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Breathing New Life Into Vapor Intrusion Concerns: EPA Issues Long-Awaited Final Guidelines


After months of anticipation, the U.S. Environmental Protection Agency (EPA) has finalized its vapor intrusion (VI) guidelines for buildings.1 Vapor intrusion is the general term for the migration of hazardous vapors from contaminated soil or groundwater through the subsurface and into nearby buildings. Potentially Responsible Parties (PRPs) face significant challenges in assessing the severity, liability, and potential mitigation and/or remediation of VI, particularly considering the prior patchwork of regulations and guidance in the area.

The newly published guidelines, which include a separate set of guidance for petroleum from underground storage tanks (USTs), are intended to bring national uniformity to the assessment and mitigation of VI. States with pre-existing VI policies will need to assess whether and how to align with the federal approach.

The new Guidance updates and finalizes EPA's draft guidelines that were issued in 2002.2 Some significant changes from the draft guidance include:

  • Recommendations to collect indoor air samples early in an investigation.
  • Recommendations for more sampling in various media/ locations (ground water, deep soil gas, sub-slab soil gas, indoor air, etc.) to enhance a "multiple lines of evidence" approach rather than simply relying on a few soil gas measurements outside a building.
  • Expanded community involvement.
  • More specific conditions for preemptive or early actions, including relocation of individuals.

One major concern for industry is that site remediations already deemed completed by government regulators may need to be reopened in order to ensure compliance with the new guidelines. Plaintiffs' attorneys are likely to seize upon these new and/or additional recommendations in an effort to suggest that sites closed according to previous standards are not adequately remediated and continue to present a threat to public health.

VI can be a concern at any building located "near" soil or groundwater contaminated with volatile chemicals. The new Guidance maintains EPA's previous definition of "near": within 100 feet, laterally or vertically, of occupied buildings, but this distance could be greater under certain site-specific conditions. Unlike some other exposure pathways, changing weather patterns—including wind and atmospheric pressure—as well as soil conditions can affect exposure risks from VI. Figure 1 illustrates the basic mechanisms involved in VI. Of particular importance are convective "stack" effects arising from building heating.

[Figure 1: Mechanisms of Vapor Intrusion]

A Closer Look at the New Guidance

The new guidelines describe a two-step process: (1) a Preliminary Analysis to develop an initial understanding of the potential for human health risks from VI, and (2) a Detailed Investigation, which is recommended if the Preliminary Analysis indicates the presence of subsurface contamination with vapor-forming chemicals underlying or near buildings. PRPs can pursue two options in order to avoid the Detailed Investigation. First, there must be a source-to-building distance greater than 100 feet. Generally any conditions that would lead to advective (pressure- or gravity-driven flow) channeling of diffusive transport, or migration of the source region may invalidate the 100 foot criterion. Other possible confounding factors include existence of an underground soil gas conduit such as a utility line, source region methane generation3, release from a pressurized pipeline, and asphalt or other soil cover.

A second option to avoid the Detailed Investigation is use of the EPA's Vapor Intrusion Screening Level (VISL) calculator to demonstrate that indoor air concentrations are too low to represent a health risk. The calculator uses chemical-specific toxicity and physical parameters together with generic source-to-building attenuation factors to estimate indoor air levels. In using the calculator, it is important to consider the representativeness of parameters such as soil type. The attenuation factors are based on 95th percentile observational data for chlorinated solvents. A separate set of attenuation factors is included in the petroleum UST guidelines that take into account aerobic degradation of petroleum hydrocarbons. Demonstrating through soil gas and soil property measurements that sufficient degradation is occurring over the vertical separation between source and building is part of the analysis for petroleum USTs.

The new Guidance indicates that, in some cases, a party may decide to remediate homes rather than incur the additional expense of multiple measurements in a detailed investigation. However, this does not consider potential tort liability.

PRPs may have some reluctance to make indoor air measurements, since these can be confounded by in-home sources as well as outdoor concentrations. In addition, indoor concentrations can vary with season and occupant behavior, implying the necessity of multiple or extended measurements. The new Guidance recommends surveying and removing indoor sources as well as measuring outdoor air concentrations. However, more complex and costly analyses, such as isotopic evaluation, may be necessary to unravel the contribution from non-VI sources.

New Guidance As Applied: Target Concentrations of TCE

Some of these challenging issues are illustrated by the Guidance's instruction regarding trichloroethylene (TCE), the most common remediation "driver" at non-petroleum VI sites. (At petroleum UST sites, the "driver" is usually benzene.)

