EPA On-line Tools for Site Assessment Calculation

Indoor Air Unit Conversion

Migration of volatile chemicals from the subsurface into overlying buildings is called vapor intrusion (VI). Volatile organic chemicals in contaminated soils or groundwater can emit vapors, which may migrate through subsurface soils and may enter the indoor air of overlying buildings. Building depressurization may cause these vapors to enter the home through cracks in the foundation. Depressurization can be caused by a combination of wind effects and stack effects, which are the result of heating within the building and/or mechanical ventilation. In extreme cases, the vapors may accumulate in dwellings to levels that may pose near-term safety hazards, such as explosion. Typically, however, vapor concentrations are present at low levels, to which long-term exposure may pose increased risk for chronic health effects. Estimation of indoor air concentrations using the Johnson-Ettinger model are commonly used in assessing vapor intrusion problems.

In evaluating indoor air concentrations of carcinogenic or toxic compounds, some confusion exists over the various units in use. Common units of indoor air measurements include micrograms (or milligrams) per meter-cubed [μg/m³ or mg/m³], micrograms (or milligrams) per liter [μg/L or mg/L], parts-per-billion-volume [ppbV] and parts-per-million-volume [ppmV]. The confusion arises in converting from mass-per-volume to parts-per-number-volume units.

This confusion arises because parts-per-billion (or million)-volume used in gas measurements is based on volume-to-volume ratio and is not the same as part-per-billion (or million) used in aqueous measurements that is based on a mass-per-mass ratio. more... (PDF, 4 pp, 198KB)

It is important to remember that [μg/L] in gas systems IS NOT equal to [ppbV], nor is [mg/L] in gas systems equal to [ppmV]. Each of these conversions is dependent upon the molecular weight of the contaminant and the temperature and pressure of the system.