Chlorine dioxide, Chlorite, Chlorate
(endorsed 2011)
Guideline
Chlorine dioxide: Based on aesthetic considerations, the concentration in drinking water should not exceed 0.4 mg/L.
Chlorite: Based on health considerations, the concentration in drinking water should not exceed 0.8 mg/L.
Chlorate: Data are insufficient to set a guideline value in drinking water.
Action to reduce chlorite is encouraged, but must not compromise disinfection, as non-disinfected water poses significantly greater risk than chlorite.
General description
Chlorine dioxide is used as a disinfectant for drinking water supplies. When added to water, it dissociates into chlorite and, to a lesser extent, chlorate. It is usually generated on site due to handling and transportation difficulties.
Chlorine dioxide is used commercially as a bleaching agent in paper production, paper pulp, and cleaning and tanning of leather. Chlorite is used in the production of paper, textiles and straw products, and in the manufacture of waxes, shellacs and varnishes. Chlorates have been used as herbicides and defoliants, and in the manufacture of dyes, matches, and explosives. Chlorate is also generated by the dissociation of hypochlorite solutions, which are used for disinfection of drinking water. Use of such solutions has become more common in Australia in recent years as use of chlorine gas has declined due to occupational health and safety considerations. Chlorate levels can be minimised by restricting storage times for hypochlorite solution (7 days maximum storage is recommended), and storing the solution under cool dark conditions.
The taste and odour threshold for chlorine dioxide in water is 0.4 mg/L. No data are available on taste and odour thresholds for chlorite and chlorate.
Typical values in Australian drinking water
Chlorine dioxide (chlorite) is rarely used as a disinfectant in Australian reticulated supplies. When used, the chlorite residual is generally maintained between 0.2 mg/L and 0.4 mg/L. It is particularly effective in the control of manganese-reducing bacteria. Few data are available on chlorate levels in Australian water supplies.
Treatment of drinking water
Chlorine dioxide can be removed from drinking water by the addition of reducing agents such as sodium bisulfite (although some studies indicate that the chlorate concentration increases as a result), by exposure to sunlight, or by the use of granular activated carbon.
Measurement
Methods are available for the determination of chlorine dioxide, chlorite and chlorate and total available chlorine (APHA et al. 2005 a,b).
Health considerations
Chlorine dioxide, chlorite, and chlorate are all absorbed rapidly by the gastrointestinal tract into blood plasma and distributed to the major organs. All compounds appear to be rapidly metabolised.
Chlorine dioxide has been shown to impair neurobehavioural and neurological development in rats exposed before birth. Experimental studies with rats and monkeys exposed to chlorine dioxide in drinking water have shown some evidence of thyroid toxicity; however, because of the studies’ limitations, it is difficult to draw firm conclusions (WHO 2005)
The primary concern with chlorite and chlorate is oxidative stress resulting in changes in red blood cells. This end point is seen in laboratory animals and, by analogy with chlorate, in humans exposed to high doses in poisoning incidents (WHO 2005).
In a study with human volunteers, no adverse effects were observed after drinking water with either chlorine dioxide or chlorite concentrations up to 5 mg/L for periods of 12 weeks (Lubbers et al. 1981).
The International Agency for Research on Cancer has concluded that chlorite is not classifiable as to its carcinogenicity in humans (Group 3, no human data and inadequate evidence in animals) (IARC 1991).
Derivation of guidelines
The guideline values were determined as follows:
i) Chlorine dioxide:
A health based guideline value has not been established for chlorine dioxide because of its rapid hydrolysis to chlorite and chlorate. The guideline for chlorite is adequately protective for potential toxicity from chlorine dioxide (the no-observed-adverse-effect level [NOAEL] of 2.9 mg/kg bw/day used to derive the tolerable daily intake for chlorite is similar to the lowest NOAELs observed for effects of chlorine dioxide on neurobehavioral and neurological development and on thyroid hormone levels). The taste and odour threshold for chlorine dioxide is 0.4 mg/L.
ii) Chlorite:
where
2.9 mg/kg body weight per day is the no-effect level from a two-generation study using rats (CMA 1997, TERA 1998).
0.8 is the proportion of total daily intake attributable to the consumption of water, based on the occasional use of chlorite in the food industry.
100 is the safety factor applied in using the results of an animal study as a basis for human exposure (10 for interspecies variations and 10 for intraspecies variations).
The World Health Organization (WHO) guideline value of 0.7 mg/L was determined using an adult body weight of 60 kg (WHO 2005). The difference is not significant.
iii) Chlorate:
Data are currently considered insufficient to set a guideline value for chlorate in Australian drinking water supplies. A provisional guideline value for chlorate of 0.7 mg/L was published by WHO in 2004 based on limited data from human volunteer studies and a short-term study in rats. Data from a long-term study in rats was subsequently published (NTP 2005) and has been used to derive a new TDI value (JECFA 2007). Given the importance of maintaining adequate disinfection of water supplies and limited options for reducing chlorate levels in supplies treated with hypochlorite, further information on the occurrence and sources of chlorate in Australian waters is needed before a guideline value can be developed.
References
APHA, AWWA, WEF (American Public Health Association, American Water Works Association, Water Environment Federation ) (2005a). Method 4500, chlorine. In: Standard Methods for the examination of water and wastewater, 21st Edition. Washington, DC, American Public Health Association, American Water Works Association, Water Environment Federation, pp 4.56-4.69.
APHA, AWWA, WEF (American Public Health Association, American Water Works Association, Water Environment Federation) (2005b) Method 4500, chlorine dioxide. In: Standard Methods for the examination of water and wastewater, 21st Edition. Washington, DC, American Public Health Association, American Water Works Association, Water Environment Federation, pp 4.77-4.81.
CMA (Chemical Manufacturers Association) (1997). Sodium Chlorite: drinking water rat two-generation reproductive toxicity study. Washington, DC (Quintiles Report CMA/17/96).
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.
JECFA (Joint FAO/ WHO Expert Committee on Food Additives) (2007). Evaluation of certain food additives and contaminants. WHO Technical Report Series 947, WHO.
Lubbers JR, Chauhan S, Bianchine JR (1981). Controlled clinical evaluations of chlorine dioxide, chlorite and chlorate in man. Fundamentals and Applied Toxicology, 1:334-338.
NTP (National Toxicology Program) (2005). Technical Report on the Toxicology and Carcinogenesis Studies of Sodium Chlorate in F344/N Rats and B6C3F1 Mice. National Institute of Health.
TERA (Toxicology Excellence for Risk Assessment) (1998). Health Risk Assessment / Characterisation of the Drinking Water Disinfection By-products Chlorine Dioxide and Chlorite. Cincinnati OH,
WHO (World Health Organization) (2004). Guidelines for Drinking-water Quality. 3rd Edition, WHO, Geneva, Switzerland.
World Health Organization (2005). Chlorite and Chlorate in Drinking-water. Background document for development of WHO guidelines for drinking-water Quality.
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