**4.2.1 Centriacinar (centrilobular, proximal acinar) emphysema**

Strongly associated with smoking, it begins in the respiratory bronchioles and spreads peripherally, primarily involving the upper and posterior parts of lungs(Leopold & Gough,1957;Thurlbeck,1991 as cited in Shapiro SD,2010). Focal emphysema, a form of centriacinar emphysema, occurs in persons with heavy exposure to inert dust such as coal dust (Morgan & Seaton,1984 as cited in Shapiro SD,2010).
