Potential buyers and sellers take note—on April 16, 2013, the U.S. Environmental Protection Agency (U.S. EPA) issued draft guidance that would require more evaluation of "vapor intrusion" (migration of any hazardous vapors from any subsurface contaminant source into indoor air). This evaluation of vapor intrusion (VI) would be done under "OSWER Final Guidance for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Sources to Indoor Air, (External Review Draft)," 04-11-2013. See footnote [1]. U.S. EPA calls the guidance a "framework for assessing vapor intrusion" for Superfund and Resource Conservation and Recovery Act hazardous waste sites using "multiple lines of evidence to support risk management decisions." (p.1.) U.S. EPA is requesting comments by May 24.

ASTM International anticipated the marketplace would require more information about vapor intrusion and issued E2600-10 "Standard Guide for Vapor Encroachment Screening on Property Involved in Real Estate Transactions" originally in 2008 and again in June 2010. Even though the 04-11-2013 draft is not designed for transactions, the U.S. EPA document will influence how businesses evaluate properties for sale.

Since 2002, U.S. EPA has focused considerable attention on the vapor intrusion pathway. Evaluating what concentrations of volatile organic chemicals have entered an enclosed structure as a gas from soil or groundwater is becoming an essential component of transactional due diligence as well as an exposure pathway that must be evaluated in a property cleanup. For example, in the 2012 OSWER Directive 9200.2-84, "Assessing Protectiveness at Sites for Vapor Intrusion," U.S. EPA instructed its Regions to analyze the vapor intrusion pathway in five-year Superfund reviews to verify that a remedy remains protective even if the vapor intrusion pathway had not been analyzed during the original remedy selection. Some commentators have predicted that U.S. EPA will reopen cleanups and require substantial and costly additional VI remedies.

Commercial and residential developers, manufacturers, responsible parties at Superfund and RCRA sites and a wide variety of industries and property owners will be affected by U.S. EPA's guidance.


The draft 04-11-2013 Final Guidance is actually one of a suite of guidances, including the "OUST Guidance," which specifically evaluates vapor intrusion in the context of RCRA-regulated underground storage tank contamination. See footnote [1].

Sections 3 and 9 of this Final Guidance provide recommendations for pre-emptive mitigation (PEM). This proactive approach can result in immediate actions, such as adjustments to building ventilation or air circulation "based on limited, but credible, subsurface and building data . . . ." to protect current occupants. (Final Guidance, p. 120, Section 9.1. See list of mitigation options for existing buildings, Table 8-1, p. 97.) Immediately acting on preliminary data may be the most cost-effective approach to resolve VI issues in a transaction. The issues addressed by the 04-11-2013 draft Final Guidance are summarized below.

  1. Should I Evaluate the Vapor Intrusion Pathway?

To answer the question of whether a property owner, lender or other entity with an interest in whether there is VI liability should evaluate the vapor intrusion pathway, the entity needs to know:

  1. Is there a source of volatile organic chemicals (VOCs)[2], radon or elemental mercury in the soil or groundwater beneath or near the building under study?
  2. Do these VOCs have a way to move from the soil or groundwater to the building?
  3. Are there routes through which the VOCs can enter the building?
  4. Are there driving forces (e.g. pressure differentials) that would draw the VOCs into the building?

If conditions one through four are present, according to the 04-11-2013 draft guidance, the owner, lender, tenant or other interested party has demonstrated, at a minimum, that a potential VI exposure pathway exists.

  1. Is the VI exposure pathway complete?

To determine if the VI exposure is complete the data must show (1) the existence of VOCs, (2) a pathway for VOCs to the building and (3) a route or force which will allow or drive VOCs into the building to reach a person.

  1. Are there VOC concentrations that would cause a health risk?

If the exposure route is complete (the vapor can reach a human), the next part of the evaluation is to determine whether VOCs entering the building are in high enough concentrations to create a human health risk. The first step in determining if there is a health risk is locating and identifying the types of VOCs. These locations and chemical characteristics are the basis for developing a Conceptual Site Model (CSM). With the CSM in hand, an assessor can develop a more comprehensive sampling and evaluation approach. The property owner, lender or other interested entity can perform more detailed sampling to establish VOC concentrations outside or around the building and apply attenuation factors to VOC concentrations inside the building. These projected indoor air concentrations can then be compared to the health risk-based numbers to see if a risk exists.

U.S. EPA has developed a Vapor Intrusion Screening Level Calculator to assist in this analysis.

See document d. in footnote [1].

  1. To Evaluate VI What Types of Sampling and What Sampling Locations Should I Select?

U.S. EPA has two levels of VI assessment: (1) preliminary analysis and (2) detailed investigation. (p. 28.)

An ASTM-type Phase I can provide the first level of information on VOC location and type to develop a CSM. In addition to standard Phase I data, an entity can collect information on symptoms from building occupants, locations of utility corridors and basement or slab integrity.

