An early examination of the geographical distribution of cancer rates across the U.S. found a link between chlorination and high rates of colon, bladder and rectal cancer. More recently, a study by researchers at the University of North Carolina School of Public Health found an association between chlorinated water drinking and retarded fetal growth. Another study at the National Cancer Institute (NCI) showed that men who smoked and also drank chlorinated water for many years suffered twice the risk of bladder cancer as smokers who drank unchlorinated water for the same period.

Many millions of people use chlorinated water, so even small carcinogenic effects could kill thousands. As a result, U.S. and European governmental agencies are setting tougher standards to reduce the level of chlorination byproducts in drinking water.

There are alternative techniques to purify water, such as ultraviolet light or ozone, but these methods can be expensive, and only chlorination leaves residual amounts of disinfectant to protect the water once it leaves the processing plant.

Even though chlorination has its dangers, from a public health standpoint the risks from not using chlorination are far worse. For example, much of Peru does not chlorinate its water, and a recent outbreak of cholera sickening 300,000 people was blamed on contaminated water.