Analysing the interplay of indoor and outdoor PM: a compositional data analysis approach

Indoor and outdoor PM (particulate matter) air pollution is a major environmental problem due to its potential impact on human health. To understand the exchange relationship between indoor and outdoor PM composition, the compositional data analysis technique has been applied to data obtained from a previous study. In this study, PM levels measured both indoors and outdoors in two California dwelling units during Fall were analysed. The results revealed two distinct scenarios. In unit 933, indoor PM contained approximately 22% fewer coarse particles than outdoor PM. This difference was attributed to both the lack of indoor PM sources and limited air exchange due to residents’ habit of keeping windows and doors closed. This likely induced gravitational settling of the coarser indoor PM fractions, enriching the finer ones. In this unit, no evidence of a direct link between indoor and outdoor PM was found, suggesting that outdoor PM did not influence indoor PM. Conversely, in unit 921, indoor PM contained approximately 72% fewer fine particles than outdoor PM. This result was attributed to the absence of indoor PM sources and continuous air exchange, as occupants routinely left windows and doors open during the day. Statistical analysis indicated a relationship between indoor and outdoor PM, suggesting that outdoor PM influenced indoor PM. The proposed method sheds light on the possible mechanism that influences the coarse and fine size fraction of indoor and outdoor PM, providing a useful technique for studying their complex relationship.