Assessing the Significance of Subsurface Contami t Vapor Migration to Enclosed Spaces — Site - Specific Alternative to Generic Estimates Vapors in enclosed spaces pose two levels of concern. First, enclosed - space vapors may be found at concentrati ons near those that pose immediate flammability and/or health risks. These sites warr ant immediate attention and response as required by most st ate and federal regulatory guidance. In the second class of sites, concentrations are lower and the concern is for longer term health risks. This report focuses exclusively on this second class of sites, where advection and diffusio n occur through a soil layer and into an enclosed space, and time is availabl e to adequately address the problem on a site - specific basis. The options cons idered in this document include the following: • direct measurement through sampli ng of enclosed - space vapors, • use of near - foundation or near - sur face soil gas sa mpling results, • use of site - specific hom ogeneous and layered soil diffusion coefficients in generic algorithms, and • assessment of bioattenuation potential