The California Environmental Protection Agency, alongside the California Department of Toxic Substances Control and the California State Water Resources Control Board, introduced the Final Draft Supplemental Guidance: Screening and Evaluating Vapor Intrusion (Supplemental VI Guidance) in February. This guidance supplements existing state protocols for vapor intrusion evaluations to better assess and manage health risks for building occupants across California.
Vapor intrusion (VI) occurs when chemical vapors migrate from contaminated soil or groundwater into buildings, posing potential health hazards. The new guidance is designed for use at all phases of investigation and cleanup, especially when new data gaps are identified, or conditions change. It provides a structured four-step screening and evaluation process:
1. Prioritize Buildings – Identify and prioritize buildings near contamination sources for VI assessment.
2. Collect Exterior Soil Gas Samples – Conduct preliminary soil gas sampling outside to assess potential risks of vapor intrusion.
3. Conduct Indoor Sampling – Where risks are identified, sample indoor air, subslab soil gas, and outdoor air to determine the extent of intrusion risks.
4. Evaluate Risk Management Needs – Assess the need for managing current and future risks based on a combination of indoor and subsurface data.
The Supplemental VI Guidance also emphasizes the importance of seasonal and spatial contamination variability and offers a framework for risk management that relies on multiple lines of evidence—such as sampling data and qualitative analysis—to make informed decisions. This comprehensive approach, which also includes public participation requirements, enables regulators and practitioners to address both current and potential future health risks more effectively, enhancing protections for building occupants across the state.
Click HERE for more environmental investigation project examples informed by this guidance.
