| Grade | Change |
| Grade of severity | |
| 0 | No fibrosis associated with bronchioles |
| 1 or I | Early fibrosis involving walls of at least one respiratory bronchiole, with or without extension into septa of adjacent alveoli; fibrosis confined to alveolated walls of respiratory bronchioles and ducts and not present in more distant alveoli. Alveolitis and inflammation similar to that caused by cigarette smoking |
| 2 or II | More severe fibrosis involving acinus: alveolar ducts and/or two or more layers of adjacent alveoli. Normal lung remains in a zone between adjacent bronchioles |
| 3 or III | Fibrosis advanced and coalescent, involves entire acinus; all lung between at least two adjacent bronchioles is affected. Some alveoli are completely obliterated |
| 4 or IV | Honeycomb remodeling and large (up to 1 cm) dilated spaces grossly visible in parenchyma |
| Grade of extent | |
| A or 1 | Only occasional bronchioles are involved. Most appear normal |
| B or 2 | "More than occasional" but less than half of bronchioles are involved |
| C or 3 | More than half of bronchioles are involved |
