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California Draft Vapor Intrusion Guidance Now Final

Updated: Apr 3, 2023

On February 23, 2023, California EPA (Cal EPA) finally released a “final draft” version of the much-awaited vapor intrusion guidance, Final Draft Supplemental Guidance: Screening and Evaluating Vapor Intrusion (VI Guidance) representing a collaboration between the Department of Toxic Substances Control (DTSC) and the State Water Resources Control Board (SWRCB). According to CalEPA, the VI Guidance is a tool to help evaluate buildings that may be prone to vapor intrusion from underlying or nearby contamination and to assess potential health risks to building occupants. In addition, the VI Guidance is intended to promote consistency among the primary state agencies responsible for regulating vapor intrusion -- DTSC and the Regional Water Quality Controls Boards (RWQCBs) under the umbrella of the SWRCB.

The Guidance identifies four steps to protect health and safety at contaminated sites. First, identify the buildings most at risk of vapor intrusion from the release. Second, screen for the presence of vapor-forming chemicals just outside the building. Third, test indoor air and beneath the building foundation. Fourth, based on testing data, act to protect the health of persons inside the building, which step in “extreme cases” could include temporary evacuation of the building.

Despite numerous comments during the public comment period objecting to the use of U.S. EPA’s conservative attenuation factor of 0.03 (which is not based on California

specific data), the VI Guidance adopts EPA’s attenuation factor. According to DTSC and SWRCB webpages, data collected from investigations using the VI Guidance may be used to build California-specific attenuation factors for use in the future. Meanwhile, at least one empirical study using California data has already indicated less conservative attenuation factors are appropriate for California sites.

It remains to be seen whether the VI Guidance will in fact promote consistency in vapor intrusion investigations between the DTSC and various Regional Water Quality Control Boards – or whether certain local agencies (such as Alameda County Public Health Department) will use the VI Guidance. Some agencies have already been using the VI Guidance, including the U.S. EPA attenuation factor, since the draft was circulated over three years ago.

In general, however, the VI Guidance serves as a reminder that the costs and liabilities of owning contaminated property is not limited to the cleanup of subsurface media but to investigating potential or actual indoor air intrusion, identifying risks associated with indoor air intrusion, and managing and mitigating those risks. Effective risk management goes beyond compliance with the VI Guidance itself but includes establishing a contingency plan for sampling outcomes and communicating risk with building occupants as appropriate.

For more information contact Catherine W. Johnson at



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