Development, radiological zone patterns, and importance of diffuse pleural thickening in relation to occupational exposure to asbestos.

The radiographic appearance of the lateral pleura was divided into an upper, a middle, and a lower zone. Bilateral changes of the pulmonary layer of the pleura (diffuse pleural thickening) within the upper pleural zones were found in 863 (71%) of 1204 workers exposed to asbestos and in 249 (40%) of 622 non-exposed controls. Downwards along the chest wall this ratio of 7:4 increased progressively up to 10:1 at the lower parts of the pleura. Bilateral diffuse pleural thickening in at least two adjacent zones on each side was found in 652 (54%) of exposed and in only 86 (14%) of unexposed subjects. The difference was even more striking when comparing bilateral involvement of all three zones (28% and 3% respectively). Unilateral change was rare (4.8% and 7.8% respectively) and often due to causes other than exposure to asbestos. Pleural findings were the earliest radiographic features detectable associated with former exposure to asbestos. Bilateral diffuse thickening in at least two adjacent zones on each side seems to be a striking feature and an early indication of former occupational asbestos damage. Modifications of the International Labour Organisation 1980 classification are proposed.