By Kelly A. Reynolds, MSPH, Ph.D.

Disinfection byproducts (DBPs) in drinking water do not occur naturally but rather are the result of treatment additives. Linked to serious human health effects such as bladder cancer, miscarriages, stillbirths and birth defects, DBPs have had a major impact on source water selection, watershed management, municipal water treatment and drinking water distribution processes. Quantification of the health risks associated with consumption of DBPs has been inconsistent, suggesting a problem in the experimental research design. A new approach in the production of DBPs for laboratory study is expected to provide more robust results and help to provide long-awaited answers on the health effects of DBPs in drinking water.

Health effects of DBPs
More than three decades of research has suggested a link between chlorination of water and adverse human health effects. In 1976, the National Cancer Institute reported on the carcinogenic effects of drinking chlorinated water on the digestive and urinary tracts.1 This report, prompted the US EPA’s 1979 regulation of trihalomethanes (a primary class of DBPs) in drinking water, aimed at reducing the lifetime exposure to the contaminants. This prompted numerous epidemiological studies showing associations between DBP levels in drinking water and adverse human health effects. Many studies showed increased bladder cancer, stillbirths, miscarriages and serious birth defects while others showed no, weak or inconsistent associations with human health effects (see On Tap, June 2004 and August 2006).

No one knows how many cases of bladder cancer or pregnancy complications are caused by treated drinking water contaminants. The US EPA estimates, however, that over 260 million individuals are exposed to DBPs. In 2009, there were 70,980 new cases of bladder cancer (and 14,330 deaths) in the US.2 US EPA has projected that the current regulation aimed at reducing DBP exposures via drinking water will prevent approximately 280 bladder cancer cases per year with a fatality rate of 26 percent. Based on bladder cancer alone, an estimated monetary benefit of $1.5 billion (USD) is expected, not counting other pregnancy and fetal development benefits that may also occur. Of the six million pregnancies per year in the US, two million pregnancy losses occur, including 1.2 million abortions, 600,000 miscarriages, 26,000 stillbirths, and 70,000 losses due to other complications during gestation (i.e., ectopic or molar pregnancies).3 Thus, between 10 and 16 percent of pregnancies result in either a miscarriage or stillbirth. Most miscarriages occur in the first trimester of pregnancy and are considered to result from natural causes. Although considered a natural part of pregnancy, the common occurrence also suggests the possibility of a ubiquitous environmental cause.

Flawed research
Drinking water supplies are complex, ever-changing environments. As source water quality, supply demand and flow patterns change throughout the day, season and year, consider- able spatial and temporal variations in DBP levels are known to occur, making it difficult to relate monitoring and occurrence data to specific exposures in a population. Highly contaminated sites could be masked by evaluation of quarterly or annual averages of system-wide DBP levels. While possibly adequate for evaluating chronic health effects, use of average quarterly DBP monitoring values do not adequately represent high- and low- dose exposure frequencies that could lead to acute health effects. Previous studies were often limited to single-agent toxicological data when in reality, consumers are exposed to complex mixtures of DBPs at varying toxicities and times of exposure. These and other experimental design challenges may account for some of the variability in the epidemiological studies.

Regulatory controls
Rather than regulating individual contaminant species, the 1979 regulation of THMs was based on the collective concentrations of four THM species: chloroform, bromodichloromethane, dibromochloromethane and bromoform. Fast forward 19 years to when the US EPA expanded the regulation of DBPs to include the sum of five haloacetic acids (HAAs) in the Stage 1 Disinfectants/Disinfection Byproducts Rule. Controversy over the rule ensued, given that other known HAA species (i.e., not part of the regulation) constituted up to 50 percent of the overall HAA content detected in finished drinking water.4 Fast forward another eight years and the same concerns of limited inclusion of primary DBPs are evident in the 2006 Stage 2 Disinfectants/ Disinfection Byproducts Rule.5 This rule, however, targets public water systems identified as having the greatest risk. The first step is for systems to monitor their distribution system to determine where the highest levels of DBPs are likely to occur, and use those locations as continuous sampling sites for Stage 2 DBP Rule-compliance monitoring. This risk-based approach applies to approximately 75,000 US systems.

While some criticize the use of mixed species of DBPs in the regulations rather than individual risk evaluation, others point out the need for a more comprehensive evaluation of a broader range of mixed DBPs. Over 600 DBPs have been identified in drinking water; however, more than half remain unidentified.6 Whether measured in defined mixtures or individually, the health effects reported in epidemiological studies do not add up, indicating the need for improved epidemiological study design and/or the reevaluation of toxicological studies.

