Sampling point densities for assessing indoor air quality

Maintaining acceptable indoor air quality (IAQ) has become a public issue, policymakers have had developed strategies to address the performance of it. Pollutant concentration measured during the occupied period of a space is a key parameter to determine IAQ. Apart from a number of sampling schemes concerning the measurement time of the average pollutant level, an 8-hour continuous measurement has been widely adopted in local IAQ assessments, with a balance of accuracy and measurement effort. It is believed that the longer the measurement time, the higher the accuracy and confidence level of the measurement can be achieved. However, the probable errors of the measurement, regarding the number of measurement points, were not addressed clearly in those assessments. This study, taking a long-term carbon dioxide measurement in a typical open-plan office building as a reference, develops a database to examine the probable errors of the measurement at different sampling point densities. The results showed that there was a strong correlation between the sampling point density and the probability of obtaining the measured quantity at certain confidence levels. Moreover, the confidence level in determining the indoor air quality in the occupied period for a ventilated space could be increased from 70% to 90% at a 5% error set limit when the sampling points were doubled to the one recommended in the HKEPD certification scheme.