Effect of low level of ozone on rat lungs. II. Morphological responses during recovery and re-exposure

Morphological studies were made of the effect of 0.8 ppm ozone on lungs of 70-day-old male rats at the end of a 3-day exposure, after 6, 9, or 30 days of recovery, and at the end of a 3-day re-exposure beginning either 6 or 27 days after the initial exposure ceased. After 6 days of recovery, the residual lesion consisted of occasional cells lining proximal alveoli that were intermediate in form between types 1 and 2 epithelial cells and a few undifferentiated bronchiolar epithelial cells. By 9 days, the only evidence of previous damage was the presence of a few immature ciliated and nonciliated (Clara) bronchiolar cells. Re-exposure to 0.8 ppm at 6 or 27 days produced lesions indistinguishable from those observed after only one exposure. The findings are in agreement with interpretations made on the basis of biochemical measurements. It was concluded that the bronchiolar and alveolar type 1 epithelial cells that repopulate damaged sites by 6 days after the end of exposure are of approximately the same degree of susceptibility to ozone as their pre-exposure counterparts.