The phrase “soil vapor intrusion” refers to the process by which volatile chemicals migrate from a subsurface source, through soil vapor and into the indoor air of a building where an adverse exposure is possible. Soil vapor, also referred to as soil gas, is the air found in the pore spaces between soil particles (Figure 1). Soil vapor can enter a building through cracks or perforations in slabs or basement floors and walls. Additional points of entry may include openings around sump pumps or where pipes and electrical wires go through the foundation, Migration occurs due to differences between interior (within the structure) and exterior (within the soil) pressures. Specifically, if the contaminant vapor migrates into the radius of influence of an existing building, then typical building pressure differentials related to heating or cooling systems can draw the contaminant vapors into or below the structure where it could accumulate.