Development of an indoor air quality index for continuous monitoring in French dwellings

An indoor air quality (IAQ) index can translate indoor measurement data of multiple parameters into a unitless rating of the IAQ. Compared to conventional measurements at specific times, online sensors allow for more frequent sampling and timely analysis. In this context, the present work aimed to develop an IAQ index based on the sensor data to accompany the second IAQ survey in French dwellings (CNL2) for the classification of the dwellings. The first set of data obtained from CNL2 consisted of measurements carried out in 296 dwellings in 2020-2021. Indoor air temperature, relative humidity, and concentrations of carbon dioxide, PM2.5, and PM10 were measured using sensors and verified using reference instruments. Therefore, the IAQ index was based on the five indicators. The index referred to French regulations and World Health Organization guidelines to determine the limit values of the measurements. For each time step, the index was calculated according to the highest ratio between the measured and the limit values among the five parameters. The index was classified into three categories, i.e., critical (> 100, thus > the limit values), good (< 50, thus < 0.5 of the limit values), and moderate (between 50 and 100). The classification of a dwelling was conducted based on the time series of its IAQ index over the whole measured period. A tolerated percentage of time, during which the IAQ index in a dwelling exceeded its category level, was defined to classify the dwelling into an IAQ category with certain exceedances over the measurement period. With a tolerated 25% time of exceedance, 6% of the instrumented dwellings were categorized as good, 44% as moderate, and 50% as critical.