Smoke contains contents that can irritate the respiratory tract, which can lead to inflammation of the Bronchi. This is termed Bronchitis. Cilia may be damaged by tar, leaving them immobile, which means that mucous is not swept away as it should be. This leads to an accumulation of microorganisms in the mucous. Smoking may also cause damage to the alveoli, leading to less efficient gas exchange due to reduction of surface area.