Accumulations of stray CO₂ in buildings are an emerging environmental hazard in portions of the Appalachian coalfields. In western Pennsylvania, potentially lethal concentrations of CO₂ (>25%), attended by low volumes of O₂ (<10%), have driven families from their homes. This CO₂ is anthropogenic in origin, although the specific source is often uncertain. Isotope geochemistry provides the most accurate technique for identifying the specific gas source.

Both organic and inorganic processes in the coalfields generate CO₂. Variations in ¹³C/¹²C of CO₂ help identify gas from these different sources. CO₂ generated from organic substrates by microbial activity is enriched in ¹²C; we measured organic d¹³CO₂ as light as –23.14 per mil. The activity of ¹⁴C assists the recognition of recent microbial gas generation. CO₂ derived from carbonate materials is enriched in ¹³C; we measured d¹³CO₂ values between –7.01 and +3.86 per mil in gases accumulated in building spaces. Groundwater chemistry supports interpretations of an inorganic CO₂ source based on stable isotope data.Three case studies from western Pennsylvania demonstrate the ambiguity of compositional gas data alone for determining a gas source, and the utility of isotopic analyses to correctly determine specific sources of stray CO₂ in buildings.