The overlooked changes: health co-benefits from the reduction of ambient carbon monoxide in China

Previous studies have primarily emphasized the emission reduction benefits of particulate matter and nitrogen oxides, while largely overlooking the substantial long-term decline in ambient carbon monoxide (CO) concentrations. This paper systematically evaluated the spatiotemporal trends, driving factors, and associated health co-benefits of ambient CO reductions across 337 Chinese cities during 2015-2023. Owing to the stringent CO concentration limits established in China's Ambient Air Quality Standard (GB 3095-2012), which are among the most rigorous worldwide, nationwide ambient CO concentrations declined by approximately 30-40% over the study period, with the largest reduction (45%) occurring in the Beijing-Tianjin-Hebei region. Meteorological normalization based on a Random Forest model, combined with the BenMAP-CE health impact assessment framework, revealed that more than 90% of the observed decline was attributable to anthropogenic emission control measures rather than meteorological variability. These improvements were primarily driven by progressively stricter emission standards, enhanced combustion efficiency, and the widespread adoption of cleaner fuels. The reduction in ambient CO exposure yielded an estimated number of deaths attributable to the difference in the modeled burden between 2015 and 2023 concentrations of approximately 110,000-360,000 nationwide, corresponding to economic benefits of CNY 110-360 billion. These findings highlight the substantial health benefits of sustained CO emission mitigation and underscore the importance of CO control strategies.