Although there is no doubt that chlorination of drinking water has reduced water-borne diseases to a minor threat for many of us in the west – there is mounting evidence that this benefit is not without considerable cost.

Chlorine - Boon or bane?

As I write this article ensconced in a hotel room in Kolcata (formerly Calcutta), amidst a visit to Ayurvedic interests in India, the importance of methods of controlling water-borne pathogens couldn’t be more at the forefront of my mind. The World Health Organization (WHO) estimates that as much as 80% of all diseases and over one-third of deaths in developing countries are caused by the consumption of contaminated water and, on average, as much as one-tenth of each person’s productive time is sacrificed to water-related diseases.[1] Diarrhoeal diseases linked with unsafe water are a primary cause of morbidity and mortality in infants and young children in developing countries. Across the globe, the WHO estimates that 1.8 billion episodes of childhood diarrhoea occur annually, mostly in developing countries. This contributes to the death of more than 3 million children and one million adults a year.[2]

In contrast, water-borne diseases are infrequent in the developed world, primarily thanks to widespread chlorination of our water supply. This technological development, which has been applied to virtually every municipal water source in the industrialised world since the 1920s, is often hailed as yielding one of the greatest public health victories of the last 100 or so years.[3 ]

Although there is no doubt that chlorination of drinking water has reduced water-borne diseases to a minor threat for many of us in the west – there is mounting evidence that this benefit it not without considerable cost. In this second article on drinking water, I have chosen to look more closely at health issues relating to the chlorination of drinking water, given that chlorine is far and away the most widely used disinfection agent. I will focus in particular on the health implications associated with chlorination by-products which are formed when chlorine reacts with organic compounds in water, rather than on chlorine itself. These by-products which include compounds belonging to the trihalomethane (THM) group, have increasingly been associated with cancer and adverse reproductive outcomes.[3]

History of chlorination

Early efforts aimed at making drinking water safe in Europe, during the late 1800s, centred on physically filtering out bacteria and other pathogens through sand. Sand filtration became the main method of water treatment in the early twentieth century after German microbiologist Robert Koch, who isolated Vibrio cholerae, the bacterium which causes cholera which had plagued European city-dwellers for centuries, showed that the bacterium could be filtered out through sand. Across the Atlantic, however, it was typhoid (caused by another bacterium, Salmonella typhi) that was the most important threat in drinking water, but partial mitigation was also achieved via sand filtration.

Although this rather crude method, which is still used in water treatment facilities today, provided some relief for Europeans and Americans, it wasn’t until chlorination was introduced as an adjunct to sand filtration in the early 1900s that these diseases were more or less eliminated from the western world.

However, its long history of use has brought with it new problems, including the development of chlorine tolerance or even resistance by certain pathogens, which require higher and higher levels of chlorination to achieve control. Furthermore, the spotlight is increasingly on chlorine and other halogens (such as fluoride) because of the growing body of evidence suggesting direct or indirect health risks. However, risk/benefit assessments, despite being fraught with confounding problems and other difficulties, continue to be used to justify use of chlorine given its low cost, and indications that the health risks caused by waterborne diseases (which include increased risk of cancer) considerably outweigh those of chlorine and its by-products.

Of the four key disinfection agents used, chlorine, chlorine dioxide, ozone and chloramine (chlorine reacted with ammonia), chlorine’s effectiveness (in the higher dose ranges) compares well with the best of the bunch, ozone, except in the case of treating protozoa like Cryptosporidia and Giardia, when it is comparatively ineffective.[4] The use of free chlorine as a disinfection agent, added as chlorine gas, or as sodium hypochlorite (bleach) or calcium hypochlorite, far outstrips the use of any other disinfection agent.

Chlorine – what happens when it’s added to water

Chlorine is generally added to the water supply to control pathogens with the aim of providing a level of free chlorine of at least 0.5 mg per litre (mg/L). Interestingly, levels of 0.6 mg/L or more may provide problems of acceptability by consumers on the basis of taste, while the WHO Drinking Water Guidelines (which many countries have adopted) specify that dosages 10 times greater than the target dose (5 mg/L) should not be exceeded and are deemed safe.[5]  However, mounting evidence (see below) suggests that where organic components (e.g. bromates, fulvates or organic matter) are present in water, such levels would be far from safe given the potential for lifetime exposure to significant levels of disinfection by-products (DFBs). These include trihalomethanes (THMs), haloacetic acids (HAAs) and haloacetonitriles (HANs), which have been associated with mutations, cancers and reproductive effects in both animal and human studies.

