chlorination of water may lead to thyroid and liver cancers, according to a study conducted by H Komulainen and colleagues at the National Public Health Institute, Kuopio and Institute of Bio-medicine, University of Helsinki, Finland. When a by-product of chlorine called mx was given to rats for two years in drinking water, it resulted in cancer at multiple sites, in both the sexes. They claim while the carcinogenic effects of mx in bacteria and mammals were known, their study has first time shown its cancer causing potential in animals (Journal of National Cancer Institute , Vol 89, No 12).

Chlorine reacts with humic substances in water and produces a large number of halogenated organic by-products. These include chlorinated and brominated trihalomethanes such as chloroform or bromodichloromethane and haloacetic acids that are abundantly found disinfection by-products as well as 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5 h )-furanone (mx), which is in lower concentrations.

The World Health Organisation (who) admits that data available on toxicity of mx are not adequate to determine its proper levels in drinking water. The who guidelines state that an overall assessment of the impact of disinfection on public health must consider not only the microbiological quality of treated water, but also the toxicity of disinfection and their reaction products. The reason is that any reactive chemical agent such as chlorine forms new compounds that may lead to long-term harmful effects.

According to Ronald L Melnick at the National Institute of Environmental Health Sciences, Research Triangle Park, N Carolina, it is important to strike a balance between potential health risks caused by mx as against the risks due to other disinfection by-products. Studies had earlier shown that trihalomethanes is carcinogenic in rats and mice. The us Environment Protection Agency (epa) has set a maximum contaminant level (mcl) for this compound.

When the carcinogenicity of mx was compared with chloroform, another drinking water disinfectant by-product, the cancer potency of mx was found to be 170 times more than chloroform. It is necessary to study other chemicals that contribute to cancer risk associated with chlorination. The epa always evaluates microbial and chemical risks for various disinfections for drinking water and periodically issues revised guidelines.

The agency is exploring methods to reduce disinfection by-product formation by either removing precursors such as total organic carbon prior to chlorination or using alternative disinfectants. In this regard, the National Institutes of Health, Bethesda has launched a two-year programme to study carcinogenic potential of mx and other chlorine by-products on humans.