Report Finds Current Data Sufficient For EPA To Finalize Risk Assessment On TCE

A new report from the National Academies' National Research Council recommends research to improve understanding of how trichloroethylene (TCE) causes cancer and other adverse health effects. However, the report concludes that enough information exists for EPA to complete a credible human health risk assessment now.

In 2001 EPA issued a draft risk assessment on TCE, a solvent widely used as a degreasing agent that is contaminating air, soil and water at several military installations and hundreds of waste sites around the country. The release of the draft risk assessment was followed by much debate about the quality of evidence on TCE and how that evidence should be assessed, with the findings opposed by the U.S. Departments of Defense and Energy. This prompted an interagency group to request that a Research Council committee review issues related to assessing the health risks from exposure to TCE. The committee was not asked to conduct a risk assessment of its own.

The evidence on cancer and other health risks from TCE exposure has strengthened since 2001, the committee stated in its July 27 report. It pointed out that research, including studies of human populations, supports the conclusion that TCE is a potential cause of kidney cancer. Research shows that the chemical may cause other kidney problems as well, but the level of exposure needed to produce kidney damage is not clear. Animal data indicate that relatively high doses of TCE are needed to induce liver toxicity and cancer. Some epidemiology studies indicate a higher incidence of liver cancer among populations exposed to TCE, but the evidence is inconsistent. Studies of people exposed to TCE at work do not show a strong association between exposure and lung tumors, the report notes.

Animal research and human population studies suggest that TCE exposure may also be associated with other health effects, such as reproductive and developmental problems, impaired neurological function, and autoimmune disease. The committee recommended studies to advance understanding of the mechanisms by which TCE causes cancer and other health problems; which populations are most sensitive to TCE's effects; and how exposure to a mixture of TCE and other chemicals affects human health.

A large body of epidemiological data on TCE and cancer is available, but a new analysis of that data is needed to better characterize the hazard that TCE presents to humans, the committee said. It found several weaknesses in the analysis that EPA used in its draft risk assessment, as well as in an analysis developed by researchers since the draft was issued. To overcome these weaknesses, the new analysis should establish clear criteria for including epidemiological studies based on objective characteristics, the committee said. It added that it would be appropriate for EPA to use a model jointly developed with the U.S. Air Force to simulate how the body metabolizes TCE, although the model does not resolve uncertainty about the mechanisms by which the chemical causes cancer.

A model is being used to extrapolate from animal studies an estimate of the cancer risk posed by TCE at low doses. The risk is extrapolated below a "point of departure," which is associated with an incremental effect, such as 5 percent more cancers. EPA should consider a range of points of departure in its risk assessment, the committee recommended. Because there is not enough evidence on how TCE triggers cancer to choose the best model for relating the body's response to different dose levels -- a so-called dose-response model -- it is appropriate under EPA's cancer guidelines to extrapolate the risk using a linear model, in which cancer risk rises in proportion to dose.

Additional information on the report, Assessing the Human Health Risks of Trichloroethylene: Key Scientific Issues, can be found at http://www.nap.edu/catalog/11707.html.

An EPA consumer fact sheet on trichloroethylene in water is available at http://www.epa.gov/safewater/contaminants/dw_contamfs/trichlor.html. Another fact sheet can be found at the Agency for Toxic Substances and Disease Registry's Web site at http://www.atsdr.cdc.gov/tfacts19.html.

This article originally appeared in the 07/01/2006 issue of Environmental Protection.

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