Healing in rat lung after subacute exposure to nitrogen dioxide.

The nature of healing was observed in rats after 1, 4, 10, 16, and 20 weeks of continuous exposure to 15 ± 2 ppm nitrogen dioxide. Animals from each group were allowed to recover for 0, 8, 20, and 52 weeks and were compared with control rats of the same age. Lung weights increased at two different times as compared with those of the control animals. The first was associated with hypertrophy of bronchiolar and adjacent alveolar epithelium. This tended to return toward normal during recovery. The second occurred in aging, recovering rats, long after contact with nitrogen oxide had ceased. This was consistent with an increase in the staining property of collagen and elastic tissue in the alveolar parenchyma, which was most noticeable in the ductal areas. Between the two increments, lung weights were equivalent to those of control animals. Healing in the longer-exposed rats left some inhomogeneity of the air spaces of the parenchyma and some partial and, possibly, complete closure by fibrosis of the terminati...