Chlorine
CAS NO 7782-50-5 (endorsed 2014)
Guideline
Based on health considerations, the guideline value for total chlorine in drinking water is 5 mg/L.
General description
Chlorine dissociates in water to form free chlorine, which consists of aqueous molecular chlorine, hypochlorous acid and hypochlorite ion. Chlorine and hypochlorites are toxic to microorganisms and are used extensively as disinfectants for drinking water supplies. Chlorine is also used to disinfect sewage and wastewater, swimming pool water, in-plant supplies, and industrial cooling water.
Chlorine has an odour threshold in drinking water of about 0.6 mg/L, but some people are particularly sensitive and can detect amounts as low as 0.2 mg/L. Water authorities may need to exceed the odour threshold value of 0.6 mg/L in order to maintain an effective disinfectant residual.
In the food industry, chlorine and hypochlorites are used for general sanitation and for odour control. Large amounts of chlorine are used in the production of industrial and domestic disinfectants and bleaches, and it is used in the synthesis of a large range of chemical compounds.
Free chlorine reacts with ammonia and certain nitrogen compounds to form combined chlorine. With ammonia, chlorine forms chloramines (monochloramine, dichloramine and nitrogen trichloride or trichloramine) (APHA 2012). Chloramines are used for disinfection but are weaker oxidising agents than free chlorine.
Free chlorine and combined chlorine may be present simultaneously (APHA 2012). The term total chlorine refers to the sum of free chlorine and combined chlorine present in a sample.
Typical values in Australian drinking water
When used as a disinfectant, the free chlorine residual in major Australian reticulated supplies ranges from 0.1 mg/L to 4 mg/L, with typical concentrations in the reticulation of about 0.2 to 0.5 mg/L.
Treatment of drinking water
Chlorine can be removed from drinking water by aeration, by exposure to sunlight, or by the addition of reducing agents such as sodium bisulfite.
Measurement
The concentration of chlorine in drinking water can be determined by several methods including the amperometric titration method (APHA Method 4500-Cl Part D 2012), DPD ferrous titrimetric method (APHA Method 4500-Cl Part F 2012) and the DPD colorimetric method (APHA Method 4500-Cl Part G 2012). The methods are subject to interferences and vary in complexity, sensitivity, precision and accuracy. Water utilities should consider Standard Methods when selecting a method (APHA 2012). The chlorine concentration should be determined immediately after sampling as chlorine is not stable in water.
Health considerations
Chlorine and hypochlorites are strong oxidising agents that readily react with organic molecules to produce a wide variety of chlorinated compounds. This reactivity makes it difficult to separate the effects of chlorine from those of its metabolites. In animal studies using a naturally occurring non-radioactive chlorine isotope, chlorine was rapidly absorbed by the gastrointestinal tract, and highest concentrations of the isotope were found in blood plasma.
It is assumed that the toxicities of aqueous solutions containing chlorine, hypochlorous acid or hypochlorite are similar since they are in dynamic equilibrium. Chlorine concentrations therefore refer to free available chlorine.
Very few toxic effects have been associated with drinking water containing high chlorine concentrations. In one report, 150 people drank water with 50 mg/L during a period of mains disinfection, with no adverse effects. Several instances have been reported where military personnel drank water with chlorine concentrations up to 32 mg/L for several months with no ill effects. Mouth irritation and momentary constriction of the throat were observed when the chlorine concentration exceeded 90 mg/L. Most people would refuse to drink water with a chlorine concentration over 25 mg/L (Muegge 1956).
A number of studies have suggested an association between water chlorination and various cancers or adverse reproductive outcomes. However, results of analytical epidemiological studies are insufficient to support a causal relationship for any of the observed associations (IPCS 2004). (See Section 6.3.2 for a discussion of disinfection by-products, and Section V - Fact Sheets on specific disinfection by-products.)
Long-term animal toxicity studies have shown no specific effects from the ingestion of chlorine. Chlorine, hypochlorous acid and hypochlorite did not act as carcinogens or tumour initiators.
Assessment of the mutagenicity of chlorine is complicated by the reactivity of chlorine. Hypochlorite was found to be mutagenic in tests with one strain of bacteria but not with another. Chromosome aberrations were reported in tests with mammalian cells.
The International Agency for Research on Cancer has concluded that hypochlorites are not classifiable as to their carcinogenicity in humans (Group 3, no human data and inadequate evidence in animals) (IARC 1991).
Derivation of guideline
The guideline value for chlorine in drinking water was determined as follows:
where
15 mg/kg body weight per day is the no-effect level from a 2-year drinking water study using rodents (NTP 1992).
70 kg is the average weight of an adult.
2 L/day is the average amount of water consumed by an adult.
100 is the safety factor in using the results of an animal study as a basis for human exposure (10 for interspecies variations and 10 for intraspecies variations).
It is assumed that all chlorine intake is from drinking water.
References
APHA Method 4500-Cl Part A (2012). Chlorine (residual): Introduction. Standard Methods for the Examination of Water and Wastewater, 22nd edition. APHA (American Public Health Association), AWWA (American Water Works Association) and WEF (Water Environment Federation), Washington, DC.
APHA Method 4500-Cl Part D (2012). Chlorine (residual): Amperometric titration method. Standard Methods for the Examination of Water and Wastewater, 22nd edition. APHA (American Public Health Association), AWWA (American Water Works Association) and WEF (Water Environment Federation), Washington, DC.
APHA Method 4500-Cl Part F (2012). Chlorine (residual): DPD Ferrous Titrimetric method. Standard Methods for the Examination of Water and Wastewater, 22nd edition. APHA (American Public Health Association), AWWA (American Water Works Association) and WEF (Water Environment Federation), Washington, DC.
APHA Method 4500-Cl Part G (2012). Chlorine (residual): DPD colorimetric method. Standard Methods for the Examination of Water and Wastewater, 22nd edition. APHA (American Public Health Association), AWWA (American Water Works Association) and WEF (Water Environment Federation), Washington, DC.
IARC (International Agency for Research on Cancer) (1991). IARC Monographs on the Evaluation of Carcinogenic Risks to Humans: chlorinated drinking water, chlorination by-products, some other halogenated compounds, cobalt and cobalt compounds. World Health Organization, International Agency for Research on Cancer, p 52.
Muegge OJ (1956). Physiological effects of heavily chlorinated drinking water. Journal of the American Water Works Association, 48:1507-9.
NTP (National Toxicology Program) (1992). Toxicology and carcinogenesis studies of chlorinated water and chloraminated water in F344/N and B6C3F1 mice (drinking water studies). National Toxicology Program, Technical Report No. 392, Publication No. 92-2847. United States Department of Health and Human Services, National Institute of Health.
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