Indoor carbon dioxide (CO2) concentrations have been used for decades to evaluate indoor air quality (IAQ) and ventilation. However, many of these applications reflect a lack of understanding of the connection between indoor CO2, ventilation rates and IAQ. In particular, a concentration of 1800 mg/m3 (1000 ppmv) has been used as a metric of IAQ and ventilation without an appreciation of its basis or application. After many years of trying to dissuade practitioners and researchers from using CO2 as a metric of ventilation and IAQ, the first author developed an approach to determine CO2 levels that can be used as more meaningful indicators. This approach is based on the fact that space types differ in their recommended or required ventilation rates, occupancy and other features that impact indoor CO2 concentrations. Rather than employ a single CO2 concentration for all spaces and occupancies, this alternative approach involves the estimation of space-specific CO2 concentrations. The concept considers the steady-state CO2 concentration that would be expected in a given space type based on its intended or expected ventilation rate per person, the time to achieve steady-state, the number of occupants as well as the rate at which they generate CO2, and the occupancy schedule as it pertains to the likelihood that steady-state will be achieved.
This alternative approach was described in a previous AIVC conference paper, with sample calculations presented for several commercial and institutional building spaces. Those calculations yielded potential CO2 concentration metrics, along with corresponding measurement times after full occupancy. Based on these analyses, it was stressed that reported CO2 concentrations for comparison to these or other metrics need to be associated with a measurement time relative to the start of occupancy as well as information about the space in question and its occupancy. Since this previous work, an online calculator has been developed to allow users to perform these calculations, and that calculator is described here. In addition, this paper applies the approach to residential buildings, which are more challenging based on their varying configurations and the large fraction of time that occupants spend in bedrooms.