F2 and chronic obstructive pulmonary disease: Kruskal–Wallis tests were used to evaluate mean aerodynamic diameter of ≥ 2.5 μm (PM2.5) exposure on the day before hospitalization; we assessed its association with changes in prothrombin time (PT) in AECOPD inpatients with different Global Initiative for Chronic Obstructive Lung Disease (GOLD) classes.