VI can arise from a variety of media: soil, soil gas, groundwater and/or underground containers. The CSM is supposed to focus the investigative efforts on the likely chemicals and sources. (See Figure 6-1, p. 48.) Therefore, an interested entity may need samples from beneath or near the building in some or all of these media. (p. 33 and p. 53.) According to U.S. EPA's guidance to have an accurate characterization, the sampling plan needs to consider a) if multiple locations should be sampled over an extended time period to account for seasonal variations in vapor concentrations, b) depth to groundwater, c) existence of a clean zone of groundwater between a VI source and the building, d) nature of the subsurface geology and e) nature of the chemicals. (pp. 53 to 56; pp. 64 to 71.) These factors have a bearing on whether VI is likely. For example, if Dense Non-Aqueous Phase Liquid (DNAPL) (sinking chemicals) is the form the chemicals are taking, the likelihood of VI is reduced greatly. In the draft guidance, U.S. EPA recommends that likely sources within 100 lateral or vertical feet of the building be included in a sampling survey. (p. 49.) Sampling beyond the 100' distance is appropriate where migration pathways like utility corridors are identified. The complexities of this approach highlight why PEM can be a very valuable tool particularly in a fast moving transaction.

  1. What Attenuation Factors Should I Consider?

Section 6.5.2. of the draft 04-11-2013 Final Guidance uses the Johnson & Ettinger definition of "vapor attenuation": reduction in volatile chemical concentrations that occurs during subsurface vapor migration, coupled with the dilution that can occur where the vapors enter a building and mix with indoor air. (p.74.) Both physical and chemical mechanisms can cause attenuation. Based on vapor intrusion studies from around the country, U.S. EPA has calculated what decreases in concentration (attenuation factors) are likely to occur in certain groundwater and soil contamination scenarios. See Table 6-1 in the draft 04-11-2013 Final Guidance. (p. 75.)

However, if the circumstances at the site are not consistent with the assumptions underlying its attenuation factors then the Agency recommends testing the indoor air. Testing the indoor air is not an ideal solution. U.S. EPA acknowledges it is difficult to isolate the evidence of vapor intrusion from volatile organic chemicals coming from indoor air sources like carpets and stored chemicals or from outdoor sources, such as vehicles and drums of solvents. (pp. 24-25; p. 62.)

Because of swings in VOC concentrations over time and across various parts of a building, "fully characterizing" a vapor intrusion problem, as U.S. EPA may do in a Superfund or RCRA situation, can be time-consuming and expensive. "[I]ndoor air concentrations and soil gas concentrations can exhibit significant temporal variations even for a single building . . . ." (p. 24.) A "full characterization" is likely to require samples from a variety of locations and media (soil, soil gas, groundwater) taken over time.[3]

  1. What Risk Assessment Approach Does the Guidance Describe?

The level of data needed to apply the risk assessment approach in the Final Guidance is substantial: "multiple lines of evidence" supporting a "sound" Conceptual Site Model and significant characterization of the subsurface vapor sources. (Section 6, pp. 78-80; Section 7 of Final Guidance pp. 81-85 which list the "multiple lines of evidence" including Phase I investigations, soil data, groundwater data, soil gas data, evidence on migration pathways like utility corridors and conditions of foundations.)


The comprehensive nature of the 04-11-2013 Final Guidance and the documents related to it form a valuable resource. The attenuation factors, if used as practical bench mark, can provide more certainty about whether VI poses a health risk. The fact that U.S. EPA compiled the factors (e.g., clean aquifers can act as barriers to VI) based on data it has collected will give them credibility. In a transaction setting, entities affected by this guidance need to understand the cost-effective options such as PEM to avoid being caught in an expensive and time-consuming "full characterization" of the VI issue. However, since the 04-11-2013 guidance was developed for regulatory programs, it says "[v]apor intrusion mitigation of buildings should not be viewed as a substitute for remediation of subsurface vapor sources." (p. 93, also p. 120.)


The Final Guidance covers full Superfund and RCRA site VI characterizations as well as simple single building VI evaluations. Conservative regulators or lenders may focus on the resource intensive "full characterization" approach as the route to the greatest certainty about VI risks. However, substantial certainty about actual VI concentrations is probably not needed in most business transactions. In deals, PEM, such as adjusting ventilation to reduce VI concentrations, is likely to be much more cost-effective than doing detailed sampling and analysis.


Preliminary investigation often will not be definitive enough to eliminate the VI pathway as a potential issue. Phase I studies tests for small projects can run from $1,500 to $5,000. Sampling of any type is beyond an ASTM Phase I scope of work. Costs to analyze indoor air samples can range from $80 to $300 per sample canister, depending on the analytical method used and suite of constituents tested for. The labor and equipment costs for collection of soil vapor and indoor air samples can range from several hundred dollars to thousands of dollars, depending on the size and complexity of the site.[4] In a 2011 transaction, the Phase I for a four-story office building occupying a little over an acre cost $2,700. Analysis of VI risk from concentrations of chemicals identified in pre-transaction soil and groundwater tests was priced at $3,500. A VI air modeling evaluation for the building was estimated to be $5,000.


New construction can address VI problems through the installation of vapor barriers and passive mitigation systems which, in our experience, are relatively inexpensive. With respect to existing structures, sub-slab ventilation techniques, much like those used for radon mitigation, are effective remediation approaches. Proactive mitigation (i.e., installing ventilation systems) has been more cost-effective than a full VI characterization followed by, in some instances, a source removal program.