1 INTRODUCTION In chronic obstructive pulmonary disease (COPD), persistent airflow obstruction is caused by high exposure to toxic gases and particles, such as cigarette smoke.1, 2 COPD is a progressive condition characterized by emphysema, chronic bronchitis and small airway obstruction, and is evidenced by dyspnoea, cough and sputum production. COPD has become a leading cause of disability and is the third most common cause of death worldwide,3, 4 accounting for 3.23 million deaths in 2019.