Short-Term Effects of Indoor Infiltration Exposure to Particulate Matter and Ozone on Mortality Risk

People spend the majority of their time indoors; however, most previous studies on the health effects of particulate matter (PM) and ozone (O₃) have used outdoor concentrations as a proxy for personal exposure, which may introduce misclassification bias. Since indoor PM and O₃ originate primarily from outdoors, estimating their indoor infiltration levels provides a closer approximation of true personal exposure. This study used data on approximately four million deaths occurring over an eight-year period in Jiangsu Province, China. The infiltration factor method and time-series analysis were employed to assess the linear and nonlinear associations of short-term indoor exposure to outdoor-origin PM₁, PM_2.5, PM₁₀, and O₃ with all-cause, cardiovascular, and respiratory mortality. In addition, the interactions between indoor PM and O₃ were investigated. The results indicate that indoor exposure to outdoor-origin PM and O₃ was positively associated with mortality, and these associations were stronger than those observed for direct outdoor exposure. Each 10 μg/m³ increase in the 2-day moving average concentration of indoor PM₁, PM_2.5, PM₁₀, and O₃ was associated with a 1.82% (95% confidence interval [CI]: 1.64, 2.01), 1.02% (95% CI: 0.91, 1.13), 0.69% (95% CI: 0.62, 0.77), and 1.79% (95% CI: 1.60, 1.99) increase in all-cause mortality, respectively. No threshold was observed in the exposure-response associations. Furthermore, significant multiplicative and additive interactions were identified between infiltrated PM and O₃. Consequently, greater attention should be directed toward indoor air quality, particularly the coordinated management of combined exposure to indoor PM and O₃, in order to better protect public health.