Concentrations, characteristics, influencing factors, and interactions of indoor and outdoor microplastics during the hot season at the intersection between tropical and subtropical zones.

Recent studies have indicated that high temperature increased outdoor microplastics (MPs) concentrations at the intersection between tropical and subtropical zones. However, it remains unclear whether this affects indoor MP concentrations and which factors change the association between indoor and outdoor MPs. This study investigated the impact of high temperatures on both indoor and outdoor MPs during the hot seasons in this climate zones and identified factors, including air pollutants, temperature, relative humidity, and occupant activities, that affect the association. The predominant shapes, size ranges, and polymer compositions of both indoor and outdoor MPs were fragments (> 50 %), 3 to 50 μm (> 75 %), and polyamide and poly(ethylene oxide) (> 80 %), respectively. The average indoor and outdoor MP concentrations were 8.0 ± 10.7 particles/m and 10.9 ± 14.1 particles/m, respectively, showing a significant correlation. Outdoor temperature was positively associated with outdoor MP concentration. Smaller sizes of MPs were observed in outdoor air than that in indoor air. Additionally, electric fan operation significantly increased the ratios of indoor and outdoor MP concentrations. These results highlight the significant role of thermal exposure in influencing outdoor MP concentrations, with outdoor air being an important contributor to indoor MPs in this climate zones. Additionally, electric fan operation further affects the relationship between indoor and outdoor MP concentrations. More research is necessary to investigate whether thermal exposure increases the risk of MPs exposure via inhalation in this climate zones and to develop appropriate heat adaptation measures for improving indoor air quality.