The Complexity Associated with Setting Non-cancer Indoor Air Remediation Goals for Trichloroethylene Based on Short-term Exposure at Groundwater Contamination Sites


Over the last few years, there has been increasing consideration of vapor intrusion in groundwater investigations, groundwater remedy selection, periodic reviews of the continued safety of already implemented groundwater remedies, environmental due diligence, and property and personal injury litigation. The frequency and complexity of vapor intrusion evaluations and re-evaluations is driven by new, more explicit, vapor intrusion guidance documents and regulations promulgated by the Environmental Protection Agency (EPA) and state regulators, voluntary industry standards (such as the ASTM International’s E1527-13, Standard Practice for Phase I Environmental Assessments (December 30, 2013) (ASTM VI Standard)), the EPA’s controversial adoption of a novel, much more stringent non-cancer toxicity factor for trichloroethylene (TCE) in 2011, and the concerns of local residents. This article focuses particularly on the problems that plumes of TCE (a common groundwater contaminant) present to regulators and potentially responsible parties (whether private companies or governmental responsible parties) attempting to address vapor intrusion, and suggests some prudent actions that the private sector might consider.

The EPA’s New TCE Cancer Potency Is Unlikely to Result in More Stringent Remediation

There is no federal indoor air standard for TCE. Screening levels and site-specific remediation goals for TCE in indoor air are calculated using the EPA’s default exposure assumptions and the 2011 cancer potency or non-cancer reference air concentration (RfC).

The potential acceptable residential indoor air remediation goals for TCE based on cancer effects range from the concentrations that correspond to the one-in-one million (10-6) lifetime risk to a one-in-ten thousand (10-4) lifetime risk (using the EPA’s standard 30-year lifetime and the EPA’s new (2011) cancer potency), i.e., from 0.43 µg/m3 to 43 µg/m3 (see EPA Region 3, 6, and 9 Superfund screening level calculations), a concentration range 2.8 times lower than the EPA’s 2009 interim recommended indoor air remediation goals (1.2 µg/m3 to 120 µg/m3). As a practical matter, the 2011 TCE cancer potency is unlikely to change indoor air remediation goals selected by regulators (compared to the 2009 guidance). Background levels of TCE in indoor air and the practical quantification limit for measuring TCE in air are generally around 1 µg/m3. In any case, remedies are selected based on a case-by-case weighing of the nine remedy selection factors (including cost effectiveness and practicality), not just risk. Indoor air concentrations below 1.2 µg/m3 are on the low end of the acceptable cancer risk range and, therefore, are unlikely to justify changing the remedy. At sites with indoor air remediation goals for TCE that have already been set, regulators are unlikely to seek to lower an existing cancer-based TCE indoor air remediation goal for the reasons cited above and because the EPA’s five-year assessment policy only requires a revision of an existing cleanup goal if the concentration is unsafe (i.e., greater than 43 µg/m3 – the 10-4 lifetime risk level). Therefore, the change in the TCE inhalation cancer potency is not likely to trigger additional remedial action, although more sites may be subject to investigation.

The EPA and States Are Interpreting Non-cancer Regulatory Risks in an Inconsistent and Confusing Manner

The EPA’s 2011 novel non-cancer air reference concentration of 2 µg/m3 is more likely to result in a more stringent TCE indoor air remediation goal, because this concentration is much more stringent than either of the 2009 recommended non-cancer remediation goals of 10 µg/m3 or 600 µg/m3. More importantly, the EPA’s new recommended non-cancer remediation goal is based on an elevated risk of heart birth defects from short-term exposure to TCE during pregnancy (albeit in one unconfirmed rat study). This focus on short-term exposure differs from the EPA’s historic use of a lifetime exposure of 30 years to calculate non-cancer risk. Neither EPA Headquarters, nor any authoritative scientific body (such as the National Academy of Sciences) has defined “short-term” in this context or provided guidance on how to monitor or calculate the measured indoor air concentration that should be compared to 2 µg/m3. In this vacuum, the governmental scientists at the EPA and state regulatory agencies seem to be adopting their own, inconsistent definitions of short-term exposure during pregnancy. For example, EPA Region 9 has required evacuation of Navy office buildings based on this interpretation of short-term exposure. EPA Region 10 and Alaska interpret short-term to trigger action only if the 21-day average exceeds 2 µg/m3. Massachusetts requires notification within two hours of obtaining knowledge that the indoor air concentration exceeds 2 µg/m3, but does not specify in its written guidance the time over which the average concentration must be measured. New Hampshire considers a 2 µg/m3 indoor air level safe for “short-term exposure to TCE during the first trimester of pregnancy,” but also provides no averaging time in its guidance.

