California Finalizes Draft Supplemental Guidance on Vapor Intrusion Screening and Evaluation

February 28, 2023

The California Department of Toxic Substances Control (DTSC), State Water Resources Control Board, and San Francisco Bay Regional Water Quality Control Board (collectively, CalEPA VI Workgroup) published their final draft supplemental guidance relating to the screening and evaluation of vapor intrusion on February 23 (Final Draft Supplemental Guidance). The plan provides clarification and additional information on the process the state is recommending for detecting vapor intrusion and applying attenuation factors in assessing contaminated sites.

The term vapor intrusion (VI) refers to the potential for vapors from contaminated groundwater and soil to migrate into indoor air. To provide a standardized method of measuring and evaluating sites with potential VI, the DTSC released its 2011 Vapor Intrusion Guidance. Over time, regulators decided to supplement this guidance with lessons learned and advances gained related to the assessment of VI. The CalEPA VI Workgroup published the first of these supplements on February 14, 2020, focused on the screening and evaluation of VI.

Despite the public comment period for this guidance ending on June 1, 2020, the CalEPA VI Workgroup delayed in publishing its final version until now. This delay caused widespread confusion in the interim as stakeholders remained unclear on what standards to apply with respect to vapor intrusion practices. To address this ambiguity, the DTSC issued a one-page advisory in August 2022, providing clarity on site screening, delineation, and migration. The August advisory, however, is now superseded by the newly published Final Draft Supplemental Guidance.


The Final Draft Supplemental Guidance is intended to add to, not replace, the DSTC’s 2011 Vapor Intrusion Guidance. It is also not intended to apply to leaking petroleum underground storage tanks, which are governed under a separate state policy. The guidance, instead, covers the assessment of existing buildings and evaluation of future buildings, constructed on open lots or redevelopment sites, to better protect building occupants from potential risks of VI.

In an effort to further these goals, the guidance establishes a four-step VI evaluation tool for existing buildings, provides an overview of VI risks posed by sewers and available screening methods for these pathways, clarifies which attenuation factors should be used in assessing contaminated sites, and discloses that data from sites evaluated using the criteria in the Final Draft Supplemental Guidance will be entered into a publicly available database to inform future regulatory efforts.


The CalEPA VI Workgroup received over 575 public comments from 71 individual letters/emails. These comments and CalEPA VI Workgroup’s responses guided many of the revisions to the final draft. Of these revisions, a few of the more important changes are listed below.

Four-Step VI Evaluation Tool

The Final Draft Supplemental Guidance clarified the four steps that should be taken when screening and evaluating buildings for VI:

  1. Evaluating a building’s proximity to a spill or known source of potential VI, when there are several buildings, to inform an investigator which building to evaluate first or whether to skip to outdoor screening or indoor air testing
  2. Measuring for vapor forming compounds (VFCs) outside a building of interest at different locations, and considering how the concentration of such compounds varies during different seasons
  3. Testing the indoor air of buildings of interest for VFC concentrations
  4. Taking appropriate remedial action in response to the screening results, including relocation in extreme cases

Only once these steps are complete does the CalEPA VI Workgroup recommend developing an overall cleanup plan that accounts for the specific site characteristics and contaminants of interest.

Sewers as a VI Pathway

The Final Draft Supplemental Guidance further recognizes that, in addition to entering through openings in a foundation, VFCs can migrate through subsurface pipe networks, including sewers, drains, etc. Because some traditional ways of testing for VI could miss VFCs that travel through sewer pipes, the CalEPA VI Workgroup recommends adopting a specialized test for such pathways. This tailored approach involves a desktop review to identify “the locations and depths of sewers and utility conduits in the vicinity of release areas or groundwater plumes” followed by field sampling investigation of sites determined to pose a greater risk of a threat to human health. These methods are described in greater detail in Attachment 3 of the Final Draft Supplemental Guidance.

Attenuation Factors

One of the more closely watched updates of the Final Draft Supplemental Guidance was the announcement of attenuation factors for VI assessments. Attenuation factors are ratios that help predict the fraction of chemicals that may pass into a building as vapors and can be used to determine whether such quantities would pose a risk to human health.

In a step away from DTSC’s 2011 VI Guidance, the CalEPA VI Workgroup clarified that while the US EPA’s 2015 attenuation factors may be relied on for initial screening of sites in California, “alternative approaches may be used if supported by adequate technical and site information.” Such information should include an evaluation of the spatial and temporal variability of VFC concentrations in various media and multiple lines of evidence, including sampling data and other qualitative and quantitative information collected prior to and during a site investigation and building evaluation. Given this flexible approach, concerns regarding a default assumption based on attenuation factors resulting in increased expenses remediating contaminated sites may be reduced if supported by the proper data.

California VI Database

To inform regulatory actions moving forward, the CalEPA VI Workgroup announced that data collected during site investigations and reported to GeoTracker will be compiled in a California database. This database will guide the development of state-specific attenuation factors that may be incorporated into future versions of this supplemental guidance. This database will be publicly available and as such, will carry with it risks often associated with publicly available data, including the possibility of users drawing incorrect statistical inferences. However, it will also provide accessibility to data that may inform facilities regarding neighboring contamination risks, in addition to providing a more robust dataset for future policymaking.


The CalEPA VI Workgroup emphasized this Final Draft Supplemental Guidance “is meant to promote Statewide consistency in site investigation and cleanup at sites where contaminants in soil gas and groundwater pose an unacceptable risk to current and future building occupants.” With a final draft now published, industry stakeholders should now have additional clarity regarding the state’s expectations in investigations and workplans. This includes the ability to present alternative attenuation factors for initial screening when supported by the requisite data.


If you have any questions or would like more information on the issues discussed in this LawFlash, please contact any of the following:

Rick R. Rothman (Los Angeles)
David K. Brown (Los Angeles)
San Francisco