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CHLORINE (as Cl2) 
CASRN: 7782505

Update: May 2004

Current Massachusetts Regulatory Limit
The MMCL is equal to the MRDL of 4 mg/L. ORS has adopted the MRDL for chlorine 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 of 4 mg/L was established under the U.S. EPA's 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 nonenforceable 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 for chlorine is equal to the MRDLG for chlorine. 

Basis for Criteria
Chlorine is a very effective disinfectant that is widely used for disinfection of water supplies. The U.S. EPA has set an MRDLG for chlorine of 4 mg/L (assuming a 70 kg adult ingests 2 L/day water) based on the RfD presented below.

RfD oral = 0.1 mg/kg/day (U.S. EPA, 1994; U.S. EPA, 1998)
UF: 100 (10 = interspecies; 10 = intraspecies)
MF: 1

The MRDLG is based on a NOAEL of 14 mg/kg/day identified in a two-year rat study in which rats were given chlorine at doses ranging from 4 to 14 mg/kg/day and mice at doses ranging from 8 to 24 mg/kg/day. Neither systemic toxicity nor change in body weight and survival were noted. An uncertainty factor of 100 was applied to account for inter- and intra- species differences (U.S. EPA, 1994). 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. Derivation of the MRDLG is:

MRDLG = 14 mg/kg/day x 70 kg x 0.8/(100 x 2 L/day) = 3.9 mg/L (4.0 mg/L).

Critical Effects
Chlorine is very reactive and thus the health effects associated with the administration of high levels of chlorine may be due to its reaction by-products. Effects noted in animals after short-term oral exposure include decreases in blood-glutathione levels, hemolysis and biochemical changes in liver. Chlorinated water has been shown to be mutagenic to bacterial strains and mammalian cells (U.S. EPA, 1994). 

Cancer Assessment
The 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. 

disinfectant byproduct

Analytical Information

Analytical Methods
Free, combined, and total chlorine: ASTM Method D1253-86;
Standard Methods 4500-Cl D, 4500-Cl F and 4500-Cl G.
Total chlorine: Standard Methods 4500-Cl E and 4500-Cl I
Free chlorine: Standard method 4500-Cl H

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

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

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 Please note that the information in IRIS may differ from that used in the derivation process as published in the Federal Register notice.

U.S. EPA. 1994. National Primary Drinking Water Regulations. Disinfectants and Disinfection Byproducts. Proposed Rule. Fed. Reg. 59:145:38668. (July 29, 1994).

U.S. EPA. December 16, 1998. National Primary Drinking Water Regulations. Disinfectants and Disinfection Byproducts. Final Rule. Fed. Reg. 63:241:69406.