Similarly, the Agency for Toxic Substance and Disease Registry (ATSDR) also adopted the EPA’s RfC as its minimum risk level for non-cancer effects, but applies it over an intermediate-duration (i.e., an advisory applicable to exposures greater than 15 days to 364 days). In December 2013, the ATSDR concluded at a specific site that in homes that “have had TCE indoor air levels greater than 21 µg/m3,” there is:

A potential for developmental effects if a women was exposed even for a fairly short period of time (i.e., under three weeks) to these levels when the fetal heart is developing during the first trimester of pregnancy. … Given the narrow time window for health effects on a developing fetus, ATSDR is concerned that unidentified or future exposures could pose an urgent public health hazard. (boldface added.)

At the Millsboro, Del. Superfund site, ATSDR compared calculated 24-hour-average concentrations of TCE in indoor air to be 2.6 µg/m3 to 4.8 µg/m3 with the EPA’s RfC non-cancer levels and stated that this comparison “suggests that there may be an increased likelihood of adverse fetal cardiac effects.” At a site in Dracut, Mass., ATSDR stated that women exposed in an office building to between 54-61 µg/m3, based on an eight-hour air sample, “may be at risk of having a child with heart problems due to TCE exposure, and may be at risk for developing autoimmune effects (decreased thymus weight) due to TCE exposure.” The Wisconsin Department of Health states that “breathing TCE entering homes … via the vapor intrusion pathway for a long time (a year or more), or during pregnancy, could harm people’s health if TCE levels exceeded 2 µg/m3” and “breathing air approaching or exceeding these levels of TCE should be avoided.” (boldface added.)

The Alliance for Risk Assessment (a collaborative effort of private-sector non-profit organizations) recommends that a range of indoor air concentrations of between 3 to 20 µg/m3 (a range with a factor of 7) be considered and recommends a remediation goal concentration of 9 µg/m3 averaged over 24 days (the period over which the heart is formed in the fetus) as a reasonable approach. So averaging times cited above range from 8 hours to 30 years and the response actions varied from a caution that the information needed to make a decision is not available, to issuance of evacuation orders.

The frequency that remedial action will be triggered varies greatly, depending upon the averaging time. For example, use of a 24-hour averaging time will trigger an indoor air remediation if the concentration on one day exceeds 2.0 µg/m3, even if 20 days out of a 21-day averaging period do not exceed this limit, or even if 10,949 days out of 30 years may have average concentrations below 2.0 µg/m3.

In summary, there is no definitive guidance on what constitutes a “short” period of exposure to TCE in indoor air or on how to measure compliance. More generally, unlike the process of selecting a remediation goal based on carcinogenic risk, there is not a long-established process for making flexible risk management decisions for non-cancer effects from short-term exposure.

Suggested Courses of Action

These unique circumstances require thoughtful and scientifically supportable responses where vapor intrusion is considered. Specifically:

  • private parties should urge the EPA to consider the scientific uncertainties, the short-term exposure policy issues, and the other unique aspects of non-cancer risks in setting indoor air action levels for TCE
  • potentially responsible parties should include in the administrative record at cleanup sites documentation of these issues
  • buyers and sellers should consider these complexities in their environmental due diligence of properties potentially impacted by TCE vapors
  • defendants in litigation should avoid potential misinterpretation of exposure to concentrations above 2.0 µg/m3 of TCE. The regulatory reference dose is not a level that is likely to cause harm to a population as a whole, no less cause an individual’s injury.


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