Pulmonary effects of prolonged ozone insult in rats. Morphometric evaluation of the central acinus

Groups of Sprague-Dawley rats were exposed to 0.2, 0.5, or 0.8 p.p.m. of ozone 8 hours a day for 20, 50, or 90 consecutive days to evaluate the prolonged effect of oxidant exposure on the centriacinar area of the lung. Morphologic evaluation revealed that epithelial changes and accumulations of macrophages in centriacinar regions at 90 days were similar to but less severe than lesions seen at 7 days. Quantification of inflammatory cells in centriacinar regions using scanning electron microscopy revealed a 5-fold increase above controls after exposure to 0.8 p.p.m. of ozone for 20 days. Both the 0.5- and 0.8 p.p.m. exposure groups had significantly (p< 0.05) increased numbers of inflammatory cells within proximal alveoli at all time periods. Morphometric analysis applied to transmission electron microscope micrographs demonstrated that the arithmetic mean thickness of the air-blood barrier was 1.20 /+- 0.13 micrometer. (mean /+ standard deviation) in controls; 2.92 /+- 0.92 micrometer. and 2.17 /+ 0.18 micrometer. in rats exposed to 0.8 p.p.m. of ozone for 20 or 90 days, respectively. The increased thickness was accompanied by an increase in the relative volume of the interstitium. These results indicate that centriacinar lesions induced by ozone diminish with continued exposure, but that significant morphologic alterations do persist through 90 days of exposure at the 0.5 and 0.8 p.p.m. levels but not at the 0.2 p.p.m. ozone level.