Occupational, environmental and iatrogenic lung disease

Extent
A	None
B	Less than 25% of the lung substance involved
C	25–50% of the lung affected
D	More than 50% of the lung diseased
Severity^a
0	No appreciable peribronchiolar fibrosis, or fibrosis confined to the bronchiolar walls
1^b	Fibrosis confined to the walls of respiratory bronchioles and the first tier of adjacent alveoli
2^b	Extension of fibrosis to involve alveolar ducts and/or two or more tiers of alveoli adjacent to the respiratory bronchiole, with sparing of at least some alveoli between adjacent bronchioles
3	Fibrotic thickening of the walls of all alveoli between at least two adjacent respiratory bronchioles
4	Honeycomb change

^{^a}

An average score is obtained for an individual case by adding the scores for each slide (0–4), then dividing by the number of slides examined

^{^b}

Grade 1 and, to a lesser extent, grade 2 need to be distinguished from smoking-induced peribronchiolar fibrosis and mixed-dust pneumoconiosis.