Long-term exposure to the by-products of chlorination in drinking water appears to raise the risk of one type of leukemia - chronic myeloid leukemia (CML) - but decrease the risk of chronic lymphoid leukemia (CLL) and other types, new research shows.

The findings, which appear in the American Journal of Epidemiology, are based on a population-based study comparing 686 leukemia cases and 3420 similar but unaffected “controls” for whom water quality information was available for 30 years or longer. The subjects were Canadian residents between 20 and 74 years of age.

Long-term exposure to the chlorination by-product triahalomethane at levels greater than 40 micrograms per liter raised the risk of CML by 72 percent, Dr. Patrick Levallois, from Institut National de Sante Publique in Sainte-Foy, Canada, and colleagues note.

By contrast, such long-term exposure cut the risk of CLL by 40 percent, the team found.

Further analysis showed a decreased risk of several leukemia subtypes as the duration of chlorination by-product exposure increased, the report indicates.

“Total trihalomethanes and bromodichloromethane may be particularly important in the etiology of CML, but the possible protective effect of chlorination disinfection by-products on CLL remains unclear,” the authors conclude. “Random error or selection bias could explain these results,” they say.

SOURCE: American Journal of Epidemiology, January 15, 2006.