Scarring of the lungs, also known as pulmonary fibrosis, occurs when lung tissue becomes damaged and scarred, making it more difficult for the lungs to work properly. The scarring associated with pulmonary fibrosis can be caused by a multitude of factors, including long-term exposure to certain toxins, certain medical conditions, radiation therapy, some medications, and certain occupations. Inhaled environmental substances, such as mold spores, bacteria, animal proteins, or other known triggers, can cause a form of pulmonary fibrosis called hypersensitivity pneumonitis. Exposure to inorganic dusts, including asbestos, silica, coal dust, beryllium, and hard metal dusts, can cause lung injury when they are inhaled on a regular basis and for a significant period of time. Autoimmune diseases, certain viral infections, gastroesophageal reflux disease (GERD), and cigarette smoking are also risk factors for pulmonary fibrosis. In some cases, the cause of pulmonary fibrosis is unknown, which is called idiopathic pulmonary fibrosis.