When chlorine is added to water it hydolyses (reacts with water) more or less completely to form hypochlorous acid (HOCl) as long as the pH is greater than 4 and the chlorine dose does not exceed 100 mg/L (20 times over the WHO guideline threshold). Hypochlorous acid under more alkaline conditions dissociates to the hypochlorite ion (OCl-), which is a considerably less efficient disinfection agent than hypochlorous acid itself. Under alkaline conditions (pH of 9), hypochlorite ions become the dominant species, while at a more acidic pH of 6.5, only about 10% will dissociate to the hypochlorite ion. Accordingly, chlorine will be considerably less efficient (but safer) as a disinfection agent in alkaline waters.[3]

Research has also shown that exposure to THMs increases dramatically (by at least 50%) in hot compared with cold water, with the by-products being absorbed both through the skin and by inhalation.[6] This suggests that exposure to THMs from showers and baths may be at least, if not more, important than exposure from water consumed orally (in drinks and foods). Chloroform, exposure and estimated internal dose due to inhalation and dermal absorption of a 10 minute shower or a half hour bath are equivalent to the dose from ingesting 2 litres of tap water.[7] Uptake is temperature dependent, so that absorption through the skin while bathing is 30 times greater in water at 40°C compared with 30°C.[8] Swimming provides a source of uptake; a one hour swim can result in a chloroform dose of 65 μg/kg, 141 times greater than that received from a 10 minute shower.[9]

Disinfection by-products associated with water chlorination

The first suggestions that chlorination of water might increase cancer risk were proposed during the 1970s, and this has led to further research on risk potential and the discovery that other adverse effects, particularly reproductive ones, might be important.


Feeding studies first showed that THMs could cause tumours of the liver, kidney and intestines in rodents during the 1970s.[10] A 1992 meta-analysis by Morris and colleagues from the Division of Biostatistics at the Medical College of Wisconsin, pooled data from 10 relevant case-control and cohort studies, took into account confounding factors and concluded that exposure to chlorination by-products in drinking water causes a 10-40% increase in bladder and colorectal cancers.[11]

However, since only three of these studies included exposure data, the Iowa Womens’ Health Study was established, using a cohort of 28, 237 post-menopausal women, to better assess this association. The results were published by Timothy Doyle and colleagues from the Division of Epidemiology, School of Public Health, University of Minnesota in 1997 and provided conclusive evidence that women who lived in communities with higher levels of chloroform (one of the most important THMs) in drinking water, were at “significantly increased risk of cancer, particularly colon cancer”.[12] This cancer happens to be the most common cancer present in both men and women in the UK.

Reproductive effects

In 2000 a group of statisticians at Imperial College London, led by Mark Nieuwenhuijsen, reviewed relevant toxicological and epidemiological evidence on the potential role of chlorination by-products in the induction of adverse reproductive effects. Effects that have been associated with chlorination by-products include spontaneous abortion, stillbirth, reduced birth weight and survival, developmental disabilities and congenital malformations of the cardiovascular and neurological systems (e.g. neural tube defects such as spina bifida).[13] The scientists showed that exposures from showers, baths, drinking water, drinks and foods caused significant absorption of chlorination by-products and these may contribute to increased risk of a diverse range of adverse reproductive effects. However they also demonstrated that the available research is inconclusive and sometimes contradictory, given that it is difficult to compare studies as most use chloroform as the key marker for chlorination by-products and different mixtures of by-products under different conditions may cause significant variations in risk.

What the UK authorities say about chlorination by-products

In a letter  from the Drinking Water Inspectorate (DWI) to sewerage companies in 1999, the DWI and Department of Health appears to have done everything it can to cover themselves by noting possible risks as well as the need to minimise exposure, while at the same time avoiding the difficult decision of banning chlorination. Michael Rouse, Chief Inspector from the DWI, quoting the Committee on Carcinogenicity of Chemicals in Food, Consumer Products and the Environment (COC)  said:

“Overall, the further epidemiological studies fail to provide persuasive evidence of a consistent relationship between chlorinated drinking-water and cancer. It remains possible that there may be an association between chlorinated drinking water and cancer which is obscured by problems such as the difficulty of obtaining an adequate estimate of exposure to chlorination by-products, misclassification of source of drinking water (including the use of bottled water), failure to take adequate account of confounding factors (such as smoking status), and errors arising from non-participation of subjects…. "We therefore consider that efforts to minimise exposure to chlorination by-products remain appropriate, providing that they do not compromise the efficiency of disinfection of drinking-water."