Improved design
Recently, a US EPA study involving researchers from four national laboratories and centers of the Office of Research and Development, along with collaborators from the water industry and academia, published a new procedure for producing mixed DBPs.6

The procedure involves concentration of natural source water using RO membranes and the addition of bromide followed by chlorination. The result is the formation of a more representative drinking water sample with a wider variety of mixed DBPs as found in real-world scenarios. Use of this formulated DBP concentrate is expected to improve toxicological studies (i.e., animal studies) and improve the risk assessment of DBPs when studied in their true form in nature—a widely mixed sample, including volatile and mixed-volatile compounds not previously preserved in laboratory stock samples.

In this improved sample creation, regulated DBPs and priority unregulated DBPs, and previously undetected and unreported haloacids and haloamides, are included, as well as non-halogenated DBPs such as nitrosodimethylamine (NDMA). Some of the DBPs found are known to be more potent human toxicants than most of those on the currently regulated list. In short, this whole-mixture sample better represents the water consumers are drinking and should improve the current toxicological database.

For accurate risk assessment, it is critical that compounds used in animal studies to quantitate the toxicological nature of an agent represent fully the public’s exposure. While the ’Four Laboratory Study‘ will help to eliminate major flaws of previous studies, the toxicology studies are years away from completion. We can expect that previous DBP toxicity data will be reexamined with the knowledge of increased exposures to a greater number of compounds that must be considered.

Importance of POU/POE treatment systems
Chlorination of drinking water supplies has been called by Life magazine one of the greatest public health discoveries in history. It is a highly effective disinfectant for most microbial pathogens and widely used around the world for effective drinking water treatment. Despite possible effects of DBPs, chlorination (along with filtration of drinking water) is largely responsible for the 50-percent increase in life expectancy over the last 100 years.7

History has shown the dangers of misguided concerns over DBPs and chlorination. In 1991, Peruvian officials, responding in part to concerns in the US related to DBP exposures in drinking water, decided to stop chlorinating the water supply. This decision resulted in a five-year epidemic of cholera that spread to 19 Latin American countries, resulting in nearly a million cases and 10,000 deaths.

Approximately 40 percent of US utilities have minimized their use of chlorine, and instead use chloramines (mixed chlorine and ammonia) in their distribution systems for control of THMs and HAA formation in the finished water. Chloramines are not as reactive as chlorine and thus do not produce the same type of byproducts. Chloramines do, however, present new risks of NDMA (a proven carcinogen) due to the biological process of nitrification (bacteria in the water utilize the ammonia) and increased lead and copper levels (due to a change in the water chemistry and increased pipe corrosion) in tap water.

DBPs are best controlled prior to their formation by eliminating the natural organic matter and bromide in raw-water sources. This will involve a widespread approach of source- water protection and management aimed at controlling algae growth, nutrient discharges and salt-water intrusion. Some of the municipal water treatment processes aimed at removing natural organic matter from water (i.e., coagulation, granular activated carbon adsorption, ultrafiltration and nanofiltration) lower THMs and HAAs but increase bromine-containing compounds, which may require more advanced treatments (i.e., reverse osmosis).4

Advanced treatment at the municipality may not be initiated, especially given the unknown health risks associated with low levels of DBPs in water. Changes in conventional processes have historically led to unforeseen consequences. As with so many contaminant risks in drinking water, DBPs are easily controlled at the point of use or point of entry. Simple granular activated carbon (GAC) systems remove many of the DBPs associated with both chlorine and chloramines. Reverse osmosis systems and other POU/POE treatment options are also available that can minimize the known and uncertain risks of drinking water consumption.


  1. National Cancer Institute (1976) Report on the carcinogenesis bioassay of chloroform. NTISPB-264-018. National Cancer Institute, Bethesda, MD.
  2. National Cancer Institute. U.S. National Institute of Health. http://
  3. American Pregnancy Association. Accessed: June 13, 2010.
  4. Singer, P.C. (2006) “Disinfection byproducts in drinking water: ad- ditional science and policy considerations in the pursuit of public health protection.” National Water Research Institute’s 2006 Clarke Lecture. San Juan Capistrano, CA. July 13, 2006.
  5. US EPA. Accessed June 13, 2010. Stage 2 DBP Rule. http://gov/safewater/disinfection/stage2/basicinformation.html
  6. Pressman, J.G., Richardson, S.D., Speth, T. F. et al. (2010) Concentration, chlorination, and chemical analysis of drinking water for disinfection byproduct mixtures health effects research: US EPA’s four lab study. Environmental Science & Technology, Epub ahead of print.
  7. Christman, K. (1998) The History of Chlorine. Water World, September 1998. Water Quality and Health Council.

About the author

Dr. Kelly A. Reynolds is an Associate Professor at the University of Arizona College of Public Health. She holds a Master of Science Degree in public health (MSPH) from the University of South Florida and a doctorate in microbiology from the University of Arizona. Reynolds has been a member of the WC&P Technical Review Committee since 1997. She can be reached via email at


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