Chlorination is a process in which toxic chlorine is used to disinfect water.   Both chlorine and certain by-products of the process are toxic.   Trihalomethanes, a major by-product, has already known to raise cancer risk.

The study led by Dr. Cristina M. Villanueva of the Municipal Institute of Medical Research in Barcelona and colleagues was meant to investigate a possible association between bladder cancer and exposure to THM in chlorinated water.

For the study, researchers enrolled 1,219 men and women with bladder cancer and 1,271 control individuals who did not have the disease.   They surveyed the participants for their exposure to chlorinated water through drinking, swimming, showering and bathing. They also tested THM in 123 communities involved in the study.

People exposed to household water with more than 49 micrograms per liter of THM, which is common in industrialized countries, were at 200 percent higher risk of bladder cancer compared to those who exposed to household water with less than 8 mcg per liter, according to the study.

Those who drank chlorinated water were 35 percent more likely to develop bladder cancer than those who did not. Swimming in chlorinated water was linked to a 57 percent increased risk of the disease.   In addition, taking longer showers or baths and living in municipalities with higher THM levels were also at increased risk of cancer.

THM, which can be absorbed through the skin or by inhalation, may have a powerful carcinogenic effect than others as unlike others it does not go to the normal detoxification process in the liver, according to the researchers.

The findings suggest for the first time that by-products induced by chlorination can be harmful when they are ingested, inhaled or absorbed through the skin, said Dr. Villanueva.

"If confirmed elsewhere, this observation has significant public health implications in relation to preventing exposure to these water contaminants," the researchers write.