By Keith A. Christman and David Fischer

Summary: Chloroform, the dominant species of trihalomethanes (THMs), has been the focus of federal disinfection by-products (DBPs) rules for over two decades. A new court ruling in Chlorine Chemistry Council et. al. vs. USEPA tossed out the chloroform goal of zero and changed the way DBPs and other compounds will be regulated in the future under the Safe Drinking Water Act.

Chloroform has been the focus of DBPs regulation since the 1970s, when it was discovered in chlorinated drinking water. Chloroform is the dominant species of the four THMs, usually representing about 80 percent of the total. Thus, enforceable standards for total trihalomethanes (TTHMs) are largely dependent on toxicity of chloroform. Early toxicological studies showed that rats given chloroform at doses thousands of times higher than the levels in drinking water developed cancer. Unless there is data to the contrary, the U.S. Environmental Protection Agency (USEPA) assumes compounds causing cancer at high doses will also cause cancer at low doses. That science was the basis for the 1979 TTHMs standard of 100 parts per billion (ppb) that was in place until December 1998, when the Stage I DBP Rule was finalized.

Under the Safe Drinking Water Act (SDWA), the USEPA is required to develop Maximum Contaminant Level Goals (MCLGs) in addition to the enforceable Maximum Contaminant Levels (MCLs). MCLGs are required by law to be “set at the level at which no known or anticipated adverse effects on the health of persons occur and which allows an adequate margin of safety.” The agency then must finalize an enforceable MCL that is as close as “feasible” to the MCLG.

The Stage I DBP Rule was the result of a decade of work by the USEPA, scientists, the water industry and others including the Chlorine Chemistry Council (CCC). The Stage I Rule was first proposed in 1994 and contained an MCLG of zero for chloroform. Based on the state of science in 1994, the agency determined there was strong evidence of chloroform’s carcinogencity and assumed, in the absence of data to the contrary, that chloroform could cause cancer at any dose. In its judgment, science didn’t support a safe threshold for chloroform’s carcinogenicity.1

New science emerges
Importantly, the state of scientific knowledge on how chloroform acts as a carcinogen grew exponentially between 1994 and 1998. Since the 1994 proposal, over 30 toxicological studies were published on chloroform. Also, a 1997 Expert Panel sponsored by the USEPA and convened by the International Life Sciences Institute reviewed the new science on chloroform and used the agency’s proposed 1996 Cancer Guidelines to make scientific recommendations about the risk assessment for chloroform. The panel was composed of 10 internationally recognized experts in toxicology and risk assessment from academia, government and industry. Its conclusions—based on information on how chloroform causes cancer at high doses in laboratory animals—deemed that chloroform was a likely human carcinogen above a certain dose, but unlikely to be carcinogenic below a certain dose. Thus, the USEPA could determine a non-zero MCLG for chloroform in drinking water that would be safe. The wealth of new data on how chloroform causes cancer in animals and the ILSI Expert Panel review prompted the agency in March 1998 to request comment on a revised chloroform MCLG of 300 ppb: “Based on the current evidence …EPA has concluded that a nonlinear approach is more appropriate …than the [default] low dose linear approach used in the 1994 proposed rule.”2

It’s difficult to overstate the importance of this scientific conclusion. Setting an MCLG for chloroform at 300 ppb would represent a significant and precedent-setting application of new science in establishing protective MCLGs. For the first time, the USEPA would be moving away from its long-held policy of establishing zero MCLGs for known or probable carcinogens. The proposed 300 ppb MCLG also strongly suggested that the Stage I TTHMs MCL of 80 ppb was “safe” with respect to chloroform and need not be lowered in Stage II to protect the public from cancer.

Non-zero MCLG rejected
The Natural Resources Defense Council among others harshly criticized the USEPA proposed revision. Ultimately, the agency chose to ignore its own scientific conclusions and finalized a zero MCLG for chloroform in the Stage I DBP Rule. It cited the need for additional review by its Science Advisory Board (SAB) and dialogue with stakeholders before proceeding with a non-zero MCLG.3 In the final rule, the USEPA acknowledged the science and its strong support for a non-zero MCLG, but refused to apply this science in setting a non-zero MCLG for chloroform.

Going to court
The USEPA’s refusal to apply this science ignored a new requirement in the 1996 SDWA Amendments that the agency “use the best available peer reviewed science” in setting standards under the act. The CCC, 10 water utilities and other parties filed suit against the USEPA for violating the SDWA’s mandate by finalizing a zero MCLG for chloroform. The litigation also piqued interest of the scientific community and the chair of a powerful Congressional committee, resulting in the filing of two amicus (friend of the court) briefs in support of the CCC’s position. One amicus brief was filed by a group of 13 eminent scientists and another by House Commerce Committee Chairman Tom Bliley. Both briefs supported using the “best available science” to finalize a non-zero MCLG for chloroform.