In my view, the COC’s assessment seems to have ignored the crucial Iowa Womens’ Health Study, but, cleverly, in their terms, this one major study does not provide evidence of a “consistent relationship” given that there are no other directly relevant of this nature. Consumers can make up their own minds about what this COC statement means.

How to reduce your risk

While risk assessments continue to favour chlorine as the agent of choice in disinfection because of its low cost and the concomitant high risk of disease from waterborne pathogens, it is unlikely that we will see elimination of chlorine and its by-products from our water supply in the near future. Although on the basis of present scientific knowledge ozonation appears more favourable, some argue that this is only because ozonation by-products have been less well studied than chlorination ones. Ultra-violet also holds promise, but cost effective treatment methods using ultra-violet disinfection have yet to be developed for large scale water treatment.

The WHO provides an interesting perspective on chlorination. It indicates that chlorination of water at point of use in developing countries could lead to a 35-39% reduction in diarrhoeal episodes, while the promotion of hygiene practices such as hand-washing would contribute to a reduction of 45%!  Since risk is directly proportionate to dosage, it is important to consider the variety of ways in which exposure to chlorination by-products can be reduced. I have listed some of the most important ways individuals can reduce their exposure below:

  • Avoid having very hot, long showers and baths, unless you have installed a full-house filtration system (such as reverse osmosis [RO]) which will eliminate the majority of DBPs
  • Use an activated carbon filter attached to your shower head to reduce levels of chlorine when showering
  • Swim in salt-water or ozone-disinfected swimming pools wherever possible
  • Filter your tap water before drinking it, using a system known to reduce concentrations of chlorination by-products, such as RO. Jug filters (containing activated carbon filters) reduce chlorine concentrations in water, but may have little effect on some of the by-products.
  • Drink bottled water from a good, natural source, but ensure it's from a source close to you and not from the other side of the world. See UNISON protest reported 21 August 2008 on this subject
  • Use DFB-free water for all drinking and cooking purposes, and where possible, for washing and swimming as well
  • When dishwashing in unfiltered tap water, use rubber gloves

What is in your drinking water?


United Nations Conference on Environment and Development (UNCED). Agenda 21: Programme of Action for Sustainable Development. Chapter 18. Protection of the Quality and Supply of Freshwater Resources, 1992.

World Health Organization. The World Health Report 2006. Bridging the Gap. WHO, Geneva, 2005.

Galal-Gorchev, H. Chlorine in water disinfection. Pure & Appl. Chem., 1996; 68: 1731-1735.

Regli S et al. In: Safety of Water Disinfection: Balancing Chemical & Microbial Risks (Ed. Craun GF), pp. 39-80. ILSI Press, Washington, DC, 1993.

Clark RM et al. In: Safety of Water Disinfection: Balancing Chemical & Microbial Risks (Ed. Craun GF), pp. 181-198. ILSI Press, Washington, DC, 1993.

World Health Organization. Rolling Revision of the WHO Drinking-Water Guidelines. WHO, Geneva, 2004.

Gordon SM, Wallace L, Callaghan P, et al. Effect of water temperature on dermal exposure to chloroform. Environ Health Perspect, 1998; 106: 337–45.

Gordon SM, Wallace L, Callaghan P, et al. Effect of water temperature on dermal exposure to chloroform. Environ Health Perspect, 1998; 106: 337–45.

Levesque B, Ayotte P, LeBlanc A, et al. Evaluation of dermal and respiratory chloroform exposure in humans. Environ Health Perspect, 1994; 102: 1082–7.

Doyle TJ, Zheng W, Hong CP, Sellers TA,  Kushi LH, Folson AR. The association of drinking water source and chlorination by-products with cancer incidence among postmenopausal women in Iowa: a prospective cohort study. Am J Public Health, 1997; 87: 1168-1176.

Nieuwenhuijsen MJ, Toledano MB, Eaton NE, Fawell J, Elliott P. Chlorination disinfection byproducts in water and their association with adverse reproductive outcomes: a review. Occup Environ Med, 2000; 57; 73-85.