Tobacco smoking is by far the main risk factor for COPD. It is responsible for 40% to 70% of COPD cases and exerts its effect by causing an inflammatory response, cilia dysfunction, and oxidative injury. Air pollution, indoor burning of biomass fuels, and occupational exposure to dusts, chemical agents, and fumes are other etiologies. Oxidative stress and an imbalance in proteases and antiproteases are also important factors in the pathogenesis of COPD, especially in patients with alpha-1 antitrypsin deficiency.