Chronic obstructive pulmonary disease (COPD) is a physiological diagnosis made on the basis of airflow obstruction. It develops when a genetically susceptible individual encounters sufficient inhaled environmental triggers. Genetic susceptibility is complex and determined by multiple alleles, with emphysema from α1-antitrypsin deficiency being a rare exception of well-described genetic risk. Cigarette smoke is the most common trigger in higher income countries, but globally the burning of biomass fuel in underventilated spaces is an important contributor to COPD.