By Katherine M. Rahill

On September 28, 2011, EPA issued a final health assessment for trichloroethylene ("TCE") to its Integrated Risk Information System ("IRIS") database.  IRIS is EPA's program through which it evaluates scientific information to determine risks of adverse human health effects as a result of exposure to environmental contaminants.  The risk information developed through the IRIS program is used by EPA to support its regulatory determinations.  The health assessments developed by EPA through this program identify both carcinogenic and non-cancer health effects associated with chemicals.

The final health assessment for TCE characterizes TCE as "carcinogenic to humans" by all routes of exposure.  Additionally, the health assessment associates TCE with a number of non-cancer health effects including harm to the central nervous system, kidneys, the liver, the male reproductive system, the immune system, and the reproductive system including a developing fetus.  While the general conclusions of EPA's final health assessment for TCE are not new, the assessment shows a clear focus on the potential risks associated with inhalation exposure to TCE from vapor intrusion.  Traditionally, EPA's focus has largely been on oral exposure through the drinking of contaminated drinking water.

For the first time, EPA has set a reference dose for chronic oral exposure and a reference concentration for chronic inhalation exposure to TCE for non-cancer health impacts.  The reference dose and reference concentration represent the daily oral exposure or continuous inhalation exposure to humans over their lifetime that EPA estimates is likely to be without an "appreciable risk" of non-cancer health impacts.  EPA has determined those levels to be 0.0005 mg/kg/day (reference dose for chronic oral exposure) and  2 ug/m³ or 0.4 ppb (reference concentration for chronic inhalation exposure).   For comparison, EPA's Toxicological Review for TCE cites the following statistics:

• Ambient air monitoring data suggest that mean levels have remained fairly constant since 1999 at about 0.3 μg/m3 (0.06 ppb).
• The median level of TCE in groundwater, based on a large survey by the USGS for 1985–2001, is 0.15 μg/L. It has also been detected in a wide variety of foods in the 1–100 μg/kg range.

EPA has also quantified risk estimates for cancer effects from oral and inhalation exposure.  Using the risk estimates, the final health assessment provides examples for lifetime exposure to drinking water and to air within EPA's risk range of 1 in 10,000 to 1 in a million as follows:  

Overall, the impact of the final health assessment is likely to be significant. EPA's own press release states that the conclusions in the final health assessment will be considered in establishing cleanup methods at NPL sites, evaluating vapor intrusion risks, revising the MCL for TCE, and developing standards for limiting the atmospheric emissions of TCE.  Additionally, the final health assessment is likely to have an effect on state standards for drinking water and vapor intrusion, especially in those states that use EPA risk estimates to set their own standards.   While EPA's acceptable risk range is 1 in 10,000 to 1 in a million, states differ on the acceptable risk allowable.  To the extent EPA's final health assessment provides a basis for a state to reevaluate its risk estimates for levels of contamination, the state may lower its standards, especially standards related to vapor intrusion risks.

For more information, the final assessment is available on EPA's website.