California regulators have announced that the comment period for a recent proposal, Supplemental Guidance: Screening and Evaluating Vapor Intrusion, has been extended to June 1, 2020, and public workshops and webinars originally scheduled for April have been postponed until further notice.

Vapor intrusion occurs when contamination moves from groundwater and soil beneath a structure into the air, accumulating in occupied areas where they can result in safety hazards or health effects. Common vapor-forming chemicals include volatile organic compounds including trichloroethylene (TCE), mercury, polychlorinated biphenyl (PCB), and certain pesticides. Testing for vapor intrusion is an important step in investigating a potential development site, and in ensuring the health and safety of existing residential and commercial buildings.

The Supplemental Guidance, which was developed collaboratively by the Department of Toxic Substances Control (DTSC), the San Francisco Bay Regional Water Quality Control Board, and the State Water Resources Control Board, is intended to increase the consistency of testing methods. It will also improve the reliability of data resulting from VI testing, which will be compiled into a planned statewide VI database through the State Water Board’s GeoTracker online system. The Supplemental Guidance also would integrate improved scientific understanding of how contamination moves from soil and groundwater to air and of short-term toxicity in certain contaminants.

The Supplemental Guidance includes recommendations for procedures for testing potentially contaminated water and soil, as well as for monitoring indoor air quality in affected structures, cover a broad range of topics, including:

  • How to prioritize testing, and to identify which buildings are the most likely to be affected by contamination.
  • Methods for evaluating multiple lines of evidence related to complex sites or facilities to understand the nature and scope of contamination.
  • Recommendations to implement data-based standards for evaluating and responding to specific risks or impacts, including updated attenuation factors (AF).
  • Mitigation and risk management recommendations.
  • Best practices for selecting locations for sampling, based on current understanding of how contaminants travel. This includes new understanding of the potential for movement in and through sewer pipes and other preferential pathways.

The Supplemental Guidance complements the procedures and practices described in existing guidance, including DTSC’s 2011 Final Guidance for Evaluation & Mitigation of Subsurface Vapor Intrusion to Indoor Air, and the San Francisco Bay Area Regional Water Quality Control Board 2014 Interim Framework. These documents remain part of current compliance guidance, as does EPA Vapor Intrusion guidance for testing and assessment of sites regulated under the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA).

Notably, the Supplemental Guidance does not apply to the evaluation of releases from underground storage tanks (USTs), which must be evaluated for vapor intrusion based on the requirements of State Water Board Resolution 2012-0062, Low-Threat Underground Storage Tank Case Closure Policy (2012).

For additional information or assistance related to vapor intrusion, USTs, or related issues, please contact Angela Levin ( or Louise Dyble (