Simply put, the cigarette smoke attracts inflammatory cells (white blood cells, including neutrophils, lymphocytes, and macrophages) into the lung. Then, the inflammatory cells release substances called proteases. The proteases dissolve the proteins in the alveolar walls (septae) and thereby destroy the septae. As a result, the alveoli join together (coalesce) to form the larger, irregular, inefficient air sacs.
It turns out that about half of all smokers develop emphysema. Mild emphysema is seen occasionally in non-smokers and may be due to passive smoking (exposure to other people smoking) and industrial air pollution. Severe emphysema, however, is seen only in smokers or in some people with rare inherited diseases (e.g., alpha-1-antitrypsin deficiency). Still, it takes about 30 years of smoking to develop fatal emphysema. This is because people usually don't die from emphysema until more than 60% of the lung tissue is affected.
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