June 01, 2013

Chasing Fumes: The Challenges Posed by Vapor Intrusion

Laurent C. Levy, Ph.D., P.E.

Afew years ago, I purchased a single-family home in Cambridge, Massachusetts. The house abutted a gas station, which I learned had a substantial file at the Massachusetts Department of Environmental Protection. That fact persuaded me to sample basement indoor air as part of my pre-purchase inspection. Good news. There was no benzene and I eventually bought the house. For the next few years I wondered whether the ethanol that was detected was tied to the gas station, urban air, or perhaps—I thought, with a beer in my hand—the previous owner’s ethanol consumption. I had just had a personal encounter with one of the challenges of vapor intrusion assessments: differentiating between background contributions to indoor air and true contributions from vapor intruding from the subsurface.

Defined as the migration of volatile chemicals from the subsurface into the indoor air of overlying structures, vapor intrusion (VI) has received increased regulatory attention over the past two decades because it can cause unacceptable health risks to the occupants of houses, apartments, and commercial buildings. In a typical scenario, hazardous volatile chemicals, such as chlorinated solvents or petroleum products, are released to the subsurface through an accidental spill or leaky storage tank at an industrial or commercial facility (e.g., refinery, manufacturing site, dry cleaner). The chemicals migrate downward and reach groundwater where they may slowly dissolve and form a contaminant plume that follows the natural groundwater movement. As the dissolved contaminant plume moves downgradient, volatile chemicals can volatilize from groundwater and travel upwards as “soil gas” or “soil vapors” to reach ground surface. Where buildings overlie ground surface, the vapors can seep through foundation cracks or move along utility conduits and contaminate the indoor air. Examples of chemicals commonly encountered in VI investigations include volatile constituents of gasoline and other petroleum products (e.g., benzene), as well as chlorinated solvents, such as trichloroethylene (TCE) used historically in manufacturing for surface treatment operations (degreasing), and tetrachloroethylene (also known as perchloroethylene or PCE) used in manufacturing and dry cleaning. TCE, PCE, and benzene are considered carcinogenic, such that relatively low levels inhaled in indoor air can pose unacceptable long-term exposure risks to occupants.

Typical VI cases often occur in urban areas with a history of mixed use. The impact of subsurface releases of volatile chemicals on industrial parcels is compounded by the potential presence of sensitive receptors in adjacent residential properties. VI has received particular attention in the northeastern United States, where there are several conditions generally favorable to VI—industrial legacy and mixed-use urban areas; land scarcity and relatively high property values that enable the conversion of former industrial sites into residential use; shallow groundwater tables; a rigorous winter season where house and apartment occupants live with windows closed; and the operation of heating systems that enhance soil gas infiltration and potential indoor air quality issues.

Because dissolved contaminant plumes can be several miles long, VI issues can arise at hundreds of properties downgradient of a chemical release. Such cases are not frequent, however, for several reasons. They require large spill volumes and a combination of subsurface conditions favorable to VI, including permeable soils, shallow groundwater, and hydraulic gradients that readily transport and maintain dissolved contamination near the groundwater table. Within thick aquifers, dissolved plumes tend to “dive” due to groundwater recharge from precipitation. In addition, non-aqueous phase liquid (NAPL) chlorinated solvents are denser than groundwater and, as a result, they tend to migrate downward and remain primarily in deeper portions of aquifers with reduced VI potential. By contrast, petroleum product-related NAPLs, such as gasoline, will remain at the water table; however, the vapors of volatile petroleum constituents will readily biodegrade before reaching ground surface provided that sufficient oxygen is available in the soils. Hence, VI-related issues tend to be limited to properties proximate to the release area.

At present, the VI pathway is routinely evaluated as part of subsurface investigations that involve volatile chemicals. Soil, exterior soil gas, and groundwater quality data (collected near the water table), depth and direction of groundwater, subsurface geology, and an evaluation of potential receptors can be used to determine whether a VI investigation is needed on- or off-site. Groundwater or soil gas screening levels deemed protective by the regulator can help assess whether the VI pathway is potentially complete and requires further evaluation. If screening levels are exceeded, a VI assessment will typically include subslab vapor sampling (i.e., soil gas sampling beneath the slabs of buildings of concern) coupled with indoor air sampling and often ambient (outdoor) air sampling. It is generally recognized that indoor air sampling alone is not sufficient to evaluate VI in consideration of background contributions of volatile compounds from ambient air (e.g., benzene and other gasoline-related constituents, which are notoriously ubiquitous) or from indoor air (e.g., upholstery, glues and solvents that may be present within the building, cigarette smoke, dry-cleaned clothing). Simultaneous subslab vapor and indoor air sampling can be used to compare chemical signatures, estimate a subslab-to-indoor air attenuation factor—the ratio of subslab vapor to indoor air concentrations—and, assuming that the attenuation factor falls within typical ranges, determine whether volatile chemicals present in indoor air are likely to have resulted from VI. Additional information can also be collected to assess the VI pathway under a multiple-line-of-evidence approach favored by guidance manuals. For example, the differential pressure between subslab and indoor air can be measured to determine whether a building is negatively or positively pressurized. The latter case, which is common for commercial facilities, limits the potential for subslab vapors to migrate into the building.

