Objective To investigate the characteristics and influencing factors of fine particulate matter (PM_2.5)pollution in residential homes in representative cities of China, and to provide a scientific basis for effective indoor health protection measures.

Methods Using a cross-sectional research design, from April 2018 to March 2019, a total of 612 homes were selected from 12 representative cities; indoor PM_2.5 concentrations, temperature, and humidity were measured during both warm and cold seasons, and the general characteristics of the homes were collected through a questionnaire survey. Indoor PM_2.5 concentrations were compared using the paired t test, one-way analysis of variance, and covariance analysis with temperature and humidity adjustment. A multivariable linear regression analysis was used to determine factors influencing indoor PM_2.5 concentrations.

Results The geometric mean of indoor PM_2.5 mass concentrations across the 12 cities was 54.0 μg/m³, and the median (P₂₅, P₇₅)was 53.9 (30.6, 94.5)μg/m³. The indoor PM_2.5 of 51.4% of the surveyed homes exceeded the standard limit for PM_2.5 (50 μg/m³)specified in the Standards for Indoor Air Quality (GB/T 18883-2022). The mass concentration of indoor PM_2.5 in the cold season was higher than that in the warm season(t=-18.14, P < 0.001). No differences were found in the PM_2.5 mass concentration between different indoor sampling points (warm season: t=0.56, P=0.578; cold season: t=0.06, P=0.956). After adjusting for temperature and humidity, there was a significant difference in the indoor PM_2.5 concentration between upwind and downwind areas in the cold season (F=5.94, P=0.015, P'=0.003). The indoor PM_2.5 concentration was significantly different for homes in different administrative regions (warm season: F=29.13, P < 0.001; cold season: F=66.89, P < 0.001). The multivariable regression analysis showed that having a decoration in the past five years, growing houseplants, and the use of central air conditioners were risk factors associated with increased indoor PM_2.5 concentrations, and OR(95%CI)were 5.700 (2.564, 12.671), 6.212 (2.515, 15.341), and 8.585 (1.969, 37.434), respectively; the use of air purifiers was a protective factor, with OR(95%CI) being 0.065 (0.027, 0.161).

Conclusion Indoor PM_2.5 pollution levels in residential homes in representative cities of China are generally high, with significant variations in seasonal and geographical distributions among cities. Having an interior decoration in recent five years and the use of central air conditioners are risk factors contributing to higher indoor PM_2.5 concentrations, while the use of air purifiers is a protective factor.