Human Health Risk Assessment 10 Narrowing Constituents of Concern to Reduce False Determination To expediate evaluation of the vapor intrusion pathway, our team examined the soil vapor analytical results collected from the light non- aqueous phase source to develop a list of constituents that might be present at concentrations that could pose an inhalation risk within residences. We were able to narrow the list of potential volatile petroleum hydrocarbons to three constituents of concern for sub-slab soil vapor and indoor air. Background sources, particularly petroleum related, are ubiquitous in indoor air from many sources, including but not limited to, building products, cleaners, paints, automotive products, and tobacco uses. These background sources complicate evaluation of the vapor intrusion pathway. Narrowing the constituents of concern within indoor air reduced the potential for a false determination of a completed vapor intrusion pathway within the nearby residences. Furthermore, our team worked with the regulatory agency and its technical expert to identify residences that were at the greatest risk due to: (1) the proximity to the significant gasoline release or (2) the building construction type. This limited the collection of indoor air, crawlspace, and sub-slab soil vapor samples to eleven structures within the residential community. Solution Helps Client Avoid Costly Mitigation and Investigation Following collection of the samples within the eleven structures, a decision matrix was developed to segregate the structures into six potential behavior types. This decision matrix simplified evaluation of the vapor intrusion pathway for each structure. Within each of the residences, the concentration of the constituents of concern were above the USEPA vapor intrusion screening levels. However, the source of these constituents was attributed to background sources in indoor and outdoor air and the vapor intrusion pathway was determined to be incomplete within each structure. This determination was supported by the multiple lines of evidence and decision matrix. The regulatory agency and its technical expert quickly approved the findings of the expedited investigation. Our innovative approach helped our client avoid costly mitigation and prolonged investigation within the residential community. Our client was able to instead focus their efforts on reducing their long-term liability and recovering the significant gasoline release beneath the residential community.