<qa>  What is the most common cause of Emphysema?Emphysema is a progressive, debilitating disease that affects nearly 3 million people in the United States or roughly one percent of the US population. The disease is characterized by destruction of lung tissue as a result of inflammation caused by exposure to noxious inhaled agents for extended periods. The most common cause of this condition is cigarette smoking, although genetic and occupational causes account for up to 10 of cases. Despite aggressive public health initiatives aimed at discouraging the use of cigarettes, smoking-related lung diseases remain a significant cause of disability and death in the United States. Currently there are 46 million smokers in the US. Due to the number of current and new smokers, emphysema is expected to remain a leading cause of morbidity and mortality in the United States for years to come. 