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CHLORINE DIOXIDE (as ClO2) 
CASRN: 10049044

Update: May 2004

Current Massachusetts Regulatory Limit
The MMCL is equal to the MRDL of 0.8 mg/L. ORS has adopted the MRDL for chlorine dioxide published by the U.S. EPA. Although this standard is termed an MMCL, this is really an MRDL value. 

Federal Regulatory Limit
The Maximum Residual Disinfectant Level (MRDL) for chlorine dioxide was established under the Disinfectants and Disinfection Byproducts Rule (Federal Register, December 16, 1998 - Volume 63, Number 241). An MRDL is an enforceable standard, analogous to an MCL, which recognizes the benefit of adding a disinfectant to drinking water on a continuous basis and maintaining a residual to control for pathogens in the distribution system. The MRDL is set as close as feasible to the Maximum Residual Disinfection Level Goal (MRDLG). The MRDLG is a non-enforceable health goal based only on health effects and does not reflect the benefit of the addition of the chemical for control of waterborne microbial contaminants. The MRDL should not be exceeded even for short periods of time due to potential health concerns. 

Basis for Criteria
Chlorine dioxide is added to water for the oxidation of taste and odor-causing organic compounds in water. It is also used for the oxidation of reduced iron and manganese and color, and as a disinfectant and algicide. It reacts with impurities in water and is quickly dissipated. U.S. EPA has set an MRDLG for chlorine dioxide of 0.8 mg/L based on the assumption that a 70 kg adult ingests 2 L/day of water. Since most exposure to chlorine dioxide is likely to come from ingestion of drinking water, a relative source contribution factor of 0.8 is applied to this value.

RfD oral = 0.03 mg/kg/day (U.S. EPA, 2000)
UF: 100 (10 = interspecies; 10 = intraspecies)
MF: 1
Confidence in RfD: Medium to High

Numerous animal studies have examined the neurodevelopmental toxicity of chlorite. These studies consistently show a LOAEL of 14 mg/kg-day and NOAEL of 3 mg/kg-day for multiple neurodevelopmental endpoints (CMA, 1996; U.S. EPA, 2000) An uncertainty factor of 3 was applied to account for inter- and intra-species differences. Derivation of the MRLDG is: MRLDG = 0.03 mg/kg/day x 70 kg/2 L/day x 0.8 = 0.8 mg/L.

Critical Effects
Chlorine dioxide in drinking water rapidly degrades to chlorite, chlorate, and chloride ion. In an epidemiology study, chlorine dioxide rapidly (within 2-4 hours) disappeared from stored water and concentrations of chlorite simultaneously increased (Michael et al., 1981, as cited in U.S. EPA, 2000). Once absorbed, chlorine dioxide and chlorite are cleared from the blood at similar rates and are similarly distributed throughout the body (Abdel-Rahman et al., 1979, 1982, as cited in IRIS, 2000). Additionally, chloride is the major in vivo degradation product for chlorine dioxide, chlorite, and chlorate. Due to these similarities, U.S. EPA has concluded that the toxicity information for chlorite is relevant to deriving an RfD for chlorine dioxide (U.S. EPA, 2000). 

Cancer Assessment
Data are inadequate for assessment of human carcinogenic potential.

U.S. EPA believes that the available cancer epidemiological data provides important information that contributes towards the weight-of-evidence evaluation of the potential health risks associated with chlorinated drinking water. However, U.S. EPA does not believe at this time that the cancer studies are sufficient to establish a causal relationship between exposure to chlorinated drinking water and cancer. 

Class
disinfectant byproduct

Analytical Information

Analytical Methods
Standard Methods 4500-ClO2 D and 4500-ClO2 E

PQLs and analytical methods may have been updated since this guidance value was last revised. Updated analytical methods for drinking water and their associated PQLs may be found at http://www.epa.gov/safewater/methods/methods.html

Other Regulatory Data
Any Health Advisories, Reference Doses (RfDs), cancer assessments or Cancer Potency Factors (CPFs) referenced in this document pertain to the derivation of the current guidance value. Updated information may be obtained from the following sources:

Health Advisories - The U.S. EPA provides guidance for shorter-term exposures for chemicals based on their non-cancer effects. Current health advisories may be more current than those used to derive MCLs and may be found at http://www.epa.gov/waterscience/drinking/standards/dwstandards.pdf

RfDs, cancer assessments and CPFs - For specific information pertaining to derivation of drinking water criteria, consult the Federal Register notice that announces the availability of the most current guidance for that chemical. In addition, information on other current RfDs and CPFs as well as cancer assessments for specific chemicals may be found in the U.S. EPA Integrated Risk Information System (IRIS) at http://www.epa.gov/iris/. Please note that the information in IRIS may differ from that used in the derivation process as published in the Federal Register notice.

References
Abdel-Rahman, M.S., Couri, D., and Jones, J.D. 1979. Chlorine dioxide metabolism in rat. J Environ Pathol Toxicol. 3:421-430.

Abdel-Rahman, M.S., Couri, D., Bull, R.J. 1982. Metabolism and pharmacokinetics of alternate drinking water disinfectants. Environ Health Perspect. 46:19-23.

CMA (Chemical Manufacturers Association). 1996. Sodium chlorite: drinking water rat two-generation reproductive toxicity study. Quintiles Report Ref. CMA/17/96.

Federal Register. December 16, 1998. Part IV. Environmental Protection Agency. 40 CFR Parts 9, 141, and 142. National Primary Drinking Water Regulations: Disinfectants and Disinfection Byproducts; Final Rule. (63 FR 69391).U.S. EPA (U.S. Environmental Protection Agency). October 12, 2000. Integrated Risk Information System (IRIS).

U.S. Environmental Protection Agency. Washington, D.C. http://cfpub.epa.gov/ncea/iris/index.cfm.

Michael, G.E., Miday, R.K., Bercz, J.P., et al. 1981. Chlorine dioxide water disinfection: a prospective epidemiology study. Arch Environ Health. 36:20-27.