Association between Long-Term Exposure to PM2.5 Inorganic Chemical Compositions and Cardiopulmonary Mortality: A 22-Year Cohort Study in Northern China

Particulate matter with diameters ≤2.5 μm (PM2.5) has been identified as a significant air pollutant contributing to premature mortality. Nevertheless, the specific compositions within PM2.5 that play the most crucial role remain unclear, especially in areas with high pollution concentrations. This study aims to investigate the individual and joint mortality risks associated with PM2.5 inorganic chemical compositions and identify primary contributors. In 1998, we conducted a prospective cohort study in four northern Chinese cities (Tianjin, Shenyang, Taiyuan, and Rizhao). Satellite-based machine learning models calculated PM2.5 inorganic chemical compositions, including sulfate (SO42–), nitrate (NO3–), ammonium (NH4+), and chloride (Cl–). A time-varying Cox proportional hazards model was applied to analyze associations between these compositions and cardiorespiratory mortality, encompassing nonaccidental causes, cardiovascular diseases (CVDs), nonmalignant respiratory diseases (RDs), and lung cancer. The quantile-based g-computation model evaluated joint exposure effects and relative contributions of the compositions. Stratified analysis was used to identify vulnerable subpopulations. During 785,807 person-years of follow-up, 5812 (15.5%) deaths occurred from nonaccidental causes, including 2932 (7.8%) from all CVDs, 479 (1.3%) from nonmalignant RDs, and 552 (1.4%) from lung cancer. Every interquartile range (IQR) increase in SO42– was associated with mortality from nonaccidental causes (hazard ratio: 1.860; 95% confidence interval: 1.809, 1.911), CVDs (1.909; 1.836, 1.985), nonmalignant RDs (2.178; 1.975, 2.403), and lung cancer (1.773; 1.624, 1.937). In the joint exposure model, a simultaneous rise of one IQR in all four compositions increased the risk of cardiorespiratory mortality by at least 36.3%, with long-term exposure to SO42– contributing the most to nonaccidental and cardiopulmonary deaths. Individuals with higher incomes and lower education levels were found to be more vulnerable. Long-term exposure to higher levels of PM2.5 inorganic compositions was associated with significantly increased cardiopulmonary mortality, with SO42– potentially being the primary contributor. These findings offer insights into how PM2.5 sources impact health, aiding the development of more effective governance measures.