Development and application of an indoor carbon dioxide metric.

Indoor carbon dioxide (CO ) concentrations have been considered for decades in evaluating indoor air quality (IAQ) and ventilation, and more recently in discussions of the risk of airborne infectious disease transmission. However, many of these applications reflect a lack of understanding of the connection between indoor CO levels, ventilation, and IAQ. For example, a single indoor concentration such as 1000 ppm is often used as a metric of IAQ and ventilation without an understanding of the significance of this or any other value. CO concentrations are of limited value as IAQ metrics, and a single concentration will not serve as a ventilation indicator for spaces with different occupancies and ventilation requirements. An approach has been developed to estimate a space-specific CO level that can serve as a metric of outdoor ventilation rates. The concept is to estimate the CO concentration that would be expected in a specific space given its intended or expected ventilation rate, the number of occupants, the rate at which they generate CO , and the time that has transpired since the space was occupied. This paper describes the approach and presents example calculations for several commercial, institutional, and residential occupancies.