Black carbon as a major contributor to ambient PM2.5-related cause-specific outpatient risks: Implications for early warning and health benefits evaluation of air quality improvement

Ambient fine particulate matter (PM 2.5 ) poses acute multi-organ health risks, but evidence on its constituent-specific early-stage morbidity risks remains limited. We conducted a nationwide time-stratified case-crossover study using over 212 million daily outpatient records for 12 major diseases from 274 Chinese cities during 2013–2017. City-specific associations between PM 2.5 and its major constituents and cause-specific outpatient risks were estimated using conditional logistic regression, and pooled by random-effects models. Robustness and independence of constituent-specific associations were tested using three complementary models. Attributable fractions of outpatient visit burdens were quantified. Black carbon (BC) showed the strongest independent associations at lag0 with most cause-specific outpatient visits, in contrast to lag0-1 with most cause-specific hospital admissions. Estimated increases (95% confidence intervals) in outpatient visits ranged from 1.33% (0.93–1.74) for chronic kidney disease (CKD) to 3.89% (3.30–4.49) for chronic obstructive pulmonary disease (COPD) per interquartile range increase (1.8 μg/m 3 ) in BC at lag0, with corresponding attributable fractions ranging from 2.38% (CKD) to 6.12% (COPD). Following stringent clean air policies since 2013, annual attributable fractions of PM 2.5 - and BC-associated outpatient visit burdens declined substantially, with relative reductions larger for outpatient visits than for hospital admissions for both PM 2.5 (40.61% vs 30.00%) and BC (28.66% vs 21.14%). In conclusion, BC is a major contributor to PM 2.5 -associated cause-specific outpatient risks. Prioritizing the control of key PM 2.5 constituents ( e.g. , BC)—rather than total PM 2.5 mass alone—may help mitigate acute multi-organ morbidity risks. Outpatient visits may serve as a more sensitive early-warning indicator for air pollution-related morbidity risks.