Lung fibrosis. Coloured X-ray of the chest of a patient with pulmonary (lung) fibrosis, or interstitial lung disease. Pulmonary fibrosis is the overgrowth of fibrous scar tissue (dark red) between the alveoli (air sacs) of the lungs. It results in loss of lung volume, impairing gaseous exchange and causing shortness of breath. There are a number of causes, including inhalation of irritants, such as asbestos; a complication of a connective tissue disease; or as a result of a pulmonary infection, such as pneumonia. Treatment is aimed at the underlying cause.