  • The VISL calculator gives a one-in-a-million excess risk of cancer for a TCE indoor air concentration of only 0.48 µg/m3 (about 90 parts-per-billion by volume). According to an EPA document4, TCE has been found in about 43% of homes not affected by VI with a 95th percentile concentration in all studies surveyed (14 studies) greater than 0.48 µg/m3. This indicates that indoor sources of TCE are often significant.
  • The VISL calculator also indicates that the target groundwater concentration of TCE is 1.2 µg/m3, about one-quarter of the Maximum Contaminant Level (MCL) drinking water standard which has been used for cleanups at many sites. At many other sites, because of poor water potability, the cleanup level has been higher. This suggests that further cleanup may be required at sites previously considered closed.
  • Finally, according to the VISL calculator, there is a target indoor air5 concentration for non-cancer effects of 2.1 µg/m3. This target concentration, intended to screen short term exposures, has been used by other agencies to develop response actions to protect women of reproductive age. For example, EPA Region IX and the San Francisco Bay Regional Water Quality Control Board (SFRWQCB) have issued their own Response Action Levels for both residential and commercial properties, as shown in Table 1.6

Table 1: Region IX and SFRWQCB guidance for TCE

Exposure Scenario

Accelerated response indoor air action level

Urgent response indoor action level


2 µg/m3

6 µg/m3

Commercial (8 hrs)

8 µg/m3

24 µg/m3

Region IX USEPA recommends that response time should be a few weeks for accelerated response action levels whereas it should be a few days for the urgent response action levels. The urgent response action may include temporary removals.

Recommendations To Minimize Litigation Risks

Vapor intrusion litigation presents uncertainty and risk for PRPs. In a recent ruling by a Minnesota federal judge, a class action related to injury from vapor intrusion was certified despite the PRP's protests that the proposed class boundary failed to consider alternative sources of contamination, and that alleged levels of contamination varied widely among class members.7 The PRP's inability to litigate such challenges before certification of a class illustrates that, even when a PRP has a potential defense regarding alternative contamination sources, it will have to incur significant cost and burden to litigate.

In order to avoid such litigation risks, PRPs should be proactive in determining whether previously closed sites are in compliance with the new Guidance and in incorporating the new Guidance into ongoing remedial projects. Like with any new set of federal standards, plaintiffs' attorneys are likely to latch on to the final Guidance as a "new standard of care" in litigation.

Additional Contributors from Exponent, Inc.

This client alert is jointly provided by Crowell & Moring LLP and Exponent, Inc. Crowell & Moring contributors are listed at the bottom of this page.

Renee Kalmes, MSPH, CIH
Exponent, Inc.
Senior Managing Scientist
Occupational & Environmental Science
+1 510.268.5007

Michael Kierski, PhD
Exponent, Inc.
Managing Scientist
Ecological & Biological Sciences
+1 608.544.2140

Brian Murphy, PhD
Exponent, Inc.
Principal Scientist
Environmental & Earth Sciences
+1 941.928.6735

About Exponent's VI Services

Exponent provides engineering and scientific consulting support to assess health and environmental exposure and risks from VI at residential, commercial, and industrial properties. These services include the evaluation of VI issues through use of soil vapor modeling and air monitoring tools and measurement data to evaluate the risks to current and future users of the site.

1 OSWER Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor Sources to Indoor Air, OSWER Publication 9200.2-154 and Technical Guide For Addressing Petroleum Vapor Intrusion At Leaking Underground Storage Tank Sites, both dated June 2015, available at

2 Environmental Protection Agency 2002, Draft Guidance for Evaluating the Vapor Intrusion to Indoor Air Pathway from Groundwater and Soils (Subsurface Vapor Intrusion Guidance (EPA530-D-02-004; November), available at

3 Methane is considered an indoor air hazard if its concentration is above 10% of the lower-explosive limit (i.e. about 0.5% by volume).

4 Background Indoor Air Concentrations of Volatile Organic Compounds in North American Residences (1990–2005): A Compilation of Statistics for Assessing Vapor Intrusion, June 2011, EPA 530-R-10-001B.

5 This concentration is intended to be one at which there would not be an adverse effect taking uncertainty factors into account. At higher concentrations there may be adverse effects.

6 July 9, 2014 Memorandum re: EPA Region 9 Response Action Levels and Recommendations to Address Near-Term Inhalation Exposures to TCE in Air from Subsurface Vapor Intrusion from Enrique Manzanilla /
Director Superfund Division, to Region 9 Superfund Division Staff and Management; SFRWQCB Interim Framework for Assessment of Vapor Intrusion at TCE-contaminated Sites in SF Bay Region (October 16, 2014).

7 Karl Ebert et al. v. General Mills Inc., No. 0:2013-cv-03341 (D. Minn. Feb. 27, 2015).

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