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Debate Continues on TCE Toxicity Estimates: TCE Remediation Plans Could be Affected

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Courtesy of AECOM

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Debate on EPA's draft toxicity factors for trichloroethene (TCE) continues both inside and outside of EPA. In August 2001, EPA published an External Review Draft TCE Health Risk Assessment with a range of recommended provisional cancer potency estimates. The upper end of the range is 40 to 70-fold more stringent than prior cancer potency estimates. Several EPA regional and program offices (Superfund, RCRA, Region 9 PRG table) are already using the upper end of the range.

However, some EPA regions and states, as well as EPA Science Advisor Paul Gilman, have expressed concern about using the provisional values, particularly on the high end. In a March 2003 briefing, EPA Region 8 toxicologists recommended not using the draft toxicity values due to method uncertainties and weaknesses and advocated using the old values until the EPA consensus review process is finished and final values are loaded on EPA’s toxicity database (IRIS). Region 4 advocates using both the old and new draft values in risk assessments until definitive guidance from EPA headquarters is provided. Still other regions and states recommend CalEPA’s potency estimate for TCE, Use of the CalEPA value follows the new hierarchy identified in the December 2003 EPA OSWER directive on human health toxicity values.

Fueling the controversy is the potential effect of the draft toxicity factors on vapor intrusion sites. Acceptable residential risk-based concentration for TCE in air derived using the provisional high end of the range is 10 to 300 times lower than average background concentrations of TCE in indoor air reported in the literature. This will result in an increased number of sites with potential TCE indoor air problems. Implications for remediation at these sites are significant.

In response to the controversy and concerns about overstating risks and increased remediation costs, the White House has formed an interagency working group to discuss with the National Academy of Sciences (NAS) performing a scientific evaluation of the EPA’s 2001 study. In addition, the Air Force, who has challenged EPA’s 2001 draft study, has requested TERA (Toxicology Excellence for Risk Assessment) to conduct a peer consultation on risk assessment issues for TCE. Input from these evaluations is expected in 2004. In the meantime, parties with TCE sites should be aware of ongoing developments in the TCE toxicity debate before finalizing remediation plans.

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