Indoor PM2.5 and Particle Number Exposure in Low‐Income Kuala Lumpur Residences: Contributions From Outdoor Sources

Outdoor air pollution has been identified as a significant source of indoor fine particulate matter. This study investigated the infiltration of outdoor PM 2.5 into homes and its potential health implications for a low‐income urban population in Kuala Lumpur. Indoor and outdoor PM 2.5 concentrations were measured, while indoor particle number concentrations (PNCs) were also assessed in 22 residences. Water‐soluble inorganic ions (WSII) concentrations were determined, primarily to calculate infiltration factors (Finfs). Results showed mean indoor and outdoor PM 2.5 concentrations of 27.36 ± 9.25 and 32.44 ± 9.38 μ g m −3 , respectively. Sulphate (SO 4 2− ) was the dominant WSII, comprising 3.98% (indoor) and 4.32% (outdoor) of the PM 2.5 mass. A strong correlation was observed between indoor and outdoor PM 2.5 levels, with an average Finf value of 0.83 ± 0.09. The average Finf for all WSII species was 0.71 ± 0.08. Indoor PNC, with a size of 0.01–1 μ m in diameter, averaged 13,581 ± 5086 # cm −3 , indicating moderate indoor particle pollution, potentially influenced by indoor sources. Using AirQ+ software, the estimated attributable cases of mortality per 100,000 at risk population due to PM 2.5 exposure were 3.88 for short‐term and 147.26 for long‐term exposure, with higher risk projected for residents of high‐floor residences compared to those on lower floors. These findings underscore the substantial impact of outdoor air pollution on indoor air quality and the significant infiltration of outdoor PM 2.5 , highlighting the urgent need for strategies to mitigate its entry into homes.