The answer is tobacco smoke. Exposure to tobacco smoke accounts for an estimated 80 to 90 percent of chronic obstructive pulmonary disease cases. The occupational exposure, passive smoking, and ambient air pollution are also risk factors but do not account for the majority of cases. In addition, the classification of stage IV of chronic obstructive pulmonary disease is defined as very severe because stage IV is very severe, Stage 0 is at risk, Stage I is mild, Stage II is moderate and Stage III is severe.
