Exposure-response relationship between ambient PM2.5 and SO2 with COPD and SAD prevalence among individuals at high risk for COPD in Yunnan Province, China: a threshold analysis - Jinliang Meng; Geyi Wen; Guoyu Ma; Ruiqi Wang; Yanyan Xu; Yunhui Zhang; Le Cai (Frontiers in Public Health, 2026)