NIRAS - Aurélie

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Chlorination and health problems



One of the first known uses of chlorine for water disinfection was by John Snow in 1850 in London after an outbreak of cholera. Then chlorine has met great success in reducing cholera, typhoid, dysentery and hepatitis A, and so lengthening the life-expectancy of humans. That is why chlorine is one of the most widely used disinfectants. It is used to kill bacteria and other microbes from water to avoid diseases caused by micro-organisms.

When chlorine is added to water, this reaction takes place:


 Hypochlorous acid is the active, killing form of chlorine. Depending on the pH value, hypochlorous acid partly expires to hypochlorite ions (OCl-):



These molecules (HOCl and OCl-) are called free chlorine molecules. So, they are available for disinfecting. They kill microorganisms by slashing through the cell walls and destroying the inner enzymes, structures and processes. The microorganisms will either die or suffer from reproductive failure. Hypochlorous acid is more reactive and is a stronger disinfectant than hypochlorite ions. In fact, it is split into hydrochloric acid (HCl) and atom air oxygen (O). The oxygen atom is a powerful disinfectant.

However, when chlorine is added to water for disinfection purposes, it usually starts reacting with dissolved organic and inorganic compounds in the water. These compounds are called “active chlorine compounds”. For example, chlorine can react with ammonia (NH3) to chloramines. After that, chlorine can no longer be used for disinfection because it has formed other products. Consequently, the dose of chlorine to apply must be high enough for a significant amount of chlorine to remain in the water (after reaction with dissolved compounds). The chlorine enquiry is a function of the amount of organic matter in the water, the pH, the contact time and the exposure.

Some researchers say that chlorine has some very serious health consequences when used as a sanitizer in swimming pools. Effects of chlorine on human health depend on the concentration of chlorine present in air, and on the duration and frequency of exposure. They also depend on the health of an individual and the environmental conditions during the exposure. Chlorine enters the body breathed in with contaminated air or when consumed with contaminated food or water. When small amounts of chlorine are breathed in during short time periods, it can affect the respiratory system (coughing, chest pains, and fluid accumulation in the lungs). Chlorine can also cause skin and eye irritations.

In fact, it is not chlorine that is dangerous for the health but its by-products: chloramines and trihalomethanes (THMs).

The chloramines are responsible for the smell and the irritant properties of the swimming-pool air. Thus, when swimmers complain of too much chlorine in the pool area, the real problem is too little free chlorine in the water (too much chlorine has been converted to chloramines). These chloramines, monochloramine (NH2Cl), dichloramine (NHCl2) and trichloramine (NCl3) are generated from the reaction of HOCl with ammonia and amino-compounds that originate from sweat and urine of the swimmers.



They are (especially the trichloramine) quite volatile and they partition easily from the water into the air. The main factors that determine the production and the air levels of these by-products are: the degree of water chlorination, the contamination of the water by nitrogen-containing compounds (which depends on the number of bathers, their behaviour and hygiene), water temperature and air recirculation. Some studies have identified trichloramine as the probable cause of respiratory problems and occupational asthma in swimmers and pool attendants, at indoor swimming pools.

The trihalomethanes are also by-products of chlorination which can be harmless for the human health. They come from the substitution of the hydrogen atoms in methane (CH4) with the halogen atoms of chlorine or bromine. The methane is a natural product of our bodies and the products of this substitution called “THMs” are for example:

ü      CHCl3      Chloroform or trichloromethane

ü      CHBr3      Bromoform

ü      CHCl2Br  Bromodichloromethane

ü      CHClBr2  Dibromochloromethane

Like the chloramines, the rate of THM production is a function of the number of swimmers, the total organic carbon content of the water, the pH and the water temperature. Besides, chloroform can be measured in the blood or urine of regular swimmers.


Several health researches have been carried out in chlorinated swimming pools. In a Dutch research in 2001, swimming pool attendants were interviewed and it showed that a lot of employees suffer from forgetfulness, fatigue, chronic colds, voice problems, eye irritations, headache, sore throat, eczema and frontal sinus inflammation. Fertility problems were also mentioned. All problems are probably caused by working conditions: work during long hours in a warm and humid environment, and with exposure to chemical substances. And when swimming pool attendants do not work, health problems vanish. Besides, in a French research undertaken by INRS, it has been shown that the first workers’ complaints appeared for a chloramine concentration of 0,5 mg/m3. All agents were complaining for a concentration of 0,7 mg/m3.


Epidemiological researches have shown that competitive swimmers have more bronchial hyperresponsiveness or asthma than other sportsmen. But the complaints disappear when they swim in outdoor swimming pools, because wind removes gasses from the air above the pool. Besides, hypochloric acid with sunlight can cause the pH value to drop. And when it drops below 3,6 swimmers can suffer from dental abrasion: tooth enamel dissolves and the teeth become brittle and sensitive.


A survey with schoolchildren has taken place in Belgium where an increase in asthma in Western Societies was observed. The scientists observed that serum concentrations of pneumoproteins were elevated in children who frequently attend chlorinated swimming pools as compared to control children. Swimming attendance explained a large part of the variance in serum pneumoproteins, when other possible factors were taken into account. The researchers considered that such increased level of serum pneumoproteins reflected increases in the alveolocapillary permeability and that this was an early indication of lung injury and risk of disease. Besides, it is important to take into account that a one year old child would absorb in one hour in a chlorinated swimming pool on average three times more chloroform than a swimming teacher in one week. 


As a consequence, it is necessary to control the formation of the chlorine by-products by having swimmers wash before entering, by treating the water, by maintaining adequate water exchange rates, and by constant dilution of the air above indoor swimming pools via good ventilation practises. Another alternative could be to use another disinfection technique.

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Started by NIRAS supervisor Sergio Fox on 27th March 2006.