| Inhalation of dusts or toxins |
| Mineral dusts - asbestos, silica, iron oxide, aluminum oxide, talc, mica, and coal |
| Toxins - NO2, sulfur dioxide, ammonia, chlorine, and phosgene |
| Drug reaction |
| Infection - viral, mycoplasma |
| Connective tissue disease, especially rheumatoid arthritis |
| Chronic rejection in heart-lung, lung, and bone marrow transplant recipients |
| Hypersensitivity reactions |
| Ulcerative colitis |
| Idiopathic |
