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Update: May 2004

Current Massachusetts Regulatory Limit
For chlorinated supplies only, the total trihalomethanes (TTHM) MMCL = 0.080 mg/L and is the sum of the concentrations of bromodichloromethane, bromoform, dibromochloromethane and chloroform.

Federal Regulatory Limit
The four trihalomethanes are byproducts of the disinfection process. Under its Disinfectants and Disinfection Byproducts Rule (63 FR 69390: December 16, 1998), U.S. EPA set the MCL for TTHMs at 0.080 mg/L. U.S. EPA did not set a Maximum Contaminant Level Goal (MCLG) for the group of TTHMs although there are MCLGs for some of the individual constituents, including bromodichloromethane (zero), bromoform (zero), dibromochloromethane (0.06 mg/L), and chloroform (0.07 mg/L).

Basis for Criteria
The four trihalomethanes (TTHMs) are byproducts of the disinfection process. The MCL of 0.080 mg/L was set based on the potential for an increased risk of cancer and other health effects (U.S. EPA, 1998a). The U.S. EPA considers that the MCL for TTHMs is feasible and achievable for a chlorinated drinking water supply. U.S. EPA also believes that by meeting MCLs for TTHMs and haloacetic acids (HAA5) (other disinfection byproducts), water suppliers will also control the formation of other disinfection byproducts not currently regulated that may also adversely affect human health (U.S. EPA, 2002).

Critical Effects
Potential health effects from ingestion of TTHMs in water include liver, kidney and central nervous system problems, as well as an increased risk of cancer. The MCL of 0.08 mg/L was set based on the potential for an increased risk of these health effects (U.S. EPA, 1998).

Cancer Assessment: U.S. EPA has not conducted a cancer assessment for the TTHMs. However, the individual TTHM constituents have been evaluated and qualitative descriptors of their carcinogenicity are provided below.

  • Bromodichloromethane is likely to be carcinogenic to humans by all routes of exposure.
  • Bromoform is likely to be carcinogenic to humans by all routes of exposure.
  • There is suggestive evidence of carcinogenicity but not sufficient to assess human carcinogenic potential for dibromochloromethane.
  • Chloroform is likely to be carcinogenic to humans by all routes of exposure under high-dose conditions that lead to cytotoxicity and regenerative hyperplasia in susceptible tissues. Chloroform is not likely to be carcinogenic to humans by all routes of exposures at a dose level that does not cause cytotoxicity and cell regeneration (U.S. EPA, 2004).

Disinfection Byproducts

Analytical Information
: See specific method 

Analytical Methods
U.S. EPA Methods 502.1; 524.2; 551.1 (U.S. EPA, 1998)

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.

Federal Register. December 16, 1998. National Primary Drinking Water Regulations. Disinfectants and Disinfection Byproducts. Final Rule. Federal Register. (63 FR 69390).

U.S. EPA (U.S. Environmental Protection Agency). July 2002. List of Contaminants and Their MCLs. U.S. EPA 816-F-02-013.

U.S. EPA (U.S. Environmental Protection Agency). Winter 2004. 2004 Edition of the Drinking Water Standards and Health Advisories.

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