Impact of black carbon exposure on length of hospital stay in asthma patients: a retrospective cohort study in yulin, China

While PM₂.₅ exposure is linked to asthma exacerbations, components-specific effects on healthcare burden remain understudied. Hospital length of stay (LOS), a critical indicator of disease severity and healthcare costs, has received limited attention in black carbon (BC) research. To quantify the association between short-term exposure to PM₂.₅ components-particularly black carbon (BC)-and LOS among asthma patients, and to identify vulnerable populations and critical exposure windows. We conducted a retrospective cohort study of 4,509 asthma patients (ICD-10: J45 ~ J46) hospitalized in Yulin, China (2018 ~ 2023). Daily PM2.5 components concentrations (BC, Organic Matter, Sulfates, Nitrates, Ammonium) were derived from 10-km resolution CMAQ model data. General linear models (GLM) and restricted cubic spline (RCS) assessed exposure-response relationships across lag0-lag10, stratified by age groups and seasons. Confounders included meteorological variables, patient demographics, and clinical characteristics. Each 1.00 µg/m³ increase in BC concentration at lag9 significantly associated with a 0.45-days prolongation in hospital stay (95%CI: 0.12 ~ 0.78). Age-stratified analysis revealed heightened vulnerability in children (effect observed at lag1 ~ 3, lag7 ~ 9) and middle-aged adults (sustained effects lag2 ~ 10), while elderly patients showed no significant associations. Seasonal analysis demonstrated that BC effects were more pronounced during non-pollen season (0.77 additional days per 1.00 µg/m³ increase), with no significant effects during pollen season. Restricted cubic spline analysis confirmed dose-response relationships, with effects plateauing at BC concentrations ≥ 1.98 µg/m³. Short-term BC exposure was significantly associated with extend asthma hospitalization duration, with critical exposure windows at lag9 and pronounced vulnerability in children and middle-aged adults during non-pollen seasons. These findings provide quantitative evidence supporting BC-specific emission controls in air quality policies and highlight the need for targeted protection of vulnerable populations, particularly in high-population regions. What is already known on this topic Asthma prevalence in China has been increasing annually, reaching 4.2% among individuals over 20 years old, yet control remain suboptimal. Environmental factors, especially air pollutants like PM₂.₅, play a crucial role in asthma onset and progression. However, most studies focus on PM₂.₅ concentrations and outpatient visits, with few examining the association between burden of disease(e.g.: hospital length of stay) and exposure of specific PM₂.₅ components(e.g.: black carbon). Notably, Regions like Yulin exhibit significantly higher asthma risks, highlighting the need to address regional disparities and the impact of specific pollutants. What this study adds this study provides evidence that exposure to black carbon (BC), a major component of PM₂.₅, significantly prolongs hospital length of stay in asthma patients, particularly among children and middle-aged individuals in the Yulin region. By focusing on specific PM₂.₅ components and hospitalization duration, it fills a critical gap in understanding how air pollution affects asthma severity across different age groups and regions. How this study might affect research, practice or policy The findings suggest that targeted reducing BC emissions could mitigate health risks associated with air pollution in high-risk areas like Yulin. Policymakers can use this insight to prioritize BC emission controls in air quality management efforts. Future research should focus on large-scale studies with precise individual exposure assessments to further elucidate the impact of specific PM₂.₅ components on asthma, guiding more effective prevention and management strategies.