Cancer myth: Chlorine and cancer

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Origin of the misconception

Chlorine (Cl) is a dangerous chemical. Inhalation of chlorine gas or drinking highly concentrated sources of chlorine (such as household bleach) can lead to vomiting, coma, and even death.[1] For this reason, many people fear that the chlorine in swimming pools and drinking water can be harmful for health, and cause cancer. This has been spread further by water filter manufacturers and makers of 'chlorine-free' pools, who often assert that chlorine can cause cancer.

Current evidence

Since 1849, when Dr John Snow first suggested that disease could be transmitted through drinking water, many methods have been used to ensure clean drinking water. Chlorination was first proposed in 1910 as a method for purifying water for troops in the field.

A consistent, safe supply of drinking water in the developed world is a major public health sanitation success. Millions of people still die from contaminated water in the third world. Without clean water, most people would not live long enough to get cancer.

Chlorine dissolves in water to form hypochlorous acid (HOCl) that partially breaks down again to form the hypochlorite ion (OCl-). Hypochlorous acid and the hypochlorite ion are toxic to microorganisms and disinfect drinking water.[2] Trihalomethanes (chloroform) and haloacetic acids are formed when chlorine reacts with organic matter in the water. Chloroform is a disinfectant by-product.[3]

The Australian Drinking Water Guidelines allow for up to 5 mg of chlorine per litre (mg/L) of drinking water. This is the same as the limit recommended by the World Health Organization (WHO). Concentrations found in Australian drinking water range from 0.1 mg/L to 4.0 mg/L, with a typical concentration of between 0.2 mg/L and 0.4 mg/L.[2] For comparison, the Western Australian Department of Health recommends maintaining chlorine in swimming pools over 2.0 to 3.0 mg/L depending on temperature.[4]

The Australian Drinking Water Guidelines report no adverse effects from the ingestion of chlorinated water. Long-term animal toxicity studies show that chlorine or its breakdown products do not act as carcinogens (cancer causing agents) or tumour initiators. According to the Australian Drinking Water Guidelines, there is very little risk from chlorine associated with drinking a lot of water. In one report, 150 people drank water with chlorine concentrations of 50 mg/L, during water mains disinfection, with no reported adverse health effects. Another study of military personnel, who often drink water with chlorine concentrations greater than 32 mg/L for extended periods, showed no adverse effects.[2]

The International Agency for the Research of Cancer (IARC) has evaluated the safety of chlorine in drinking water and concluded that there is insufficient evidence for its carcinogenicity (ability to cause cancer) in animals and humans. It has classified chlorine as neither a carcinogen nor a possible carcinogen.[5]

Swimming pools are chlorinated using hypochlorite salts (sodium hypochlorite or calcium hypochlorite). The safety of hypochlorite salts has also been evaluated by IARC, who have concluded that there is inadequate evidence for the carcinogenicity of hypochlorite in animals. A similar conclusion could not be reached for humans because of the absence of human studies.[5]

The safety of chloroform in drinking water has also been studied. Assessment is complicated because chloroform is present in air and food, and some people are exposed to chloroform at work. Also, the presence of chloroform in drinking water is associated with other disinfection breakdown products. For these reasons, the effect of chloroform in drinking water is hard to determine. IARC have concluded that chloroform is possibly carcinogenic to humans.[6]


There is no evidence for the myth that drinking chlorinated water or swimming in chlorinated pools can cause cancer. However, chlorine and chlorine gas can aggravate respiratory conditions and high concentrations of chlorine can lead to many health complications.

Further reading

Australian Drinking Water Guidelines
National Health and Medical Research Council


  1. International Occupational Safety and Health Information Centre (CIS), Chlorine, in International Chemical Safety Cards 2000, International Programme on Chemical Safety (IPCS) and European Commission (EC),.
  2. National Health and Medical Research Council (NHMRC), Australian Drinking Water Guidelines, National Health and Medical Research Council (NHMRC), Editor 2011, Commonwealth of Australia.
  3. US Environmental Protection Agency (EPA). Disinfection byproduct information. Information collection rule (ICR) 2 February 2008; Available from: [Accessed 2 February 2013].
  4. Environmental Health Directorate - Department of Health (WA). Keeping Your Swimming Pool and Spa Healthy: Environmental Health Guide. 2006; Available from: [Accessed 01 February 2013].
  5. International Agency for Research on Cancer. Working Group on the Evaluation of Carcinogenic Risks to Humans, Chlorinated Drinking-water; Chlorination By-products; Some Other Halogenated Compounds; Cobalt and Cobalt Compounds. IARC monographs on the evaluation of carcinogenic risks to humans ;. Vol. 52. 1991, Lyon: International Agency for Research on Cancer,. Available from [Accessed 01 February 2013].
  6. International Agency for Research on Cancer. Working Group on the Evaluation of Carcinogenic Risks to Humans, Some Chemicals that Cause Tumours of the Kidney or Urinary Bladder in Rodents and Some Other Substances. IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, ed. IARC. Vol. 73. 2006, Lyon, France: IARC. Available from [Accessed 01 February 2013].


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