To study the effect of ozone in a chronically damaged lung, we used a bleomycin (BLM) induced pulmonary fibrosis model. Both endotracheal instillation of BLM and O 3 exposure both produce lung inflammation and fibrosis. Oxidative stress would be a common mechanism of damage for both BLM and O 3 . Our aim was to assess lung injury induced by 5 and 60 days of intermittent exposure to 0.25 ppm O 3 in rats with bleomycininduced pulmonary fibrosis. Thirty-day-old Sprague Dawley rats were

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... ly instilled with BLM (1 U/100 g body weight) and, 30 days later, exposed to 0.25 ppm O 3 (0.25 ppm 4 h per day, 5 days a week). Histopatology controls were instilled with saline and breathing room air. Histopathological evaluation of lungs was done 5 and 60 days after O 3 exposure. BLM-induced lung damage did not change after 60 days of intermittent O 3 exposure. Five days of O 3 exposure increased the mean score of BLM-induced pulmonary inflammation and fibrosis (p=0.06). Frequency of bronchopneumonia increased from 1/7 to 6/6 (p <0.001), suggesting that a short-term exposure to O 3 in a previously damaged lung might be a risk factor for developing further lung injury.