Three groups of 96 female Wistar rats exposed for up to 2.5 years to clean air, unfiltered or filtered Diesel engine exhaust, respectively, were examined histopathologically. In addition, 120 rats were exposed for the same length of time to coal oven flue gas mixed with pitch fumes pyrolyzed under nitrogen. This experiment was designed to enable a morphological comparison of the lung changes induced by the two different types of exhausts. Most of the rats in the Diesel exhaust group had pulmonary deposits of large amounts of carbonaceous particles phagocytosed by alveolar macrophages and accompanied by severe chronic inflammatory changes characterized by alveolar septal thickening, bronchiolo-alveolar hyperplasia and alveolar lipoproteinosis. Squamous metaplasia was observed in 65% of the animals. Primary tumours occurred in 15 rats (apprx. 16%). Six rats had bronchiolo-alveolar adenomas, 7 had squamous cell tumours and 2 had both bronchiolo-alveolar adenomas and squamous cell tumours. In an effort to clarify the malignant potential of the squamous cell tumours, criteria for their classification were established on the basis of behaviour and cytological appearance. Eight of the squamous cell tumours were classified as benign, keratinizing, cystic tumours and one was classified as a grade I carcinoma. In contrast to the Diesel exhaust exposure group, the lungs of the rats exposed to coal oven flue gas mixed with pyrolyzed pitch had much less severe inflammatory changes, but developed 20 squamous cell tumours (apprx. 17%). These tumours were classified as 16 benign, keratinizing, cystic tumours and 4 carcinomas (2 each of grade I and II). In addition, one bronchiolo-alveolar adenoma and one adenocarcinoma occurred. No primary lung tumours were observed in the control group or in the group exposed to filtered Diesel exhaust. A comparison of the incidence and morphological features of the pulmonary lesions obtained suggested different processes of tumour induction by the two types of exhausts with unfiltered Diesel exhaust having more promotional effect and PAH-enriched coal oven flue gas being a more complete carcinogen.
Animals Carcinoma, Squamous Cell