Outdoor air quality: CO2 at ambient concentrations is not a direct air pollutant in the sense that it is harmful to health or vegetation. Because it is released in combustion processes, measurements of CO2 can serve as a proxy for other air pollutants related to combustion. The current background CO2 concentration is approaching 400 ppm.
Indoor air quality: CO2 is a proxy for indoor pollutants emitted by humans, and correlates with human metabolic activity. Carbon dioxide at levels that are unusually high indoors may cause occupants to grow drowsy, to get headaches, or to function at lower activity levels. Humans are the main indoor source of carbon dioxide in most buildings. Indoor CO2 levels are an indicator of the adequacy of outdoor air ventilation relative to indoor occupant density and metabolic activity.