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Last Edited: 8/11/2010

Drinking Water Contaminants: Disinfection Byproducts (DBP)

For more information on disinfection byproducts (DBPs) and current regulations, view the Environmental Protection Agency's (EPA) Disinfection Byproduct Information (PDF).  View data on DBP levels in drinking water for California.

What are disinfection byproducts and how do they get in our water?

Water may contain microorganisms such as viruses and bacteria that can cause serious illness (gastrointestinal disorders or diarrhea) and even death. Public water suppliers disinfect their water to kill these microorganisms. 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 are a family of chemicals formed when these disinfectants react with naturally occurring organic matter and other substances in the source water. The levels of disinfection byproducts depend upon the nature of the source water, the type of treatment to remove particles and organic matter, and type and concentration of disinfection.

DBPs include haloacetic acids (HAA) and trihalomethanes (THM). HAA5 is the sum of five HAAs: monochloroacetic, dichloroacetic, trichloroacetic, monobromo-acetic, and dibromoacetic acids. TTHM is the sum of four chlorine and bromine-containing trihalomethanes: chloroform, bromodichloromethane, dibromochloromethane, bromoform..

There are many known DBPs, as well as possibly as yet unidentified DBPs in disinfected water. Since TTHM and HAA5 typically occur at higher levels than other known DBPs in drinking water, their presence may be an indicator for a range of DBPs.

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What are the health effects of disinfection byproducts?

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

When people are exposed to disinfection byproducts at high levels over many years, they may develop bladder cancer or problems with their liver, kidneys, or circulatory system. There may also be a connection between exposure to DBPs during pregnancy and miscarriages, premature births, low birth weight, and birth defects.

There are several ways that DBPs can get into the human body:

  • Ingestion: drinking water with DPBs.
  • Inhalation: breathing in DBPs that are in the air. DBP may be released into the air when you use your tap water. This can happen when you are taking a shower or washing dishes. And the hotter the water is, the more likely it is that DBPs will be released into the air. DBPs can also get into the air when you boil your tap water, such as when you make tea or soup.
  • Absorption: DBPs can be absorbed through the skin when bathing and showering. For most people, only very small amounts of DBPs get into the body through the skin. However, these amounts can increase as your contact time with water increases, for example, if you typically take long baths or swim frequently in public pools.

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How much disinfection byproduct is allowed in drinking water?

All community water systems that disinfect their water (i.e. all surface water and most groundwater systems) are required to monitor for disinfection byproducts in the distribution system. Monitoring requirements and the number of samples a system collects varies based on source water type, treatment type, the number of treatment plants, historical DBP levels and the size of the population served.

Establishment of regulations in 1979

DBP regulations have been slowly phasing in since 1979. Safe Drinking Water Act regulation of DBPs began with the 1979 Total Trihalomethane (TTHM) Rule. This rule set an interim maximum concentration level (MCL) for TTHM of 100 µg/L for community water systems (CWS) serving 10,000 or more people. Smaller water systems were not regulated at first.

Expansion of regulations in 1998

In 1998, US EPA expanded the regulations. It reduced the MCL for TTHM to 80 µg/L; added MCL for HAA5 of 60 µg/L; set MCLs for other DBPs; and extended the regulation to cover all CWSs that disinfect. The new regulations were phased in over time. Large water systems had to comply as of 2002, and other systems as of 2004. In 2006, the new rule did not alter MCLs, but changed how compliance with MCLs will be calculated. The rule required that Public Water Systems evaluate their distribution systems for appropriate sampling locations. The results of this evaluation may affect the number and location of samples. The first reporting deadline for compliance with this regulation was in 2006, but it will be a number of years before the rule requires the new compliance calculations based on routine DBP samples.

Current regulations

Today, the maximum allowable contaminant level is 60 µg/L for HAA5 and 80 µg/L for TTHM. These levels are calculated as running annual averages. A running annual average is the arithmetic average of sampling results calculated at the end of every quarter for the previous consecutive four-quarter period. Compliance is achieved when the running annual averages, not individual samples, are below the MCLs.

For more details on monitoring and reporting of DBPs, see the EPA Reference Guide on Stage 1 DBP Rule (PDF)

See Drinking Water Monitoring for more information on how water samples are collected.

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How do I know if there are disinfection byproducts in my drinking water?

EPA requires that water systems use water treatment techniques that protect people from disease-causing microorganisms while also reducing the formation of disinfection byproducts and their harmful effects.

The levels of disinfection byproducts depend upon the nature of the source water and type of disinfection and can change with seasons of the year, rainfall, and distance from the treatment plant to the consumer’s tap. Surface water sources such as reservoirs and streams are more likely to have higher disinfection byproducts levels than ground water sources. If you get your drinking water from a private drinking water well, disinfection byproducts are unlikely to be present in the water.

If your public water system has notified you of a disinfection byproduct violation, it does not mean that the people who consume the system's water will become sick. Also, because of the tighter regulations on DBPs that are beginning to go into effect, it does not necessarily mean that your exposure to DBPs has increased.

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