In June 2015, the United States Environmental Protection Agency (EPA) released its long awaited final guidance aimed at providing a protocol and process for the assessment and mitigation of vapor intrusion (VI) at properties across the country. The guidance, which applies to Underground Storage Tank sites as well as standard Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) and Resource Conservation and Recovery Act (RCRA) sites, consists of two documents:

  • Office of Solid Waste and Emergency Response (OSWER) Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway From Subsurface Vapor Sources to Indoor Air (OSWER VI Guide)
  • Technical Guide for Addressing Petroleum Vapor Intrusion at Leaking Underground Storage Tank Sites (PVI Guide)

Both Guides are available at

The OSWER and PVI Guides provide direction on developing VI conceptual site models, detailed VI investigation, risk assessment/management, VI mitigation and risk communication. The OSWER VI Guide replaces EPA's 2002 Draft VI Guidance, but if you were hoping for a clear path forward, you will be disappointed. Instead, the Guides formally adopt the multiple line of evidence approach, and provide extensive technical detail in an effort to comprehensively address VI.

If you have any interest in a contaminated or potentially contaminated property that may fall under EPA’s jurisdiction, the following elements of the OSWER and PVI Guides are important to understand:

  • Early Preemptive Mitigation: Throughout the Guides, there is an emphasis on early preemptive mitigation during the evaluation process – and even as an alternative to lengthy drawn-out VI assessments – in order to alleviate exposure and protect human health. Such an emphasis can cause complications in situations where multiple sources are involved, especially in a residential setting. Having to provide a remedy before confirming a nexus with the source at issue can put the liability cart before the horse. Nevertheless, the Guides promote a preliminary analysis that relies on available information, followed up by a more detailed investigation that focuses on subsurface conditions. Further, addressing the source causing any actual VI remains a requirement under CERCLA/RCRA, and terminating VI mitigation is contingent on demonstrating the source of vapors (e.g., contaminated soil and/or groundwater) has been treated. Thus, it is likely that EPA may seek “interim remedies,” but companies are likely to push back despite the language in the Guides.
  • Who’s in Charge: EPA or OSHA? EPA recommends using human health risk assessment methods consistent with EPA guidance and current chemical toxicity to evaluate potential risks to building occupants when there is a complete or potentially complete VI pathway. EPA does not recommend use of Occupational Safety and Health Administration (OSHA) permissible exposure limits (PELs) to assess human health risk posed to workers by VI, nor does it support a no-further-action determination for VI in nonresidential buildings. Although the new jurisdictional reality is still unclear, in the past EPA has agreed to the use of PELs for VI assessments at OSHA compliant facilities where chemicals of concern are used in facility processes and the workers are informed and trained regarding the relevant chemicals. At this juncture, it is reasonable to conclude that this will remain a valid approach.
  • Continual Review of Closed Sites for VI Purposes: EPA frequently is raising the need for VI assessments during five-year reviews of Superfund sites, particularly in situations where VI was not previously considered. Even where VI was considered previously, EPA is identifying the need for updated VI assessments to ensure consistency with current VI guidance, chemical toxicity and assessment of human health risks. Perhaps even more significantly, the Guides recommend that site managers consider whether unacceptable levels of vapors can occur in the future if site conditions change.
  • A Technical Push for Multiple Lines of Evidence = More Data! Many states already adopted the multiple lines of evidence approach. Mathematical modeling of VI is no longer sufficient and limited sampling events are now confirmed to be outside of the accepted standard of care. The Guides suggest the presence of five conditions for a vapor intrusion pathway to be complete: (1) a subsurface source of vapor-forming chemicals near the building; (2) a migration route; (3) a building that will permit the entry of soil gas; (4) the presence of the same chemicals in the indoor air; and (5) occupancy of the building (although one can argue that the fifth condition is something EPA often ignores, as “potential” seems to be as good as “actual” these days). A few of the key technical components EPA discusses in the Guides that will feed into these evaluations include the following:
    • EPA recognizes that rapid biodegradation of petroleum hydrocarbons occurs over relatively short distances, even in the presence of Light Non-Aqueous Phase Liquid (LNAPL), when sufficient oxygen and soil moisture exist in the unsaturated zone;
    • EPA establishes inclusion zones, or horizontal and vertical separation distances, beyond which VI is not of concern, except where preferential vapor migration pathways exist;
    • EPA recommends that time-integrated measurements from multiple sampling events be used to assess VI exposures to account for variability in soil gas and indoor air concentrations that can occur due to changing weather and building operation conditions; and
    • EPA requires evaluation of acute, or short-term, exposures in the assessment of VI as well as chronic, or long-term, exposures.

The overall takeaway is that VI must be considered any time there is volatile subsurface contamination present for any property that could be subject to the federal regulatory scheme. Thus, VI has to be considered as part of all transactions involving the transfer or lease of any property, and the cost of CERCLA and RCRA assessments and clean-ups are only likely to continue to increase. There may never again be any true meaning to “No Further Action,” especially for any CERCLA site that is part of a five-year review process. Thus, it is important to structure a team that is well versed in all aspects of VI and to develop a plan to assess and address any potential for such an issue early in the process.