A panel of scientific experts, convened July 24 by the National Toxicology Program (NTP), recommended that six chemical compounds known as haloacetic acids (HAAs) be classified in the Report on Carcinogens as reasonably anticipated to be carcinogens. HAAs are byproducts created when chlorine, chloramine, or chlorine dioxide are used to disinfect drinking water.
"Disinfecting water is essential for a safe drinking water supply," said NTP Associate Director John Bucher, Ph.D., "but reactions from the chemicals used can be complex, and we need to understand whether there are health implications from the disinfection process."
The recommendation was based on a scientific literature review conducted by the NTP Office of the Report on Carcinogens (ORoC). The literature review was part of a larger NTP effort to assess the potential toxicity of disinfection byproducts nominated by the U.S. Environmental Protection Agency (EPA).
The report will help EPA assess the effects of those byproducts on human health and determine whether drinking water regulations should be updated. Currently, the EPA Office of Water regulates the levels of five HAAs and some trihalomethanes in public drinking water.
Widespread exposure prompted review
Approximately 250 million people, or 80 percent of the U.S. population, are exposed to disinfection byproducts through public water supplies, according to EPA estimates presented during the review by Sanford Garner, Ph.D., from Integrated Laboratory Systems, Inc. Water from public supplies is also used to make a variety of drinks and products. Private drinking wells that are not disinfected do not contain these chemicals, he pointed out.
Although HAAs can form when chlorine, chloramine, or chlorine dioxide are used, these treatments provide lasting disinfection throughout a water system.
Treatment approaches such as ultraviolet light and reverse osmosis do not form HAAs. However, they are more costly and do not provide residual disinfection, especially on the scale of large water utilities.
Garner also noted that many water treatment facilities achieve levels below current EPA limits for the five HAAs that are regulated. Reaching such levels can be more difficult for smaller water treatment facilities that may have financial constraints and older infrastructure.
Evidence of potential carcinogenicity
ORoC reviewed the scientific evidence of potential carcinogenicity for 13 HAAs before recommending six as reasonably anticipated to be carcinogens. To determine which chemical compounds may contribute to cancer formation in the body, ORoC systematically examines experimental cancer studies in animals and humans, as well as mechanistic studies and studies that provide other relevant data.
In the case of the 13 compounds under review, human data were considered inadequate to evaluate them. However, ORoC found that animal studies reported cancerous tumors in multiple species or body sites after exposure to four of the HAAs — dichloroacetic acid, dibromoacetic acid, bromodichloroacetic acid, and bromochloroacetic acid.
For two others, chlorodibromoacetic acid and tribromoacetic acid, there was scientific evidence that the compounds could be transformed into carcinogenic substances through metabolism in the body.
(Virginia Guidry, Ph.D., is a technical writer and public information specialist in the NIEHS Office of Communications and Public Liaison and a regular contributor to the Environmental Factor.)