When volatile chemicals are released onto properties, they can permeate the soil and underlying groundwater. Over time, they can migrate into buildings in gaseous form, resulting in vapor intrusion (VI). Depending on the concentrations of these chemicals and their toxicity, the volatile chemicals in the indoor air might produce human health concerns.

While the issue is global, California regulatory agencies have developed new, supplemental VI guidance for conducting evaluations of existing buildings. Building upon the state’s 2011 vapor intrusion guidance, it specifically addresses VI concerns to occupants of existing buildings, especially from chemicals like trichloroethylene that research shows can potentially affect fetuses of pregnant women at low doses. The supplemental guidance does not cover vapor investigations in general, such as for understanding site contaminant issues as a whole or for complying with regulatory site cleanup requirements.

In the past, differing regulatory frameworks from various agencies sometimes led to inconsistent approaches for evaluating VI in buildings at contaminant release sites. To improve consistency, supplemental guidance was developed through a consortium of VI professionals from across the California Environmental Protection Agency (CalEPA) and prepared by individuals from the Department of Toxic Substances Control (DTSC), the San Francisco Bay Regional Water Quality Control Board and the State Water Resources Control Board, among others. This concerted effort will lead to greater consistency within California for regulating VI at existing facilities.

Key elements of this VI guidance include:

  • Adopting the Environmental Protection Agency’s (EPA’s) empirically derived default attenuation factors of 0.03 for soil gas/subslab soil gas and 0.001 for groundwater until California-specific values become available.
  • Recommending a four-step process for evaluating VI in occupied buildings.
  • Containing new discussion on the importance of sampling contaminated vapor conduits, such as sewers, storm drains, or pipes, as potential VI sources.

The four-step VI evaluation process begins by prioritizing the order in which buildings should be investigated and deciding on a sampling approach. Occupied buildings that may present imminent VI health threats through toxicity, fire or explosion hazards are of top concern. If any of these threats exist, immediate action is needed. Buildings should then be ordered for investigation based on their occupancy, proximity to the contamination and possible connections to contaminated vapor conduits.

Depending on available site data, the investigation may proceed to either step 2 or step 3. The guidance explains how to evaluate potential health risks using data from each step, which was not incorporated into the 2011 guidance. The new guidance has stricter attenuation factors, derived by the EPA from data studies over time, to estimate how much of the subsurface vapor concentrations lessen before entering indoor air. This change could push more buildings into further VI assessment as a health protective approach.

The second step involves collecting shallow soil gas samples outside the building, but near the foundation perimeter, especially near contaminant sources. This step would be suitable under certain conditions; for example, if the volatile chemical releases are relatively small, the chemicals released are not very toxic or there is a limited pathway for vapors to migrate from below ground to indoor air.

Step 3 moves into a much more comprehensive assessment with concurrent sampling of indoor air, outdoor air and vapor from just beneath the building foundation (subslab). For each medium, guidance is provided on the sample locations, depths, frequency (to account for temporal variability due to building or seasonal changes) and number of samples to collect. This guidance includes additional information on how to conduct pre-sampling building surveys, including field screening with either field instruments or grab samples.

If health risk estimates after step 3 are greater than acceptable levels, then the VI evaluation continues to step 4, which addresses managing risks to reduce exposures. Approaches can be short-term, such as increasing building ventilation rates, or long-term, such as installing subslab venting systems.

The new guidance further describes the importance of collecting multiple lines of evidence (LOE), which could be qualitative as well as quantitative, in a VI evaluation. In addition to sample data, LOE include site history and chemical use, building construction, chemical release distance from buildings where people could be exposed, and other related information. The guidance offers adjustments to VI evaluations to account for different building types, such as large buildings, multistory buildings or buildings with parking structures below the occupied spaces. If site or building conditions change, future exposures should also be considered. Given the complexity of VI evaluations, the guidance suggests a multidisciplinary, professional team be assembled to provide sound, scientific judgment to protect human health.

The latest guidance reflects a shift toward more current, scientifically defensible methods to protect people from VI threats. The draft supplemental guidance is available for review and public comment until June 1, 2020. The agencies will then review and address comments and release a finalized version. With aligned regulatory expectations across state agencies, landowners and developers now have a clearer path to screening and evaluating VI at occupied buildings.


Every environmentally impacted site is different, so your approach to understanding site conditions, contaminants and regulatory drivers must be carefully developed.

Learn More

Kanan Patel-Coleman, a project manager and senior risk assessor at Burns & McDonnell, works with a team of human and ecological risk assessors to address environmental exposures from chemical releases. She has experience in health risk assessments, vapor intrusion investigations, product safety assessments and project management.