Excerpt from the EPA:
Vapor intrusion generally occurs when there is a migration of volatile chemicals from contaminated groundwater or soil into an overlying building. Volatile chemicals can emit vapors that may migrate through subsurface soils and into indoor air spaces of overlying buildings in ways similar to that of radon gas seeping into homes.
In extreme cases, the vapors may accumulate in dwellings or occupied buildings to levels that may pose near-term safety hazards (e.g., explosion), acute health effects, or aesthetic problems (e.g., odors). In buildings with low concentrations of volatile chemicals, the main concern is whether the chemicals may pose an unacceptable risk of chronic health effects due to long-term exposure to these low levels.
The simple conceptual model of the vapor intrusion pathway that is presented in Figure 1 below illustrates a source of contamination in soil and groundwater and the upward movement of VOCs from this source toward and into buildings.
(Source: The 2008 Brownfields Technology Primer: Vapor Intrusion Considerations for Redevelopment, EPA 542-R-08-001.)