Indoor Carbon Dioxide Concentrations in Ventilation and Indoor Air Quality Standards

Indoor carbon dioxide (CO2) concentrations have played a role in discussions of ventilation and indoor air quality (IAQ) since the 18th century. Those discussions have evolved over the years to focus on the impacts of CO2 concentrations on building occupants, how these concentrations relate to occupant perception of bioeffluents, the use of indoor CO2 concentrations to estimate ventilation rates, and CO2–based demand control ventilation. This paper reviews how indoor CO2 has been dealt with in ventilation and IAQ standards in the context of these issues.
While measured indoor CO2 concentrations are rarely close to health guidelines, much confusion has resulted regarding CO2 in ventilation and IAQ standards. For example, an indoor CO2 concentration of 1800 mg/m3 (roughly equivalent to 1000 ppmv) has become a de facto standard in many discussions without a sound understanding of its basis or significance. And while there have been anecdotal associations of CO2 concentrations in this range with occupant symptoms such as stuffiness and discomfort, research results do not support these associations with CO2 itself. Several studies have shown associations of elevated CO2 levels with occupant symptoms, but these findings are likely due to lower ventilation rates elevating the concentrations of other more important contaminants along with the CO2.
The relevance of CO2 concentrations to ventilation and IAQ standards is based primarily on two factors: their relation to indoor levels of bioeffluents and associated odors, and their relation to ventilation rates per person. Several studies of bioeffluent odor perception in chambers and buildings have shown correlations between dissatisfaction with these odors and both ventilation rate per person and CO2 level. Also, ventilation rates and indoor CO2 levels are related based on a single-zone mass balance of CO2. However, many individuals use CO2 concentrations to estimate building ventilation rates without understanding the associated mass balance theory and the assumptions on which it is based. This paper reviews these concepts and discusses the role of indoor CO2 in various ventilation and IAQ standards.