Once collected, indoor air data are used to assess exposure risks to occupants. Assuming risks are unacceptable and clearly attributable to VI, mitigation and follow-up monitoring typically need to be implemented. Mitigation often consists of a subslab depressurization system (similar to a radon reduction system), which can be installed at a cost of a few thousand dollars for a typical home.

A key technical issue associated with VI assessments relates to the variability in concentrations and attenuation factors. Different heating systems and building configurations (e.g., basement or crawlspace construction, slab condition and evidence of cracking, presence of sumps or utility openings) may result in a wide range of indoor air concentrations even if the subslab vapor concentrations are similar. In addition, volatile chemical concentrations in soil gas and, to some extent, indoor air tend to vary considerably spatially and temporally. In the subsurface, local variations in geology and moisture content will affect the distribution of contaminants. Ultimately, subslab vapor samples collected beneath two adjacent homes—or beneath the same home—may show very different concentrations. Likewise, it is possible to observe order-of-magnitude differences in concentration between subslab vapor and exterior soil gas, such that regulators can be reluctant to rely on exterior soil gas data alone. Seasonal variations will also affect VI potential. Winter is generally considered a worst-case condition because the operation of a heating system may draw vapors from the subslab into indoor air. Summer months may also be a concern, however, because subsurface soils tend to be dryer, which can enhance vapor transport, soil gas concentrations, and the resulting VI potential. Seasonal fluctuations of the water table depth can also have an impact on subslab vapor concentrations, especially under shallow groundwater conditions.

Variability is obviously a concern to regulators and, as a result, VI guidance documents often recommend sampling subslab vapor from several locations within a given building, even a typical single-family residence. Likewise, indoor air samples must routinely be collected from at least two locations (e.g., different floors) and as part of several sampling events to address seasonal effects. Given concerns for variability and uncertainty, the potential for lengthy investigations, and the modest cost of VI mitigation relative to assessment, it is tempting to err on the side of caution and consider preemptive mitigation, an option that has been advocated by public stakeholders (e.g., Center for Public Environmental Oversight) and is expected to be part of the U.S. Environmental Protection Agency (EPA) final VI guidance.

A Brief History of Vapor Intrusion Guidance

Setting aside concerns related to the intrusion of naturally occurring radon, which dates back to the 1980s, or explosive vapors associated with substantial petroleum releases, concerns for the impacts of subsurface contaminants on indoor air quality and early VI evaluations can be traced back to the early 1990s in the context of Superfund sites. Within a few years, Massachusetts, Connecticut, and several other states started to include the VI pathway as part of their contaminated site management programs. In parallel, in 1995, the American Society for Testing and Materials (ASTM) developed a guide for applying risk-based corrective action (RBCA) at petroleum release sites, which advocated the use of a tiered evaluation approach. Assuming generic Tier 1 screening levels from “look-up” tables were exceeded, cleanup levels could be developed on the basis of a site-specific risk assessment protective of human health. Among other pathways, the RBCA approach included an evaluation of the “vapor inhalation pathway” arising from the potential for vapors from contaminated soils or groundwater to migrate to enclosed, human-occupied spaces. ASTM later generalized the approach to chemical release sites. The ASTM documents still form the basis for VI guidelines from several states that do not have stand-alone VI guidance documents (e.g., Alabama, Iowa, Louisiana, Missouri, Oklahoma, Utah).

Beginning in 2000, large-scale VI sites, such as Redfield, Colorado, and Endicott, New York, received national attention. In 2002, EPA’s Office of Solid Waste and Emergency Response (OSWER) released the draft version of its VI guidance. In the next ten years, EPA continued to publish VI-related documents and tools, including a VI database, a compilation of background indoor air studies, and a screening level calculator. See comprehensive resource compilation on EPA’s Clean-Up Information [Clu-In] website. EPA also released VI-specific guidelines for five-year reviews of Superfund sites and recently considered the addition of the VI pathway to the hazard ranking system (HRS) for listing sites on the National Priorities List.

