Smoking induces a range of histological changes in the lung. It causes inflammation, with increased numbers of macrophages and neutrophils. The ciliated epithelium of the bronchi undergoes squamous metaplasia, which reduces mucus clearance. Chronic exposure leads to the destruction of alveolar walls and the development of emphysema. Additionally, the risk of carcinoma increases due to DNA damage and the subsequent dysplasia and neoplasia of epithelial cells.
