A Perfect Storm Batters Risk Management Decisions for TCE

This new interim policy means that a chronic toxic effect could occur across a 24 hour period of time and any average 24-hour exposure measurement above the RfC is cause for prompt action. This is unprecedented and scientifically indefensible.

• By David R. Gillay
• Jan 15, 2014

A confluence of rare events batters risk management decisions concerning how short-term or acute exposure to trichloroethylene (TCE) vapors are addressed at thousands of contaminated sites across the country. A perfect storm of an update to the 1987 human health risk assessment for TCE, including controversial short-term limits based on non-cancer inhalation endpoints; highly scrutinized revisions to U.S. EPA's 2002 draft federal vapor intrusion policy; and recent recommendations of major public health and environmental regulatory agencies fuel the controversy with diverging short-term limits for TCE vapors.

The industrial solvent TCE is among the most ubiquitous volatile chlorinated compounds found in our nation's groundwater. As a consequence of its persistence and volatility, TCE vapors can migrate from groundwater (and soil) into overlying structures and create concerns about the safety of indoor air levels. Despite more than 20 years of intense assessment, our understanding of its toxicity and potential cancer and non-cancer effects to human health continues to evolve.

The balance of this article summarizes the three rare events noted above and highlights new guidance on how to interpret non-cancer inhalation endpoints for TCE vapors.

First, in September 2011, EPA published a revised human health assessment for TCE, including new non-cancer toxicity guidelines for lifetime airborne exposure known as the Reference Concentration (RfC). The RfC is an estimate (with uncertainty spanning perhaps an order of magnitude) of a continuous inhalation exposure to people (including sensitive subgroups) that is expected to occur without harmful affect over a lifetime. US EPA derived a RfC of 2 µg/m³ using a controversial short-term oral study in female rats. The critical effect(s) of the oral support study was fetal heart malformations, commonly referred to as "developmental" effects. The developmental study was conducted across a short period of time (approximately 22 days) and thus raises the question of whether or not a non-cancer toxic effect can occur to pregnant women across a similar time period in the early stages of pregnancy. Because this is a development effect, the critical period of exposure is considered to be within an approximate three-week period in the first trimester of pregnancy during which the heart develops. EPA acknowledges a lack of scientific information on the exact critical period of exposure when the heart develops and conservatively assumes that an adverse development effect could occur over a period as limited as 24 hours.

Second, the new RfC is critically important when assessing the vapor intrusion pathway and the potential affect of TCE vapors on the safety of indoor air levels. This "inhalation" pathway is currently the primary mechanism of exposure to TCE. EPA has studied the vapor intrusion exposure pathway for more than a decade and released new draft guidance in April 2013. This new draft guidance was highly scrutinized by a wide variety of stakeholders, including Fortune 500 companies, federal agencies, and state regulators. The draft guidance profoundly changes how TCE vapors are measured, assessed, and mitigated. These changes, coupled with the new RfC, are significantly increasing the costs and timeframes for addressing TCE vapors.

Third, regulators and public health agencies offer contrasting interpretations of how to use TCE's new RfC in risk management decisions. EPA headquarters is fully aware of this situation but has not issued a national policy or guidance on how to manage short-term non-cancer risk. This has created a vacuum, and several EPA Regions and states require immediate or prompt action when indoor air exceeds 2 µg/m³ for residential exposure across any given 24-hour period. On Dec. 3, 2013, EPA Region 9 issued new guidelines with recommended interim short-term indoor air action limits for TCE. EPA stated:

[i]n the event the indoor air TCE concentration related to subsurface vapor intrusion is detected above the prompt response action levels [2 µg/m³ for residential and 9 µg/m³ for a commercial 8-hr workday], then interim mitigation measures should be evaluated and implemented quickly, and their effectiveness (defined as a reduction of the TCE indoor air concentration to below [the applicable action level]) confirmed promptly (e.g., all actions completed and confirmed within a few weeks).

This new interim policy means that a chronic toxic effect could occur across a 24-hour period of time and any average 24-hour exposure measurement above the RfC is cause for prompt action. This is unprecedented and scientifically indefensible. Even though Region 9's new guidelines are not legally binding, far too many regulators view these guidelines as setting forth "requirements" that must be performed. These are very low levels of exposure enforced across a very short exposure period of time, and many complications result when trying to accurately measure and comply. Typically, when comparing exposures to the chronic RfC, sampling is designed to represent long term or years of exposure, and data are commonly averaged from multi-sampling events across months or years. In contrast, other federal regulatory agencies responsible for public health see the toxicity quite differently and seem to recognize these challenges.

Two major public health agencies offer differing interpretations of EPA's new, stringent RfC for short-term inhalation risk management decisions. ATSDR recently determined that 21 µg/m³ is an acceptable short-term limit for intermediate exposure (14 days up to 365 days). In March 2012, EPA targeted TCE, among other chemicals, for further assessment under the Toxic Substances Control Act (TSCA). EPA's Office of Pollution Prevention and Toxics (OPPT) manages programs under TSCA and, among other things, OPPT evaluates new and existing chemicals and their risks and finds ways to prevent or reduce pollution before it gets into the environment. OPPT recently completed its draft risk assessment focused on TCE's use as a degreaser in small commercial settings and by consumers in residential settings and its use as a clear protective coating spray by arts and crafts hobbyists. OPPT ignored the new RfC and the fetal heart malformations study used to set the RfC. The comparable level for any developmental effect in the TSCA risk assessment was about 50 times greater than the 21 µg/m³ ATSDR level.

The new, stringent RfC means that major risk management decisions for TCE will be driven by non-cancer toxicity rather than potential cancer risk. This is unprecedented, and there are no policies or procedures that address how to manage short-term exposure to non-cancer risk. Many erudite development toxicologists believe that the use of the RfC to evaluate short-term exposures for TCE is not scientifically defensible. In addition, the residential prompt action level of 2 µg/m³ is within the upper percentiles of common indoor air background and further complicates interpreting sampling data. As you may note, this article does not discuss the significant implications to commercial/industrial land use or how the RfC squares with OSHA's TCE levels for the workplace. To address these complex issues, the Alliance for Risk Assessment (ARA) recently published practical guidance and sponsored a national webcast on how to interpret TCE's non-cancer endpoints, clarified issues surrounding the potential developmental cardiac malformation, and explored the margin of safety used to set the TCE RfC.

It is possible to weather the TCE storm. A multidisciplinary and experienced team consisting of legal counsel, toxicologists, vapor intrusion experts, and risk assessors is required. The ARA is a vital team member and offers an excellent forum for interfacing with the public and regulators.