In 2009, EPA’s Office of the Inspector General noted that the lack of a final VI guidance was impeding indoor air risk evaluation efforts. Although committed to release its final VI guidance by the end of 2012, EPA failed to meet that deadline. In November 2012, a June 2012 working draft of the guidance was circulated by InsideEPA. Then, in April 2013, EPA formally released an “external review draft” final guidance for public comments.

The April 2013 draft final guidance reflects advances in the field over the past ten years that certain states have already included in their own guidance (e.g., New Jersey, New Hampshire, Massachusetts). Consistent with most recent state guidance documents, EPA’s draft final guidance recommends a multiple-line-of-evidence approach relying on empirical data (e.g., paired subslab vapor/indoor air data with a spatial and temporal variability assessment) and cautions against using indoor air modeling (i.e., predicting indoor air concentrations from soil gas or groundwater data) as a single line of evidence for ruling out the VI pathway. To responsible parties, this could mean facing increased investigation costs and increasing public participation efforts even if groundwater or exterior soil gas data suggest exposure risks are limited.

Certain provisions contained in EPA’s April 2013 draft final guidance reflect a less conservative stance than in the 2002 draft guidance. For example, EPA’s proposed generic subslab-to-indoor air attenuation factor of 0.03 is lower than the value of 0.1 recommended in 2002, meaning that more attenuation to indoor air is expected for a given subslab vapor concentration and that, accordingly, subslab vapor screening levels should increase. The April 2013 document also acknowledges differences between VI related to chlorinated compounds and VI related to petroleum products (i.e., petroleum VI or PVI) in consideration of the aerobic biodegradation potential of volatile petroleum constituents. The document points to a companion PVI guidance developed by OSWER’s Office of Underground Storage Tanks (OUST), also released in April 2013 as an external review draft. See www.epa.gov/oust/cat/pvi. Of particular interest, the PVI guidance provides vertical separation distance criteria between contamination and building slab beyond which PVI is generally not expected to pose a threat to overlying buildings because clean, biologically active soils are expected to attenuate vapors.

From EPA Guidance to State Guidance

In parallel to—or perhaps in the absence of—EPA’s final VI guidance, twenty-four states have published stand-alone VI guidance manuals or draft documents since 2004 (see map and resource list) along with various federal entities and organizations (e.g., U.S. Department of Defense, U.S. Postal Service, Interstate Technology & Regulatory Council [ITRC]). While the documents reflect generally consistent frameworks, such as a phased approach relying on preliminary screening and gathering of multiple lines of evidence to characterize the VI pathway, individual states’ approaches have led to a guidance patchwork that can be difficult to navigate with assessment criteria that may vary broadly between states. For example, there is a two order-of-magnitude range in subslab-to-indoor air attenuation factors used by states to derive subslab vapor screening levels. The more conservative states, which use an attenuation factor of 0.1, either relied on EPA’s 2002 draft guidance or used the 95th percentile value derived from EPA’s draft VI database study released in 2008 (e.g., Alaska, Minnesota, Kansas, North Carolina, Ohio, Washington). However, the VI database study was finalized in 2012 with a less prescriptive 95th percentile attenuation factor (0.03), which has not been reflected in the states’ documents.

Adding to the variability in attenuation factors, the states have used very differing approaches for developing indoor air screening levels. See illustration. Most states derive screening levels using a risk-based approach; however, certain states also account for background indoor air concentrations (e.g., Connecticut, Massachusetts, New Hampshire, New York, Virginia, Vermont) or analytical reporting limits (e.g., Massachusetts, New Hampshire). Values derived using a risk-based approach may rely on different cancer risks, non-cancer hazard indexes, or toxicity values. For example, certain states (e.g., Minnesota, Tennessee, Texas) do not consider naphthalene to be carcinogenic and use indoor air screening levels that are orders of magnitude above states with more conservative levels for naphthalene. For the latter states, which view naphthalene as carcinogenic, the screening number is a fraction of what is typically found in background indoor air. With respect to TCE and PCE, states have been generally slow at updating their indoor air screening levels to reflect the Integrated Risk Information System (IRIS) assessments published by EPA in 2011 and 2012, respectively. This has been particularly problematic for PCE for which EPA has lowered the inhalation unit risk (IUR)—a toxicity value used to assess cancer risk—by a factor of about twenty, meaning that the indoor air screening level should generally increase by the same factor.

