Tracer gas techniques have been used to study air exchange in mechanically ventilated office buildings for many years. The analysis of the concentration of carbon dioxide (CO²) generated by building occupants has been suggested as an alternate means of evaluating building air exchange and ventilation system performance. Various techniques for CO² analysis have been proposed. These include measuring the decay rate of CO² concentration after the occupants leave the building, analyzing real-time CO² concentration data in conjunction with a CO² mass balance equation, and using instantaneous CO² concentration readings to directly determine air exchange rates. Local CO² concentrations have also been suggested as a means of evaluating ventilation effectiveness. These techniques require specific assumptions and unique conditions in order to yield reliable information on building air exchange characteristics, and these requirements may not always be met in office buildings.

This paper examines the relationship between CO² concentration and building air exchange. A dataset of simultaneously measured air exchange rates, based on sulfur hexafluoride (SF⁶) decay, and CO² concentrations obtained in three office buildings is employed to examine this relationship. The results indicate that CO² decay rates can provide reliable estimates of building air exchange rates, but that there may be significant error associated with the use of instantaneous concentrations to determine air exchange rates.