The new study is the first to suggest that chlorine is harmful to humans when ingested or absorbed through the skin, according to study leader Cristina M. Villanueva of the Municipal Institute of Medical Research in Barcelona and her colleagues.
Chlorine itself is not harmful, but its byproducts increase the risk of cancer. Trihalomethanes are the most prevalent by-product, and they can be absorbed into the body through the skin or by inhalation. When THM is absorbed through the skin or into the lungs, they hold stronger carcinogenic properties because they arenít detoxified through the liver, Villanueva and her team found in their research.
Villanueva and her team surveyed 1,219 individuals with bladder cancer and 1,271 control individuals without the disease, polling them about their exposure to chlorinated water, including their bathing, swimming and tap water drinking habits.
The researchers also looked at the THM levels in the water systems of 123 municipalities.
People who live in households with more than 49 micrograms per liter of THM were at double the risk of bladder cancer versus households that have below 8 micrograms per liter of THM.
In industrialized countries, the common level is 50 micrograms per liter, the researchers note.
The researchers also found that use of swimming pools increased the risk by 57 percent and that people who drank chlorinated water held a 35 percent greater risk. Taking long showers and bathing also increases the risk in households that has water with higher levels of THM.
In the United States, an estimated 67,160 new cases of bladder cancer are expected to occur in 2007, and 13,750 deaths, according to statistics from the American Cancer Society.
"If confirmed elsewhere, this observation has significant public health implications in relation to preventing exposure to these water contaminants," the researchers said in their report.
The study was published in the January issue of the American Journal of Epidemiology.