High concentrations of stray carbon dioxide in buildings are an emerging environmental hazard in some areas of the Appalachian coalfields. In western Pennsylvania, potentially lethal concentrations of CO2 (>25%) and low volumes of O2 (<10%) have caused several private homes to become uninhabitable. The CO2 is primarily anthropogenic, although the specific source is commonly uncertain. Carbon isotope geochemistry provides the most useful technique for identifying the specific gas source.
Carbon dioxide derived from organic substrates by microbial activity is enriched in 12C, whereas CO2 derived from carbonate materials is enriched in 13C. The activity of 14C can support the identification of geologically recent microbial gas generation. Ground-water chemistry can support interpretations of an inorganic CO2 source based on stable-isotope data. Case studies from western Pennsylvania demonstrate the utility of isotopic analyses in identifying specific sources of stray CO2 in buildings.