Certain states have continued to rely on their own, more conservative toxicity evaluation (e.g., California, Massachusetts). It is also possible that there may be some reluctance to substantially raise the screening level of PCE in consideration of the effect the change could have on public perception and ongoing VI investigations. We are aware of a site where the state regulator continues to use EPA’s former PCE screening level, combined with a conservative subslab-to-indoor air attenuation factor of 0.1, for the sake of adding a “factor of safety,” even though the state’s VI guidance specifically calls for using current EPA values. EPA’s indoor air screening levels are already designed to be health protective with residential exposure scenarios that assume almost continuous exposure (24 hours per day, 350 days per year) and toxicity values that account for data uncertainties and sensitive receptors.

Overly stringent screening levels will obviously trigger VI investigations where these may be unnecessary. Compare, for example, the VI-related groundwater screening level for benzene in Vermont (5.2 micrograms per liter [μg/L]) and the corresponding value in New Hampshire (2,900 μg/L). It is hard to believe that two states with similar climates, locations, and sizes could have screening levels that vary by a factor of 500. When groundwater concentrations slightly exceed very conservative screening levels, responsible parties may be legitimately concerned with obtaining access to nearby properties and collecting subslab vapor and indoor air samples to assess the VI pathway. Under these circumstances, it is probable that indoor air concentrations will not suggest an exposure risk; however, there may be serious concerns for bad publicity, added investigation costs, and the threat of toxic tort and property damage lawsuits.

Most state guidance manuals tend to focus on chlorinated compounds in residential settings. This is not a surprise considering the sensitive nature of residential receptors and the persistence of chlorinated compounds in the environment. EPA’s VI database reflects this trend with 97 percent of data associated with chlorinated compounds and 85 percent of buildings sampled corresponding to residential settings. Certain states have started to include detailed considerations for petroleum compounds (e.g., Michigan, New Jersey) and adjust attenuation factors and/or screening levels (e.g., New Hampshire, Vermont) to account for their biodegradation potential. It is likely that additional states will make further changes after EPA’s PVI guidance is finalized and ITRC releases its own PVI guidance.

Addressing VI issues in commercial/industrial settings present several challenges. The first issue is a technical one. Most states have developed commercial/industrial screening levels that account for reduced exposure time (typically, eight hours per day for 250 days per year, consistent with EPA’s approach). However, with few exceptions (e.g., California, Hawaii, Oregon, Wisconsin), states typically provide a single generic attenuation factor that applies to both residential and commercial structures. This can lead to over-conservatism when characterizing VI at commercial buildings because these buildings are typically larger and positively pressurized. Therefore, under similar subsurface contamination conditions, there is more subsurface to indoor air attenuation in commercial settings; and the potential for the VI pathway to be complete is less likely than in residential settings. Initially, EPA had recognized this issue. The June 2012 working draft circulated by InsideEPA provided generic attenuation factors that were specific to commercial buildings and assumed about three times more attenuation than for residential structures. Unfortunately, the nonresidential attenuation factors were eliminated from the April 2013 draft final guidance based on the absence of sufficient empirical data to derive them.

The second issue relating to VI in commercial buildings relates to the applicability of indoor air screening levels in occupational settings. State VI guidance documents generally recognize that the permissible exposure limits (PELs) used by the Occupational Safety and Health Administration (OSHA) are not appropriate to assess VI-related indoor air risks to workers in industrial/commercial settings when the investigated chemicals are not used within the facility and indoor air is degraded by subsurface contamination (e.g., California). Certain guidance documents consider different categories of workers and suggest that VI screening levels (and not PELs) should apply whenever worker’s exposure to a given chemical is not voluntary and the worker is not trained (e.g., New Hampshire, Vermont).

Using VI screening levels rather than occupational levels would have little consequence if those levels were equivalent; however, there can be a three to six order-of-magnitude difference between OSHA PELs and the far more stringent risk-based indoor air screening levels used by EPA and the states. For example, the PEL for TCE is 535,000 micrograms per cubic meter (μg/m3), while EPA’s industrial indoor air screening level is 3.0 μg/m3 for a ten to six cancer risk and 8.8 μg/m3 for the non-cancer risk. Recognizing that OSHA levels may not protect all workers, EPA Regions IX and X have developed interim short-term exposure limits for TCE on the order of a few μg/m3 based on EPA’s 2011 IRIS assessment. The short-term values rely on the new reference concentration (RfC) of 2 μg/m3 for TCE—a measure of its noncarcinogenic toxicity—to protect a fetus from heart defects that may be associated with a pregnant woman’s exposure to TCE, and would apply to both residential and commercial settings.

