Prevalence, risk factors, and mortality of COPD in young people in the USA: results from a population-based retrospective cohort oleh Yun Li, Qing Zhang, Yuan Gao, Jinhai Huang, Zihui Wang, Nanshan Zhong, Fengyan Wang, Jinping Zheng, Yi Gao, Junfeng Lin, Lunfang Tan, Shuyi Liu, Shiyin Peng, Heai Fang, Yuting Weng, Shiyin Li