EPA's Assessment of Trichloroethylene Completed

October 3, 2011

On September 28, 2011, EPA released the final IRIS risk assessment for trichloroethylene (TCE), concluding that the chemical is “carcinogenic in humans by all routes of exposure.”  IRIS assessments often form the basis for standards set by states and EPA for safe drinking water, air emissions, and contaminated property cleanups.  

TCE, a widely used solvent favored in many industrial applications as a metal degreaser, was already classified as “highly likely to cause cancer.” The final IRIS assessment, however, goes much further by designating TCE “carcinogenic” and identifying all routes of exposure as posing cancer risks.

The IRIS review process in general, and the IRIS review of TCE in particular, has been the subject of considerable controversy because of EPA's use of very conservative hazard and exposure models.  In the case of TCE, the Agency’s determination in 2001 that TCE is mutagenic triggered use of its conservative linear non-threshold model for assessing cancer risks. Using that model, EPA estimates that the risk of cancer from inhalation of TCE is 4 × 10-6 per μg/m3 (based on human kidney cancer risks and adjusted for potential risk for non-Hodgkins lymphoma and liver cancer). The oral risk for cancer is 5 × 10-2 per mg/kg/day, resulting from a physiologically based pharmacokinetic model. 

The final IRIS risk assessment for TCE could lead to more stringent cleanup standards for TCE-contaminated environmental media.  TCE is a contaminant of concern, typically in groundwater, at 761 Superfund sites, as well as 1750 contaminated military installations. EPA often bases the groundwater cleanup standard on the drinking water standard, i.e., "maximum contaminant level" or "MCL," which presently is 5 ppb for TCE. The final IRIS assessment may cause EPA to reduce the MCL for TCE, which in turn would lead to a more stringent groundwater cleanup standard. More stringent cleanup may also be required at sites where TCE in soil is penetrating building foundations, causing a "vapor intrusion" problem.

Tougher standards for TCE will increase cleanup costs by billions of dollars. Commenters, such as the Halogenated Solvents Industry Alliance, have questioned the soundness of the revised TCE risk assessment. Given the enormous stakes, legal challenge appears likely.  

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