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