Illustration of the lungs of a smoker. The surface of the left lung (right) is darkened by chemicals from smoking cigarettes. The right lung (left) is shown in cross-section and reveals the inflamed red walls of the bronchi (tubes). The bronchi are lined with hair-like cilia, which beat to remove particles from the lungs. However, the toxic particles in cigarette smoke kill the cilia, leading to an increase in the number of mucus producing cells. This extra mucus now blocks the airways leading to inflammation and destruction of the alveoli (air sacs), which inflate and leave dark black holes. The destruction of the alveoli is irreversible, and decreases the lung function.