A group of disorders that cause progressive scarring of lung tissue. Interstitial lung disease may be caused by long-term exposure to hazardous materials, such as asbestos or coal dust, or it can be caused by an auto-immune disease such as rheumatoid ahritis. Once lung scarring occurs, it's generally irreversible. Ref: guyton and hall textbook of medical physiology 12 edition page number:327,328,329