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Abstract Indoor air pollution poses a significant environmental concern, leading to adverse health effects. Fine particulates (PM2.5) observed indoors exhibit high variability, influenced by both indoor emission sources and the infiltration of outdoor particles through open spaces and the incomplete building insulation. This study examines the relationship between indoor and outdoor PM2.5 levels using data from a network of citizen-operated low-cost air quality sensors, deployed in Legionowo, Poland. Our results showed that generally, indoor PM2.5 was lower than outdoor levels, with occasional peaks attributed to potential indoor emission sources. Statistical analysis identified emission events, particularly during cooking and household-heating periods, occurring more frequently from October to April. In the absence of indoor sources, outdoor particles accounted for 29–75% of indoor particle concentration, highlighting the significance of infiltration. This study shows how citizen-generated data using low-cost sensors, after post-processing, can provide decision-ready information as for example outdoor particulate matter infiltration factors for each building. This information can help decision-makers in devising effective interventions such as prioritizing indoor ventilation or addressing outdoor pollution sources.
Salamalikis et al. (Wed,) studied this question.