Changes to the Supplemental Vapor Intrusion Guidance

By April 11, 2023 No Comments

Changes to the Supplemental Vapor Intrusion Guidance

On February 23, 2023 the release of the Final Draft Supplemental Guidance: Screening and Evaluating Vapor Intrusion (Supplemental VI Guidance) was created jointly by the California Environmental Protection Agency, Department of Toxic Substances Control (DTSC) and the State Water Resource Control Board (SWRCB). The following will be a summary of the changes to the guidance and recap on what has remained unchanged.


The goal of Supplemental VI Guidance is to create statewide consistency in site investigation and cleanup of sites where subsurface contaminants in soil gas and groundwater pose a significant threat to the current building and occupants.  There is a four –step process for the screening of buildings for vapor intrusion and assessing potential health risks for open cases as delineated by the Supplemental VI Guidance. The four steps include:

  • Step 1 – Decide which buildings should be tested first and how.
  • Step 2 – Screen buildings from outside.
  • Step 3 – Test indoor air.
  • Step 4 – Act to protect people’s health.

The Supplemental VI Guidance also provides information and recommendations on Using USEPA 2015 Attenuation Factors (AFs), Considering sewers and utility pathways as a potential VI migration route and pathway of exposure when these utilities intersect contaminated subsurface media, and Building a California-specific VI database.  Please refer to the Final Draft Supplemental Guidance: Screening and Evaluating Vapor Intrusion for more detailed information.

Revisions to the Supplemental Guidance

Some of following key changes to the Supplemental Guidance are:

  • Clarification on the use of the Supplemental Guidance (Introduction);
  • Clarification of building prioritization (Step 1);
  • Clarification of soil gas sampling depths (Step 2);
  • Clarification on heating, ventilation, and air conditioning (HVAC) operation during indoor air sampling (Step 3);
  • Addition of post-screening approaches to refine current and future VI human health risk assessment (Step 4);
  • Addition of lines of evidence (LOE) attachment (Attachment 1);
  • Final Draft Supplemental Guidance: Screening and Evaluating Vapor Intrusion February 2023; and
  • Clarification of alignment between the Petroleum-Specific Considerations Attachment (Attachment 2) with the State Water Board’s Low-Threat Underground Storage Tank Case Closure Policy.

Recommendations that have not changed from the previous 2020 draft:

  • Early assessment of VI risk to occupants of buildings during the investigation phase of a cleanup project;
  • Expedited response action for immediate threat to human health;
  • Appropriate and predictable sampling approach to generate high-quality data to evaluate potential VI risk at a building;
  • Use of United States Environmental Protection Agency (USEPA) VI attenuation factors (AFs) for the initial screening of buildings;
  • Development of a high-quality, building-specific dataset needed for improved risk management decisions;
  • Assessment of future VI risk using subsurface data;
  • Inclusion of the high-quality VI data collected using the Supplemental Guidance in the GeoTracker database; and
  • Public engagement early and throughout the site investigation.

Site-wide assessment, cleanup goals, mitigation, long-term monitoring, remediation, and case/site closure are outside the scope of the Supplemental Guidance

A note on Attenuation Factors:

There is an ongoing debate within the regulatory and consulting communities regarding the AFs utilized by the USEPA and California EPA / DTSC/ SWRCB.  The current AFs were proposed in the 2020 Draft Supplemental Guidance and heavily debated.  Although heavily debated and acknowledged by the regulatory community as flawed, the AF of 0.03 has been upheld in this 2023 update.  Here at McAlister GeoScience, we recommend our clients take the option provided in the guidance to develop a site-specific AF.  The use of the provided AF of 0.03 will most likely generate a situation where a vapor intrusion risk is a false positive and not truly a risk at your specific site.

From the 2023 updated guidance:  

Vapor Intrusion Attenuation Factors
Attenuation factors are used to estimate how much of the vapors underground or in
groundwater end up in the indoor air. The attenuation factors in the U.S. EPA guidance may
be used in California. These factors are based on a large study of buildings at contaminated
sites throughout the United States, including several in California. These attenuation factors
are commonly used nationwide.


The Workgroup is aware of the strengths and limitations of USEPA’s AFs.  …
Limitations of the USEPA dataset include a) very few California sites are in the database,
b) 75 percent of residential homes in the database have basements but only
5 percent of homes in California have basements, c) USEPA did not evaluate
commercial/industrial buildings due to insufficient data, and d) groundwater and
indoor air paired measurements had poor spatial correlation.

Again, Here at McAlister GeoScience, we recommend our clients take the option provided in the guidance to develop a site-specific AF.

For more information

The Supplemental Guidance and previous February 2020 Draft Supplement guidance can be found at the State Water Board’s and DTSC’s websites:

State Water Board:


As always, if you have any questions on whether your site may be in an area of vapor intrusion or for any other questions you can always reach us at



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