This figure depicts the migration of volatile chemicals from contaminated soil and groundwater plumes into buildings. Volatile chemicals can enter buildings through cracks in the foundation and openings for utility lines. Atmospheric conditions and building ventilation influence vapor intrusion.
Source: Vapor Intrusion Considerations for Redevelopment, EPA 542-R-08-001 (2008)
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 (similar to the way radon gas seeps into homes). Volatile chemicals may include volatile organic compounds, select semi-volatile organic compounds, and some inorganic analytes such as elemental mercury, radon, and hydrogen sulfide.
In extreme cases, vapors accumulate in occupied buildings to levels that can pose near-term safety hazards (e.g. explosions), acute health effects, or aesthetic problems (e.g. odors). However, chemical concentration levels are usually low or, depending on site-specific conditions, vapors may not be present at easily- detectable concentrations. In buildings with low concentrations of volatile chemicals, the main concern is whether or not the chemicals pose an unacceptable risk of significant chronic health effects due to long-term exposure.