ACE2 and chronic obstructive pulmonary disease: Tobacco smoke increases ACE2 expression (15, 25), which is likely one reason why COVID-19 patients with chronic obstructive pulmonary disease or who are current smokers have increased ACE2 expression in bronchial epithelial cells in the respiratory tract compared with healthy subjects (15, 26, 27, 28, 29, 30, 31, 32).