Using carbon dioxide concentration to assess indoor air quality in offices

Carbon dioxide (CO2) concentration is commonly used as a surrogate indicator for assessing indoor air quality (IAQ) and ventilation efficiency. The measurement and analysis of average indoor CO2 concentration is useful to understand the performance of a ventilation system. However, uncertainties due to the ventilation rate effectiveness as well as the initial state, build-up, fluctuation, and decay of indoor CO2 concentration from occupant load variation have not been addressed in many measurements; considerable measurement efforts are obligatory. This study, taking the long-term CO2 measurement and occupant profile of a typical open-plan office building as reference, has developed a database to examine the probable errors of an alternative sampling scheme for assessing indoor CO2 level in which the average pollutant concentration is obtained from two short sampling periods in two sampling sessions when the building is occupied. The results show that the assessed CO2 level deviates from the long-term average and the probable errors correlate with the required measurement time and occupant load profile. At certain confidence levels, the potential reductions in measurement time of the alternative scheme, as compared with an 8-h continuous one, will be up to 30%, some 20% less than the reduction of measurement time for assessing indoor radon level.