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Questions and Answers on Health Effects of Disinfection Byproducts

May, 2004

1. What are disinfection byproducts?

Disinfection byproducts (DBPs) form when disinfectants used to treat drinking water react with naturally occurring materials in the water. The predominant byproducts that result from use of chlorine as a disinfectant are trihalomethanes (chloroform, bromoform, bromodichloromethane, and dibromochloromethane) and haloacetic acids (monochloro-, dichloro-, trichloro-, monobromo-, dibromo-). Trihalomethanes (THMs) and haloacetic acids (HAAs) form when chlorine reacts with organic and inorganic material in source water (which comes from decomposing plant material, pesticides, etc.). The amount of trihalomethanes and haloacetic acids in drinking water can change from day to day, depending on the season, water temperature, amount of chlorine added, the amount of plant material in the water, and a variety of other factors.

2. Why is drinking water chlorinated?

Chlorinating tap water is critical to protect the public from disease-causing microorganisms. Drinking water is chlorinated to kill bacteria and viruses that cause serious illnesses and, in some cases, death. Chlorination of drinking water has benefited public health enormously by lowering the rates of infectious diseases (for example, typhoid, hepatitis and cholera) spread through untreated water. In the beginning of the last century tens of thousands of people died from disease-causing microorganisms in the water supply.

3. What about disinfectants other than chlorine?

There are a number of other disinfectants that are effective at killing disease-causing microorganisms, including ozone, chlorine dioxide, chloramines, and ultraviolet (UV) light. With the exception of UV light, these alternative disinfectants produce different types of disinfection byproducts that also have potential risks.

Water is first disinfected at the treatment plant and is then distributed to homes through a system of pipes. Ozone, chlorine dioxide, and UV light work very well in the treatment plant as alternatives to chlorine. The water in the distribution system must also be continuously exposed to a disinfectant. However, there is no way to continuously produce these disinfectants in the distribution system. Therefore, water utilities have to use either chlorine or chloramine (which are made from mixing chlorine with ammonia) as a secondary disinfectant. Chloramine can help reduce some disinfection byproducts, such as trihalomethanes and haloacetic acids, but may not be the perfect solution. Each water system has unique characteristics that must be considered when choosing a disinfectant.

4. What health risks are posed by disinfection byproducts such as TTHMs and HAAs?

EPA has regulated DBPs since 1979 to address health risks posed by a potential association between chlorinated drinking water and cancer. With respect to reproductive and developmental effects, we do not know for sure. Little is known about reproductive and developmental effects and the role of environmental exposures from these disinfection byproducts. However, the evidence available at this time raises concern about a possible link between chlorinated drinking water and reproductive and developmental effects. Although uncertain, this evidence has prompted EPA to begin developing a new drinking water regulation (the Stage 2 Disinfection Byproduct Rule) that will provide an incremental step towards mitigating potential reproductive and developmental risks and further reduce risks from cancer.

5. I am pregnant. Should I stop drinking my tap water?

First, remember that drinking lots of liquids during pregnancy is very important. Keep following the advice that your health care provider has given you. The existing studies on reproductive and developmental effects from exposure to chlorinated byproducts are inconclusive. If chlorinated byproducts are ultimately shown to cause reproductive and developmental effects, these risks would probably be small relative to other potential risks such as smoking or alcohol consumption. Also, your particular drinking water utility may have low levels of chlorinated byproducts which may alleviate your concern. If you are concerned about the safety of your tap water, you can review the annual water quality report you receive from your utility or ask your water utility about the concentrations of total trihalomethanes (TTHMs) and haloacetic acids (HAA5) and compare them to the current standard (see below for question on current standard). For personal health advice, you should contact your health care provider. If you have other general questions you can contact the Safe Drinking Water Hotline at 1-800-426-4791.

6. Does a pregnant woman who receives drinking water from a private well need to worry about TTHMs?

Non-chlorinated private well water is unlikely to contain TTHMs or HAAs.

7. What would EPA recommend based on the current science?

To protect public health, EPA continues to strongly support both the disinfection of drinking water to reduce the risk of waterborne disease and the reduction of disinfection byproducts. If you are concerned about the safety of your tap water, you should review your annual water quality report you receive from your utility or ask your utility about the concentrations of TTHMs and HAAs in your system. For personal health advice, you should contact your health care provider. If you have other general questions you can contact the Safe Drinking Water Hotline at 1-800-426-4791.

8. How did the Agency weigh risks vs. benefits for disinfectant byproducts?

Disinfection byproducts are a special case because decreasing disinfection byproduct risk could increase risks from disease-causing microorganisms. Eliminating or significantly decreasing disinfection to stop disinfection byproduct formation would seriously compromise overall public health protection. The Agency's priority is maintaining protection from disease-causing microorganisms. However, there are a number of things that water systems can do to reduce the levels of disinfection byproducts in drinking water (such as decreasing the amount of disinfectant and removing as much organic material as possible prior to disinfection).

9. How does EPA regulate TTHMs?

EPA has worked collaboratively with stakeholders during the last 10 years in developing the current and forthcoming regulations for disinfection byproducts. These regulations will decrease risks from disinfection byproducts in drinking water nationwide. EPA's current standards for disinfection byproducts provide the safest balance between the need to disinfect drinking water while providing a healthy margin of safety to all, including our most vulnerable citizens.

The Agency first regulated TTHMs in 1979 at 100 ppb, or µg/L, for systems serving more than 10,000 people. The Agency revised this rule when it issued the Stage 1 Disinfectants and Disinfection Byproducts Rule (Stage 1 DBPR) in December of 1998. The Stage 1 DBPR is the first of a staged set of rules that will reduce the allowable levels of disinfectants and disinfection byproducts (DBPs; including TTHMs) in drinking water. The Stage 1 DBPR set the maximum contaminant level for TTHMs at 80 µg/L. This standard had to be met by the end of 2002 for systems serving 10,000 people and by the end of 2004 for systems serving less than 10,000 people.

EPA proposed the Stage 2 DBPR in August 2003. This rule will reduce peaks of TTHMs and other DBPs in the distribution system. EPA believes that this regulation will further decrease potential cancer, reproductive, and developmental risks. EPA intends to finalize the Stage 2 DBPR in 2005.

10. What research is EPA conducting with regard to health risks that pregnant women might be concerned with?

Epidemiology and toxicology research to evaluate this potential threat to pregnant women has become a major focus of the EPA’s drinking water health effects research program in recent years. This year, EPA is spending over $1 million on studies related to reproductive and developmental effects from drinking water contaminants. EPA is actively conducting research both in-house and collaboratively with other researchers. In addition, EPA has established many research partnerships to address this issue. We have collaborations with other federal organizations (such as the Centers for Disease Protection and Prevention (CDC)), research organizations (such as American Water Works Association Research Foundation (AWWARF)), and various universities across the country.

Additional details on the types of studies that are being conducted on DBP cancer, reproductive, developmental and other non cancer health effects may be found on the Drinking Water Research Information Network (DRINK) web site at http://www.epa.gov/safewater/drink.


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