Detailed site investigations to assess potential inhalation exposure and risk to human health associated with the migration of petroleum hydrocarbon vapors from the subsurface to indoor air are frequently undertaken at leaking underground storage tank (UST) sites, yet documented occurrences of petroleum vapor intrusion are extremely rare. Additional assessments are largely driven by low screening‐level concentrations derived from vapor transport modeling that does not consider biodegradation. To address this issue, screening criteria were developed from soil‐gas measurements at hundreds of petroleum UST sites spanning a range of environmental conditions, geographic regions, and a 16‐year time period (1995 to 2011). The data were evaluated to define vertical separation (screening) distances from the source, beyond which, the potential for vapor intrusion can be considered negligible. The screening distances were derived explicitly from benzene data using specified soil‐gas screening levels of 30, 50, and 100 µg/m3 and nonparametric Kaplan‐Meier statistics. Results indicate that more than 95% of benzene concentrations in soil gas are ≤30 µg/m3 at any distance above a dissolved‐phase hydrocarbon source. Dissolved‐phase petroleum hydrocarbon sources are therefore unlikely to pose a risk for vapor intrusion unless groundwater (including capillary fringe) comes in contact with a building foundation. For light nonaqueous‐phase liquid (LNAPL) hydrocarbon sources, more than 95% of benzene concentrations in soil gas are ≤30 µg/m3 for vertical screening distances of 13 ft (4 m) or greater. The screening distances derived from this analysis are markedly different from 30 to 100 ft (10 to 30 m) vertical distances commonly found cited in regulatory guidance, even with specific allowances to account for uncertainty in the hydrocarbon source depth or location. Consideration of these screening distances in vapor intrusion guidance would help eliminate unnecessary site characterization at petroleum UST sites and allow more effective and sustainable use of limited resources.

Vapor intrusion is associated with subsurface sources of both chlorinated VOCs and petroleum VOCs; however, petroleum vapor intrusion has been reported to occur under a narrower range of hydrogeologic settings. Research conducted over the last several years including field studies, evaluation of large datasets, and modeling studies, has provided an improved understanding of the differences in vapor intrusion associated with chlorinated VOCs and petroleum VOCs. When oxygen is present in the vadose zone, aerobic biodegradation typically results in rapid attenuation of petroleum VOCs diffusing upwards from deeper sources. At many building sites, wind-driven advection and/or building pressure fluctuations provide sufficient oxygen transport below the foundation to support this aerobic biodegradation. In such cases, there is limited potential for vapor intrusion from dissolved sources of petroleum VOCs unless preferential migration pathways are present. These findings support a framework for the evaluation of vapor intrusion at petroleum hydrocarbon sites that involves simple screening for preferential pathways at sites with sufficient vertical separation between the building and a dissolved source (e.g., 3 m) or a LNAPL source (e.g., 10 m), but a more intensive investigation at sites with petroleum sources in closer proximity to the building.