The California Environmental Protection Agency’s (Cal/EPA’s) Department of Toxic Substances Control (DTSC ) has issued final guidance on subsurface vapor intrusion into indoor air. The guidance, released October 12, 2011, details how developers and site cleanup contractors should address and mitigate volatile organic compound (VOC ) emissions that enter buildings from contaminated soil. DTSC requires that human health risks be evaluated at sites under its oversight and include exposure from vapor intrusion in the evaluation if VOC s are present.

Evaluating an indoor air exposure pathway involves characterizing the nature and extent of subsurface VOC contamination, obtaining appropriate environmental data, using fate and transport models to predict indoor air concentrations from vapor intrusion, and conducting indoor air sampling if necessary. The guidance, according to DTSC , outlines the technical aspects of evaluating the exposure pathway and provides recommendations on elements that should be included in a site investigation. DTSC developed the guidance over several years and considered more than 370 public comments on earlier drafts in preparing the final document.