Children, the environment and cancer

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Specific environmental contaminants - IV

Chlorinated water

Risks of water chlorination

The majority of scientific studies have concluded that exposure to chlorinated water over long periods of time slightly increases the risk of bladder cancer. Some studies have found links to colorectal and possibly some other cancers as well. In a review conducted by a working group made up of scientific experts in 1998, 10 of 11 studies found an increased risk of bladder cancer. Of these, 5 were statistically significant - meaning the results were unlikely to be due to chance. All 5 found an increased risk of bladder cancer. This expert working group concluded that it was possible or probable that chlorinated water caused an increased risk of bladder cancer. A more recent review of the best available research confirmed the association with bladder cancer, especially among men.

Though chlorine is a strong chemical, chlorine itself is not the main cause for concern in drinking water. When chlorine combines with organic matter (such as dead leaves and soil) present in untreated water, it forms new chemicals that remain in the water. These are called chlorine disinfection “by-products”. The amount and types of by-products that are formed depends on several factors, including the quality of the untreated water and the amount of chlorine used to treat the water.

Several chlorination by-products are considered possible cancer-causing substances. Trihalomethanes (THMs) and haloacetic acids (HAAs) are the most common chlorination by-products. Animal studies on individual THMs and HAAs have found that some can cause cancer in laboratory animals.

THMs are the most-studied group of chlorination by-products. Chloroform is an example of a THM, and the International Agency for Research on Cancer (IARC) considers it a possible cancer-causing substance in humans. People can be exposed to low levels of THMs not just from drinking water, but also from breathing in water vapour during showering, bathing, swimming and other uses.

Though the scientific community has more information on THMs than on other chlorination by-products, many believe that other chlorination by-products may also be leading to the increased risk of cancer. More research is needed.

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