The additional rat study will extend the exposures to lower doses of MX to better characterize the shape of the dose-response curve, especially in the low dose region.
The mouse study will be complementary to the Finnish and NTP rat studies, providing data in both sexes of two species for additional use in assessing human risk.
The National Toxicology Program, headquartered at the National Institute of Environmental Health Sciences, coordinates the study of the adverse effects of chemicals within the Department of Health and Human Services.
MX and five other by-products of drinking water disinfection will be studied. The Environmental Protection Agency proposed the six to NTP for study as part of the EPA-National Institute of Environmental Health Sciences’ Drinking Water Disinfection By-Products Initiative.
The further study of MX became of particular interest today with a report in the current (June 18, 1997) Journal of the National Cancer Institute of the results of the rat study of Hannu Komulainen et al of the National Public Health Institute, Finland. (Assistance in analyzing the tumors was provided by Ronald Herbert, D.V.M., Ph.D., of the U.S. NTP and NIEHS.)
MX is among the by-products of reactions between chlorine and organic matter suspended in the water being disinfected.
In an accompanying editorial in the Journal, NIEHS and EPA scientists compared the cancer potency and occurrence in drinking water of MX to two other carcinogenic by-products called trihalomethanes that EPA regulates and monitors. Although MX was found to be a more potent carcinogen, the limited monitoring done thus far suggests it is present at much lower concentrations than the others. Thus, according to the editorial, "the combination of cancer potency and exposure indicates that potential risks for MX may be about 1/10 that of the trihalomethanes."
The editorial by Ronald L. Melnick, Ph.D., and Gary A. Boorman, Ph.D., DVM, both of NIEHS, and Vicki Dellarco, Ph.D., of EPA, says that the cancer risk associated with lifetime exposure to the highest reported level of MX in drinking water might lead to two additional cases of cancer per million people per year.
However, the authors add that water chlorination is one of the major disease prevention achievements of the 20th century.
"Stopping water chlorination in the absence of an equally effective disinfection program is not a sensible choice," they say, noting that Peru had 300,000 cases of cholera when it interrupted adequate disinfection of drinking water in the early 1990s.
The editorial writers say the "potential public health concerns" raised by the rat study should be taken seriously and that better estimates of MX concentrations in U.S. drinking water supplies are needed.
Melnick, Boorman and Dellarco point out, "The potential risk from MX in drinking water must be weighed against the benefits of chlorination as a proven disease prevention strategy, as well as the potential risks of water disinfection by-products that are considered in EPA's drinking water regulations."
After a German demonstration in 1892 by Robert Koch of the value of filtration against cholera, and the Jersey City, N.J., demonstration of chlorination in 1908, the combination of filtration and chlorination went into wide use and deadly intestinal diseases went into dramatic decline. Wheeling, W.Va., which had long had 155-200 cases of typhoid per 100,000 population, saw cases drop to just seven in the first three month of 1919 -- and then climb again when chlorination was discontinued for a short time.
The effect of chlorination on typhoid was only a part of the picture. One estimate is that for every waterborne typhoid death there had been several from other waterborne causes -- before water treatment.