Portable air purification as a strategy to mitigate indoor exposure to biomass-burning PM_2.5 and toxic constituents in northern Italy: A case study

This study investigated the effectiveness of a portable air purifier in reducing indoor fine particulate matter (PM_2.5, d_p < 2.5 μm) concentrations and associated oxidative potential in a suburban area of Milan in northern Italy, during the peak residential heating season, when biomass combustion is a major contributor to air pollution. Simultaneous indoor and outdoor PM_2.5 sampling was conducted using a filter-based method under both purifier-off and on conditions, followed by detailed chemical and toxicological characterization. Outdoor PM_2.5 levels averaged 34.75 μg/m³, exceeding the WHO 24-h guideline of 15 μg/m³, while average indoor concentrations under purifier-off conditions reached 13.49 μg/m³. However, the operation of the purifier reduced indoor PM_2.5 to an average of 5.53 μg/m³, corresponding to a 2.4-fold decrease compared to the purifier-off conditions. Oxidative potential measured by the dithiothreitol (DTT) assay also declined from 0.41 to 0.17 nmol/min/m³, representing a 58% reduction compared to purifier-off and 85% relative to outdoor air. Mass-balance analysis further showed that the infiltration factor decreased from 0.51 to 0.15, indicating a 70% reduction in persistence of particles penetrated from outdoor to indoor. These findings demonstrate that portable air purifiers can be highly effective in mitigating the exposure burden from indoor PM_2.5 and its oxidative potential, offering critical insights for reducing real-world exposures in biomass-impacted environments.