a long period of time.

Disinfection byproducts (DBP) are formed when disinfectants are used to inactivate microbial contaminants in water and react with materials, primarily organic matter, in the water. Several hundred DBPs in over a dozen chemical classes have been identified. Most commonly, DBPs form when chlorine reacts with naturally occurring organic matter in the source water. (Modified from the National Environmental Public Health Tracking Network (NEPHTN) Nationally Consistent Data and Measures (NCDM) DBP indicator document, version 4)

DPB levels tend to be highest in water derived from surface sources because ground water generally has little organic matter. Ground water can, however, produce relatively high levels of the more brominated DBPs when the water, due to either geological circumstances or salt water intrusion in coastal areas, has elevated levels of bromide. (Modified from the NEPHTN NCDM DBP indicator document, version 4)


Sum of annual average/annual maximum of DBP concentrations for drinking water source samples. Sum of estimated annual state population, based on the mean/maximum sample concentration, where applicable.


Not applicable. There are no rate measures in this DBP indicator.

Data Interpretation Issues

The current measures are derived for community water systems (CWS) only. Transient non-community water systems, which are regulated by the Environmental Protection Agency (EPA), may also be an important source of DBPs exposure. Measures do not account for the variability in sampling, numbers of sampling repeats, and variability within systems. Concentrations in drinking water cannot be directly converted to exposure because water consumption varies by climate, level of physical activity, and between people (EPA 2004). Due to errors in estimating populations, the measures may overestimate or underestimate the number of affected people. (Modified from the National Environmental Public Health Tracking Network (NEPHTN) Nationally Consistent Data and Measures (NCDM) DBP indicator document, version 4)

The Safe Drinking Water Act compliance data include only a handful of the hundreds of known disingection byproducts (DBP), most of which occur in chemical classes other than trihalomethanes (THM) and haoacetic acids (HAA) (Weinberg et al. 2002). While compliance sampling for THMs and HAAs are directed at the DBPs thought to be the most commonly produced by chlorination, non-regulated DBPs exist even among the THMs and HAAs. (Modified from the NEPHTN NCDM DBP indicator document, version 4)

Concern has also been expressed about iodinated THMs and HAAs which, while present in lower concentrations than the brominated and chlorinated THMs, are thought to be toxic at lower doses (e.g. Plewa et al. 2004). THMs and HAAs may not be the most comprehensive indicators of DBP levels in water, because alternative disinfection methods produce different DBPs and may result in high levels of these unregulated DBPs. Little is known about the quantitative occurrence of these DBPs in the distribution system (Richardson et al. 2002, Krasner et al. 2006). While the health effects of different DBPs may vary, with some suspected to be hazardous, few have been characterized for their effects on human health (Woo et al. 2002). (Modified from the NEPHTN NCDM DBP indicator document, version 4)

DBP levels vary seasonally. Quarterly samples may not capture maximum levels and may not even adequately reflect short term levels. Therefore, they may be inadequate for estimating exposure during critical periods of a pregnancy which may be as short as two to three weeks, especially if peak exposure matters more than average exposure. Furthermore, these fluctuations make it difficult to characterize levels with a single number, such as an annual average, and thus pose challenges to the development of meaningful synopses of patterns and trends. (Modified from the NEPHTN NCDM DBP indicator document, version 4)

Why Is This Important?

People drink and use water every day. The majority of Americans are provided with high quality drinking water. About 90% of people in the U.S. (262 million in 2006) get their water from a community water system versus a smaller water supply such as a household well. The U.S. Environmental Protection Agency (EPA) sets regulations for treating and monitoring drinking water delivered by community water systems. Currently, there are water quality standards and monitoring requirements for over 90 contaminants. Drinking water protection programs play a critical role in ensuring high quality drinking water and protecting the public's health.

Because people drink and use water every day, contaminants in drinking water have the potential to affect many people. The number of people served by a community water system varies from at least 25 people to hundreds of thousands. Community water systems in the U.S. provide among the highest quality drinking water in the world. However, some contaminants are present at low levels, and it is still possible that drinking water can become contaminated at higher levels. If a person is exposed to a high enough level of a contaminant, they may become ill. Effects can be seen by the duration (time) of the exposure. Short-term or long-term effects depend on the specific contaminant, the level of contaminant in the water, and the person's individual susceptibility. As additional information is obtained about how specific contaminants affect public health, standards may change in order to better protect public health.

The information provided above is from the Utah Department of Health's Center for Health Data IBIS-PH web site ( The information published on this website may be reproduced without permission. Please use the following citation: "Retrieved Thu, 02 July 2015 10:33:25 from Utah Department of Health, Center for Health Data, Indicator-Based Information System for Public Health Web site:".

Content updated: Mon, 30 Jun 2014 16:19:15 MDT