Roles of long-term exposure to ambient PM2.5 mass and its chemical components in the trajectory of chronic disease multimorbidity: Evidence from a large longitudinal study

• The first study investigating PM 2.5 chemical constituents and chronic multimorbidity. • Long-term exposure to PM 2.5 mass strongly promotes multimorbidity development. • Different PM 2.5 components had varying effects on multimorbidity development, with BC having the greatest effect. • Multimorbidity burden might be greatly averted if PM 2.5 levels drop to 35 μg/m³. Against the backdrop of global aging, rising risks of chronic diseases and multimorbidity pose considerable public health challenges. To investigate the association of long-term exposure to fine particulate matter (PM 2.5 ) and its chemical components with multimorbidity development, this prospective cohort study included 8,257 participants from the China Health and Retirement Longitudinal Study (CHARLS) cohort (2011–2020). PM 2.5 mass and its chemical components (sulfate [SO 4 2- ], nitrate [NO 3 - ], ammonium [NH 4 + ], organic matter [OM], and black carbon [BC]) were evaluated by city-level three-year average exposures. Multistate models were used to estimate the hazard ratios (HRs) for chronic disease accumulation and death, and the corresponding population-attributable fractions (PAFs) attributable to PM 2.5 mass and its chemical components were also calculated. For single chronic disease patients, each standard deviation (SD = 14.2 μg/m³) increase in PM 2.5 mass concentration was associated with HRs of 1.46 and 2.14 for basic multimorbidity and death, respectively. Among those with basic multimorbidity, each SD increase yielded HRs of 1.31 for complex multimorbidity and 2.20 for death. BC showed the strongest association with multimorbidity development, followed by SO 4 2- , OM, NH 4 + , and NO 3 - . Achieving China's Grade II PM 2.5 standard (35 μg/m³) could avert 19.32% of transitions from one disease to basic multimorbidity and 13.86% of transitions from basic to complex multimorbidity. Long-term exposure to PM 2.5 mass and its chemical components was associated with increased risks across all disease stages of multimorbidity development. Targeted air pollution control, especially BC reduction, is crucial to mitigate associated health burdens.