Certain states have already followed suit at the beginning of 2013 (e.g., Massachusetts, New Hampshire, New Jersey) and lowered their imminent hazard or rapid action levels for TCE on the basis of the new RfC. There is growing concern among potentially responsible parties that the TCE short-term exposure limit is needlessly over-protective and that EPA’s OSWER may eventually adopt this value—and possibly short-term values for other contaminants—further complicating remediation and contaminated site redevelopment efforts. The noncarcinogenic toxicity of TCE and short-term exposure peaks could be driving risk management decisions and result in quasi-continuous monitoring or preemptive mitigation.

The Path Forward

In attempting to address the VI pathway, federal and state authorities tackle the difficult task of ensuring public health without harming economic development—while incorporating new findings relating to the toxicity of volatile chemicals and VI assessment techniques. As they attempt to redevelop contaminated properties, developers and lenders address the dilemma of collecting indoor air data from yet-to-be constructed buildings or relying on indoor air modeling that may not achieve regulatory closure. Responsible parties find themselves with the prospect of having to reinvestigate sites previously thought to be closed or on the remediation path. Industries face the possibility of having to comply with indoor air targets that vary considerably from state to state and are a fraction of those acceptable a decade ago. Attorneys, environmental consultants, and their clients must navigate through a constantly changing maze of VI guidance documents. While we acknowledge that states and federal entities have their specificities and priorities, we question the need of having as many guidance manuals as there are states; and we believe that we would all benefit from harmonizing VI characterization efforts.

If we were to provide a VI wish list, that list would include several components. First, regarding VI screening levels and attenuation factors, we believe that all states should: (1) eliminate screening level tables within VI guidance documents and, instead, use stand-alone tables that are posted on the states’ websites and changed periodically in response to EPA’s updates; (2) explicitly account for background levels in indoor air screening levels by relying either on EPA’s background compilation study, a state-level study, or area specific studies (e.g., urban, suburban, or rural); (3) use consistent attenuation factors, exposure assumptions, and toxicity values without a state-added “factor of safety”; (4) for groundwater screening levels, account for local average soil temperature for deriving contaminant volatility properties; and (5) develop generic attenuation factors specific to commercial buildings. To assist with those efforts, a website could be built that provides a comprehensive and up-to-date comparison of screening levels and other key assessment criteria between states.

Second, we believe EPA’s VI database should be expanded. This database has provided considerable information about the distribution of subsurface-to-indoor air attenuation factors. As previously noted, however, the database focuses primarily on chlorinated compounds and residential settings. Moreover, about half of the roughly 3,000 subsurface-indoor air data pairs collected from about 900 buildings pertain to only two sites (Endicott and Redfield). Substantial benefit could be derived from expanding EPA’s database to include: (1) additional sites, including “large” sites; (2) more data for industrial/commercial settings; (3) additional paired subsurface-indoor air data for petroleum compounds; (4) paired subslab/exterior soil gas data collected near the structure to better assess the representativeness of exterior sampling; and (5) multiple data series to further evaluate the temporal and spatial variability of concentrations and attenuation factors.

Third, for responsible parties, there is some value in collecting exterior soil gas data and using modeling as a possible approach for ruling out the VI pathway without sampling subslab vapor or indoor air. EPA and the states should recognize the benefits of this approach, especially in settings where the completeness of the VI pathway is unlikely even though groundwater screening levels are exceeded. Detailed criteria or guidelines should be defined under which this approach may be acceptable.

Fourth, EPA needs to further explain its position on OSHA PELs. In the 2002 draft VI guidance, EPA indicated that it did not expect its guidance to be used for settings that are primarily occupational. The April 2013 draft final guidance document is open-ended on this matter. There is a need to clarify the extent to which OSHA PELs can be used. EPA and the states should also indicate whether PELs can be applied in instances where chemicals of concern are no longer used at an industrial or commercial facility, but indoor air issues are present and believed to be related to building materials (e.g., concrete) rather than the subsurface itself.

Finally, EPA should clarify and unify its approach to short-term exposure levels (and states, hopefully, will then adopt consistent guidelines). If short-term exposure to certain chemicals, such as TCE, is confirmed to cause health effects, EPA should consider publishing short-term exposure levels in addition to indoor air screening levels, propose sampling strategies (e.g., frequency and duration) that are consistent with the short-term window, and define settings or conditions under which short-term peak exceedance may be acceptable.

Laurent C. Levy, Ph.D., P.E.

Mr. Levy is a senior project manager at Gradient, an environmental consulting firm located in Cambridge, Massachusetts. He may be reached at llevy@gradientcorp.com.