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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 control 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 and then transformed into hypochlorites and hypochlorite and hypochlorous acids. All three species exist in equilibrium with each other, their relative amounts varying with the pH. They are rapidly degraded and therefore do not not bioaccumulate and do not persist in the environment.
As stated by IPCS, the noteworthy biocidal attributes of chlorine have been somewhat offset by the formation of disinfectant by-products of public health concern during the chlorination process. As a consequence, alternative chemical disinfectants, such as ozone (O3), chlorine dioxide (ClO2) and chloramines (NH2Cl, monochloramine), were increasingly being used; however, each of them has been shown to form its own set of by-products. Although the microbiological quality of drinking-water cannot be compromised, there was a need to better understand the chemistry, toxicology and epidemiology of chemical disinfectants and their associated Derived By Products in order to develop a better understanding of the health risks (microbial and chemical) associated with drinking water and to seek a balance between microbial and chemical risks.
Disinfectants and their by-products were already evaluated in Guidelines for drinking-water quality, and guideline values were recommended by WHO (1993, 1996, 1998).
The overall conclusion of the most recent assessments was that there is no need for further information and/or testing, and that for the protection of human health and the environment there is no need for risk reduction measures beyond those that are already being applied.
This was in particular the conclusion of the Joint Research Centre (JRC) of the European Union, for workers, humans exposed via the environment, combined exposure, or consumers exposure (including household products, drinking water, as well as adults or children of 1 year old in swimming pools).
These conclusions, which remain valid in 2017, are based on various evaluations of international organizations (whose references are given above), in particular the risk assessment made by the EU in 2007 and the opinion on the human health part of this risk assessment in 2008 by the Health and Environment Committee SCHER of the DG SANCO (now DG SANTE) of European Commission.
The IPCS reported also highlighted that the health risks from these by-products at the levels at which they occur in drinking-water are extremely small in comparison with the risks associated with inadequate disinfection. Thus, it is important that disinfection would not be compromised in attempting to control such by-products. Where local circumstances require that a choice should 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.
Chlorine gas and sodium hypochlorite (chlorine bleach) are used as drinking water disinfectants, swimming pool disinfectants, waste water treatment or cooling water disinfection. Sodium hypochlorite is more specifically used for household and laundry cleaning, sanitation, deodorizing and disinfection, municipal water, sewage and swimming pool disinfection, medical environment disinfection, disinfection purposes in food industry and food manipulation, textile industry and pulp and paper bleaching, chemical synthesis, as a multisite fungicide in agriculture and horticulture. Chlorine bleach should never be mixed with acid cleaners or other acids, as toxic fumes may result.
The main industrial uses of chlorine are on the site of production and include mainly, production of inorganic chemicals, including sodium hypochlorite, PVC and non-chlorinated polymer production. Organic products production includes chlorinated solvents, chloromethane, epichlorohydrin, chlorinated paraffins, chloro and chloro-oxygenated derivatives. Many of the halogenated organic disinfectants by products were evaluated by IPCS in an UNEP-WHO report cited above.
Elemental chlorine used offsite is mainly used as an intermediate in dyestuff and pesticide production. Small other industrial offsite use elemental chlorine as an intermediate.
The major routes of exposure to chlorine and sodium hypochlorite (its solutions are also named “bleach” or “eau de Javel”) are through drinking-water, food, and contact with items either bleached or disinfected with them. The human population 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.
For occupational exposure, the relevant route of exposure is inhalation. Chlorine in the form of hypochlorous acid/hypochlorite ion (HOCl/OCl-) reacts with bromide ion, oxidizing it to hypobromous acid/hypobromite ion (HOBr/OBr-). Hypochlorous acid (a more powerful oxidant) and hypobromous acid (a more effective halogenating agent) react collectively with natural organic material present in water to form chlorine by-products including trihalomethanes, haloacetic acids and others.
Disinfectants by-products such as chloramines, 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, 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).
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 exposure risk is essentially related to occupational exposure but measured levels of chlorine in chlor-alkali manufacturing plants and in plants using chlorine as chemical intermediate are in most cases lower than the occupational exposure limit for chlorine. (1.5 mg/m3).
Regarding exposure of humans and animals to chlorine in drinking-water, the WHO report stated that no specific adverse treatment related effects exposed have been observed.
For the disinfection by-products of sodium hypochlorite, the toxicological data available suggested that the likelihood of adverse effects is not significantly different between the described disinfectant options.
Chlorine is of no concern with regard to mutagenicity and carcinogenicity. The International Agency of Research on Cancer (IARC), which is associated to the WHO has concluded that hypochlorite related molecules are not classifiable as to their carcinogenicity to humans (Group 3).
None of the chlorination by-products studied to date is a potent carcinogen at concentrations normally found in drinking-water. In an exceptionally long seven-generation toxicity study, the incidence of malignant tumours in rats consuming drinking-water with a free chlorine concentration of 100 mg/L did not differ from that in controls. An UK government scientific advisory committee on carcinogenicity1 who reviewed the available evidence in 2007 concluded that “the evidence for a causal association between cancer and exposure to chlorination by-products is limited and any such association is unlikely to be strong”.
Evidence of an association between exposure to disinfection by-products and adverse health effects seems most consistent for bladder cancer although the causal nature of the association remains inconclusive. The results of published studies did not provide convincing evidence that chlorinated water or the trihalomethanes by-products cause adverse pregnancy outcomes. Although some associations have been found in more recent studies on THM focusing on brominated species and other by-products such as haloacetic acids, the EU Scientific Committtee SCHER supported the conclusion that there was no evidence for developmental or reproductive toxicity of sodium hypochlorite based on the available database on hypochlorite and chlorine.
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 is no direct consumer use of chlorine and consequently no direct public exposure is expected. In specific occupational cases in which measured concentrations can be higher than the limit (i.e. during maintenance), adequate protection equipment must be worn and strict safety procedures are applied.
1 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.
The WHO guideline value for chlorine is 5 mg/L (rounded figure). It should be noted that this value is conservative. Interstingly, 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.
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.
For the aquatic compartment, the conclusion of the risk assessment for chlorine was that there is no need for risk reduction measures beyond those that are being applied already. This conclusion applies to chlorine and its aqueous transformation products, hypochlorous acid and hypochlorite. 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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