Does water chlorination cause cancer?

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Chlorination of drinking water as means of disinfection[1] was started in 1905. Hints of the existence of disease-causing agents in drinking water were already seen during the cholera epidemic in London in 1854, at a time bacteria were not yet recognized as causing infections. Certainty was achieved during the cholera epidemic in Hamburg in 1892, when the pathogenicity of bacteria was already known. Hamburg and Altona, twin cities, both took their drinking water from the river Elbe, but Altona filtered their water through sand filters while Hamburg used untreated water. The cholera epidemic affected almost exclusively the inhabitants of Hamburg: 17,000 people suffered the disease and 8,606 of them died.

Water chlorination has been called one of health promoting measures responsible for saving most lives. It was only relatively recently that suspicions were raised about possible health problems associated with chlorination. There was an alarm in the United States in the 1970s when high concentrations of chloroform and related chlorinated organic compounds were found in the water of the Mississippi River. Initially these trihalomethanes were thought to be industrial emissions, but subsequently they were found to increase due to chlorination of drinking water. Since then, chlorination side products have been investigated all over the world.

Risk of cancer

When the mutagenicity[2] of drinking water was studied during the 1980s, other chlorinated compounds were found in addition to the trihalomethanes. One such group, chlorinated furanones including "MX compound", has been actively investigated. The potential cancer risks of MX and other organic chlorinated compounds formed during water chlorination have been studied especially in Finland very thoroughly. Relatively large populations have been studied to reveal whether or not the established mutagenicity is associated with a true cancer risk. Mechanistic studies have attempted to find out the ways by which MX might cause tumours.

The long-term consumption of mutagenic water seemed to increase the human risk of some cancers to some extent. The most problematic tap waters are those derived from humus-rich surface water (from lakes and rivers with marshes and coniferous forests in their drainage basins). There are hundreds of impurities in mutagenic drinking water; the main classes are trihalomethanes, chlorinated acetic acids, and chlorinated furanones. It was calculated that when the situation was at its worst between the 1950s and 1970s, mutagenicity was associated with 50–100 cancers in the population of 5 million Finns. Since then, water purification technology has been dramatically improved, and additionally the use of ground water has increased. This has clearly decreased the mutagenicity levels, but in regions which draw water from coniferous forest areas this might still be a problem.

There is a level of impurities beyond which it is difficult to go even with the improved technology. The problem is now well known and characterised, and the health risk can be minimized by various technical solutions. The necessity of disinfection has been proven repeatedly. An example of warning against playing with fire was the cholera epidemic which occurred in Peru in 1991. Worried by an American cancer study, the Peruvian authorities stopped chlorination in several towns, and result was an epidemic of cholera affecting hundreds of thousands of people, with almost 9,000 deaths.


Many alternative technologies have been proposed to replace chlorine disinfection. Usually water is disinfected twice: before purification to prevent slime growth at the waterworks, and after purification before pumping the water into the pipelines. The first, prechlorination step, is often replaced by ozone treatment, and ultraviolet irradiation has also been used to kill microbes. Both of these techniques suffer from the disadvantage of providing only a short-lived effect.

In order to achieve continuous protection in the pipelines, chlorine is needed, but it is now most often used in more subtle forms than chlorine gas. Alternatives are chlorine dioxide and chloramines. These release chlorine slowly also in the pipelines and guarantee a germ-free water delivery to the customers. It is also essential that any organic compounds such as humus are effectively removed before disinfection to eliminate the raw material needed for the creation of organic chlorination side products.

Water disinfection is a typical case of balancing benefits and risks. The chemicals used to achieve disinfection have their own risks, but these risks can be considered low in comparison with the microbial risks associated with the absence of disinfection.

Notes and references

  1. Disinfectants are used to remove non-living factors and materials produced by microbes causing disease. Usually they do not destroy germs or the most resistant organisms thus differing from sterilization, which is done by using an autoclave and a 120 ºC temperature or a long boiling time.
  2. See the chapter "Mutagenesis, carcinogenesis and other scary words – what do they really mean?"

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