This article reviews the sources, levels, and interpretation of CO2 and VOC concentrations in buildings. The use of an enrichment factor to identify emission sources and potential IAQ problems is suggested. The indicator is evaluated by collecting concentration data in two office buildings and by reviewing recent air-quality studies in the literature. Results show that bioeffluent and occupant-related emissions often account for a large share of VOC emissions; the suggested indicator clearly shows when other sources are significant. Several issues that influence the interpretation of IAQ indicators are discussed, including the use of pollutant surrogates, temporal and spatial variability in large buildings, VOC measurement technique, and outdoor pollutant levels.