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Chlorine and sodium hypochlorite hazards and risks: no need for further risk reduction measures

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Context - Chlorine and sodium hypochlorite are widely used for disinfection and bleaching, among other things to disinfect drinking water.

It appears that the usage guidelines recommend since several years are still adequate in 2017.

This is a faithful synthesis and summary of several scientific consensus reports. For the full list of sources, refer to the references.

  • Source document:WHO (2002) / IPCS (EU (2008) / (2000) / ECHA (2009)
  • Summary & Details: GreenFacts
Latest update: 5 May 2017


Chlorine is produced in large quantities for use as disinfectants and bleach for both domestic and industrial purposes. It is also widely used to disinfect drinking-water and swimming pool water and to eliminate bacteria and odours in the food industry. When chlorine gas is used for disinfection in swimming pools and in drinking water treatment, it is added to water where it is transformed in particular into sodium hypochlorite and hypochlorous acids. All different forms exist in equilibrium with each other, the relative amounts varying with the acidity level (pH) of the water. They are rapidly degraded in presence of organic matter and therefore do not not bioaccumulate and do not persist in the environment.

As stated by the International Programme on Chemical Safety (IPCS), the noteworthy biocidal attributes of chlorine have been somewhat offset by the formation of disinfectant by-products of public health concern such as chloroform during the chlorination process. As a consequence, alternative chemical disinfectants, such as ozone (O3), chlorine dioxide (ClO2) and monochloramine are increasingly used; however, each of them has been shown to form its own set of by-products.

What are the overall conclusions of the human risk assessments related to exposure to chlorine and sodium hypochlorite?

The overall conclusion of the most recent assessments is that there is no need for further information and/or testing and no need for risk reduction measures beyond those, which are being applied already for the protection of human health and the environment.

The IPCS report1 insisted on the fact that the risks to health from by-products at the levels at which they occur in drinking-water are extremely small in comparison with the risks associated with inadequate disinfection. It is important that disinfection would not be compromised in attempting to control such by-products. Where local circumstances require that a choice be made between microbiological limits or limits for disinfectants and DBPs, the microbiological quality must always take precedence. The report concluded that efficient disinfection must never be compromised.


How are we exposed to chlorine and sodium hypochlorite?

The major routes of exposure to chlorine and sodium hypochlorite (it solutions are also named “bleach” or “eau de Javel”) are through drinking water, food, and contact with items either bleached or disinfected with them. People may also be exposed to chlorine at the workplace (during manufacturing and use of chlorine as chemical intermediate), during use in water disinfection, and indirectly via the environment.

Disinfectants by-products can be controlled through removal of their precursor organic molecules and removal or modified disinfection practice. Natural organic material can be removed from water before treatment through source water protection by coagulation, use of granular activated carbon, membrane filtration, and ozone biofiltration, as well as by simple hygiene measures in swimming pools (such as showering before entering the pool and avoiding urinating).

What are the potential effects of chlorine and sodium hypochlorite on human health?

Above acceptable exposure limits, human exposure to chlorine gas may lead to local effects on the upper respiratory tract due to the corrosive effects of chlorine, such a risk is essentially related to occupational exposure.

Regarding exposure of humans and animals to chlorine in drinking water, the WHO report stated that no specific adverse treatment related effects have been observed.

Chlorine is of no concern with regard to mutagenicity and carcinogenicity and none of the chlorination by-products studied to date is a carcinogen at concentrations normally found in drinking-water. In an exceptionally long (7 generations) rat toxicity study, the incidence of malignant tumours in the animals consuming drinking-water with a chlorine level of 100 mg/L did not differ from that in controls animals2.

However, discussions still remain for bladder cancer even if the evidence of an association, although the causal nature of the association remains inconclusive.

The Scientific Committee on Health and Environmental Risks (SCHER) of DG SANTE (EU Commission) supported the conclusion that, based on the available database on hypochlorite and chlorine, there presently is no evidence for developmental or reproductive toxicity of sodium hypochlorite.

Meanwhile, it has been reported that, in some conditions, particularly in young children, asthma can be triggered by exposure to chlorinated water, via the chloramines by-products, even if the scientific evidence does not clearly support that recreational swimming increases the risk of childhood asthma.

Episodes of dermatitis have also been associated with exposure to chlorine and hypochlorite but all these elements were not yet discussed in the latest reference reports available.

There has been some association with dermatitis, although the evidence has not been discussed in consensus reference documents.

2 COC - UK Committee on Carcinogenicity of chemicals in food consumer products and the environment.. Annual Report 2008 - Second Statement on Chlorinated Drinking Water and Cancer.

What is the guideline value for chlorine in water?

The guideline value for chlorine defined by the WHO is 5 mg/L. It should be noted that this value is conservative. Interestingly, most individuals are able to taste chlorine or its by-products (e.g. chloramines) at concentrations below 5 mg/L and some at levels as low as 0.3 mg/L.

What about the risks of chlorine and sodium hypochlorite for the environment?

Sodium hypochlorite is very toxic to aquatic organisms. However, as the substance is extremely reactive, any sodium hypochlorite that is poured into the drain from household use will react with organic matter and will be removed before reaching the environment. Chlorine does not persist in the atmosphere either.

Regarding chlorinated by-products, the EU risk assessment considered tests on whole effluent from chlorinated raw sewage and observed that the halogenated by-products present did not increase the toxicity or reduce the biodegradability of the effluent. As this represents a “realistic worst case”, there should be no cause for concern for halogenated by-products generated by aqueous use of chlorine. The same is concluded for the atmospheric compartment.


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