I’ve conducted research on global climate change and on nuclear waste disposal, but vapor intrusion (VI) is the most challenging topic I’ve worked on during my 40-year career. VI’s technical challenges relate to its multimedia nature and the need to understand pollutant fate and transport both above and below ground.
Trichloroethylene, or TCE, is a widely used industrial chemical frequently found at Superfund and other hazardous waste sites as a contaminant in soil and groundwater. It is volatile, so it can be present as a vapor and hence the need for VI studies. The amount of TCE or other chemicals in groundwater required to trigger the need for a VI study can be surprisingly low. Screening levels for TCE may even be below the drinking water standard of 5 µg/L. In other words, it is possible that shallow groundwater at a site may be deemed safe to drink but not safe to live atop.
In recent years, potentially responsible parties (PRPs) face increased challenges because VI studies must often account for concerns about TCE exposures in increments of days or weeks rather than typical exposure scenarios that span multiple years. Real-time measurements will likely contribute to shaping the TCE discussion moving forward.
But how is TCE currently regulated? Which TCE sources are at issue, and are trigger levels appropriate in all cases? How do we measure their impacts effectively? Is the underlying science good? What’s next?
Underlying Science and Current Regulatory Responses
U.S. regulations and short-term exposure triggers vary from state to state. Concerns over short-term exposure are based on various published studies that link TCE to cardiac development issues in animals and humans. The most prominent study is Johnson et al. (2003), which looked at exposure of fetal rats via drinking water.
Eleven states (Alaska, California, Colorado, Connecticut, Massachusetts, Michigan, Minnesota, New Hampshire, New Jersey, New York, and Ohio) have issued trigger levels in recent years to address short-term exposure to TCE. Some U.S. EPA regions—for example, EPA Region IX—also take short-term exposure into account. These concentrations are quite low, as the following examples illustrate.
In some jurisdictions, residential occupants may be required to evacuate homes if indoor air results exceed 2 µg/m3 (0.4 part per-billion [ppb]), and some states use a tiered set of action levels. For example, Ohio has action levels for accelerated response (>2.1 µg/m3), urgent response (>6.3 µg/m3), and imminent hazard response (>20 µg/m3). Values for industrial exposures generally are three to four times higher, and a common screening level for non-residential exposures is 8.8 µg/m3.
Sources and Confounding Factors
In general, environmental agencies only have jurisdiction over TCE in indoor air if that TCE originates from subsurface contamination. They do not have jurisdiction over exposures resulting from storage and/or use of TCE in the workplace.
Occupational exposures are regulated by OSHA or other labor agencies, but the acceptable levels for workers are far higher than the environmental screening levels cited earlier. For example, the eight-hour, time-weighted OSHA PEL is 537,000 µg/m3. The PEL is intended to be protective of healthy, adult workers, whereas the environmental screening levels cited above are intended to be protective of a wider range of people. Even so, one might reasonably expect the various levels to agree to within one or two orders of magnitude rather than the observed five orders of magnitude difference.
Many companies have phased out large-scale use of TCE, but it is not uncommon for maintenance or shop workers to keep spray cans containing TCE for occasional use. TCE can also be found in office buildings via emissions from HVAC maintenance, copy machine toner cartridges, or other sources that are not readily apparent in building surveys and inspections.
Discrepancies
The wide discrepancy in action levels between occupational and environmental agencies is troubling to many researchers who feel it is logically indefensible that the health impacts of a chemical are dependent on the spatial source of the chemical rather than solely on the dose.
Furthermore, environmental agencies do not have jurisdiction over voluntary exposures in residences arising from the storage and/or use of TCE-containing consumer products. For example, TCE may be present at elevated levels in degreasers, spot removers, gun cleaners, brake cleaners, or engine cleaners. Over time, availability of TCE-containing products may be further restricted through some combination of risk management decisions by vendors, regulation by government agencies, and individual choices by consumers.
Finding Indoor Sources of TCE
If TCE is detected in indoor air, the acceptable concentration depends on whether the source is VI versus occupational exposure versus voluntary exposure via consumer products. The response therefore also depends on whether the TCE is present due to VI or use by the building occupants. Consequently, there is strong interest among PRPs and other stakeholders to find and address sources of TCE as quickly as possible.
Measuring Impacts Effectively in Real Time: Gas Chromatography
Various portable analyzers have been used for this purpose, but issues with sensitivity, specificity, or reliability have been noted. One promising approach is the Gas Chromatograph (GC) method. It allows impacts to be measured in real time, and it meets the industry need for timely, accurate, and very sensitive TCE measurements. GC has long been used in the laboratory setting, but in the field, GC can be a unique, customized tool that yields real-time measurements and allows better characterization of TCE. It is especially effective when measuring indoor air impacts.
AECOM recently performed a field trial to compare the performance of a field GC and two commercially available portable analyzers capable of detecting TCE at relatively low levels. Real-time TCE measurements were made at multiple buildings at a major industrial facility. The side-by-side measurements indicated that all three measurement options were capable of identifying indoor sources and preferential pathways. The field GC can be used to identify concentration gradients to help find sources and pathways. The field GC was also capable of directly determining if a given space was above or below the stringent screening levels used by the cognizant state agency.
What’s Next? GC and the Underlying Science
Various researchers have attempted to duplicate the Johnson et al. (2003) results and have not detected fetal heart defects. The most recent study is DeSesso et al. (2019), with additional supporting information found in Coder (2019). The current science therefore suggests the regulatory framework for TCE may be overly conservative, and this may serve as impetus to reconsider the regulation of short-term TCE exposures. For example, recent trends have been for more states to include short-term TCE exposure considerations in their VI policies. Whether this trend will continue is now an open question. Because GC offers timely, accurate, and very sensitive measurements, it will continue to satisfy the keen stakeholder interest in immediately finding and addressing TCE sources.