Effects of China's Clean Air Act on Frailty Levels Among Middle-Aged and Older Adults: A Population-Based Quasi-Experimental Study

Background: Air pollution is a frailty risk factor, yet the frailty-related health benefits of China's air pollution control policy, the Clean Air Action (CCAA), are unclear. Frailty progression and transitions differ among robust, prefrail, and frail adults. This study aimed to evaluate the CCAA's effect on frailty levels among robust, prefrail, and frail Chinese adults. Methods: Using propensity score matching with difference-in-differences analysis, we studied 9 788 adults aged >= 45 from the 2011 and 2018 China Health and Retirement Longitudinal Study. The Frailty Index (FI), summarizing 32 health deficits, quantifies frailty level (range: 0-1 scores). Frailty was defined as FI >= 0.25, prefrailty as FI 0.10-0.25, and robust as FI <= 0.10. We examined frailty transitions between these states (robust, prefrail, and frail) from 2011 to 2018. Based on provincial particulate matter reduction targets, participants were assigned to intervention (>10% reduction) or control (<= 10%) groups and categorized as robust, prefrail, or frail pre-CCAA implementation. Results: The CCAA significantly reduced FI scores among preimplementation robust individuals by 0.0205 and among prefrail individuals by 0.0114, with no significant changes in frail individuals. Frailty transition analyses confirmed specific benefits of the CCAA, which significantly reduced worsening from robust to prefrail or frail by 7.0% and prefrail to frail by 3.9%. However, it did not facilitate the improvement from frail to prefrail/robust or from prefrail back to robust. No significant subgroup differences were observed across age, gender, Hukou, education, and social participation. Conclusions: CCAA has been associated with a reduction in frailty deterioration in robust and prefrail populations.