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INDOOR AIR CONCENTRATION UNIT CONVERSIONS
Citation:
TILLMAN, F. AND J. W. WEAVER. INDOOR AIR CONCENTRATION UNIT CONVERSIONS. U.S. Environmental Protection Agency, Washington, DC, EPA/600/C-05/007, 2005.
Impact/Purpose:
To assess quantitative and qualitative methodologies evaluation of vapor intrusion, including quantification of uncertainties, to be used in the decision making process.
Description:
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 can migrate through subsurface soils and may enter the indoor air of overlying buildings. In extreme cases, the vapors may accumulate in dwellings to levels that 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. 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, micrograms (or milligrams) per liter, parts-per-billion-volume [ppbV] and parts-per-million-volume [ppmV], as well as percent [%]. The 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. Thus, micrograms per liter in gas systems IS NOT equal to [ppbV], nor is milligrams per liter 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. This on-line calculator (http://www.epa.gov/athens/learn2model/part-two/onsite/ia_unit_conversion.htm) permits conversions between all common units of indoor air concentration micrograms per liter, milligrams per liter, micrograms per meter-cubed, milligrams per meter-cubed, [ppmV], [ppbV] and percent [%] based on user inputs of contaminant type (with automatic look-up of molecular weight), indoor temperature and pressure.