This paper discusses the impact on the inhalation exposure pathway from vapor intrusion (VI) of volatile organic chemicals (VOCs). VI occurs when vapor-phase contaminants migrate from subsurface sources into buildings. The primary concerns regarding VI are immediate threats to safety (e.g., explosive concentrations of petroleum vapors or methane) and possible adverse health effects from inhalation exposure to toxic chemicals. This paper focuses primarily on the latter concern, although the reader should recognize that in high enough concentrations, petroleum compounds and methane (a biodegradation product) can collect in buildings, leading to imminent explosive hazards. The information in this paper focuses on small-scale Subtitle I UST sites as opposed to sites with large sources (e.g., refineries and tank farms); however, you can use this information to inform decisions at non-Subtitle I petroleum releases. There are two classes of VOCs that together account for a large number of soil and groundwater contamination sites in the United States: (1) Petroleum hydrocarbons (PHCs) such as gasoline, diesel, and jet fuel, and (2) Chlorinated solvents such as the dry cleaning chemical tetrachloroethylene (perchloroethylene, or PCE) and the degreasing solvents trichloroethylene (TCE), 1,1,1-trichloroethane (TCA), and PCE.