On Feb. 11, 2000, a three-judge panel of the U.S. Court of Appeals for the District of Columbia Circuit heard oral arguments. The court was clearly frustrated by the USEPA’s explanations for not applying the science in finalizing a zero MCLG for chloroform. This frustration was illustrated best by Judge Laurence H. Silberman’s question to the government attorney: “Are you suggesting it was politically difficult so you didn’t want to come out so quickly with what the science suggested?”

Following oral argument, the agency took the extraordinary step of asking the court to vacate the zero MCLG and not issue an opinion. It based this request on recent conclusions of its Science Advisory Board, which essentially endorsed the USEPA’s non-linear approach to estimating chloroform risk, thus supporting a non-zero MCLG.4 In its brief, the agency stated it no longer believed it could defend its zero MCLG. Although vacating the zero MCLG was certainly an important part of the relief CCC and other petitioners sought, the council asked the court to reject the motion and issue an opinion so as to leave no doubt that the agency is bound by the legal and scientific constraints of the SDWA. On March 31, 2000, the court denied the USEPA’s motion and issued its ruling to vacate the MCLG.5

In ruling that the agency violated the SDWA’s mandate to use the best available science, the court made clear that best available science is the scientific evidence available at the time of a rulemaking. Whether it represents the USEPA’s ultimate scientific conclusions is irrelevant. The possibility of contradiction based on future scientific data or peer review, even by its own Science Advisory Board, aren’t legitimate bases for rejecting science that currently exists. As the court noted, “All scientific conclusions are subject to some doubt….” The court ordered that the MCLG for chloroform be vacated and remanded the MCLG to the agency for a future rulemaking.

In subsequent legal briefs, the USEPA argued that it would revise the MCLG in the Stage II DBP rulemaking and finalize a non-zero MCLG. The ruling and science on which the ruling was based implies that the current 80 ppb MCL for TTHMs is safe based on USEPA’s calculations of a non-zero MCLG between 70 ppb and 300 ppb. The agency previously calculated this range assuming that 25-to-75 percent of a person’s chloroform exposure comes from drinking water.3

Future implications
The ruling has important implications for future DBP regulations. Since MCLs are required by law to be “as close as feasible to” the MCLG and the science for chloroform supports an MCLG no lower than 70 ppb, the current 80 ppb MCL for TTHMs under stage I of the DBP rule is sufficiently protective. Science suggests that there are no health benefits from further lowering the MCL for TTHMs, which are overwhelmingly chloroform. Thus, there’s no scientific or legal basis for lowering the TTHM standard in the Stage II Microbials/Disinfection By-Products (M/DBP) Rule. In fact, the M/DBP Stage II Federal Advisory Committee has recommended that the TTHM MCL remain at 80 ppb.

The court’s ruling has had an immediate impact on other USEPA rulemakings—the proposed California Toxics Rule had a 5.7 ppb goal based on the default linear mode of action for chloroform. However, in light of the court’s opinion, the final rule didn’t include human health criterion for chloroform. Instead, the agency will issue revised criterion at some future date based on the now judicially endorsed non-linear mode of action.

In the future, other rulemakings under the Safe Drinking Water Act are likely to be affected by the U.S. Court of Appeal’s ruling on chloroform as a disinfection by-product. All rules under the SDWA will need to be based on the best available science. In the case of chloroform, the USEPA’s own scientific conclusions require a non-zero MCLG.


  1. USEPA, “Disinfectants and Disinfection Byproducts; Proposed Rule,” Federal Register, July 29, 1994, pp. 38668-38829.
  2. USEPA, “National Primary Drinking Water Regulations: Disinfectants and
    Disinfection By-Products Notice of Data Availability; Proposed Rule,” Federal Register,
    March 31, 1998, pp. 15673-15692
  3. USEPA, “National Primary Drinking Water Regulations: Disinfectants and
    Disinfection By-Products Final Rule,” Federal Register, Dec. 16, 1998, pp. 69390-69475.
  4. USEPA Science Advisory Board, “Review of the EPA’s Draft Chloroform Risk Assessment,” April 2000.
  5. U.S. Court of Appeals for the District of Columbia Circuit, Decision, Case Number 98-1627, Chlorine Chemistry Council and Chemical Manufacturers Association, Petitioners v. Environmental Protection Agency, Respondent, March 31, 2000.

About the authors
Keith A. Christman is senior director of disinfection and government relations for the Chlorine Chemistry Council located in Arlington, Va. David Fischer is managing counsel for the council. Both can be contacted at (703) 741-5000. Christman also can be reached at email: Fisher can also be reached at email:


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