Long-lasting effects of chronic ozone exposure on rat nasal epithelium

Ozone, the principal oxidant pollutant in photochemical smog, causes airway epithelial injury in the upper and lower respiratory tract of laboratory a nimals. We have recently reported that long-term inhalation exposure to ozo ne causes mucous-cell metaplasia (MCM) in the surface epithelium lining the nasal airways of F344 rats. The principal objective of the present study w as to determine the persistence of ozone-induced MCM in the nasal epitheliu m after the end of a chronic exposure. Male F344/N rats were exposed to 0, 0.25, or 0.5 ppm ozone, for 8 h/d, 7 d/wk for 13 wk. Animals were killed 8 h, 4 wk, or 13 wk after the end of the chronic exposure. Ozone-related alte rations in the nasal epithelium were qualitatively and quantitatively chara cterized through histochemistry, image analysis, and morphometric technique s. Some rats were exposed for an additional 8 h to 0.5 ppm ozone at 13 wk a fter the end of the chronic exposure to determine whether previous ozone ex posure results in persistent changes in the sensitivity of nasal epithelium to acute injury. At the end of the chronic exposure, hyperplasia was prese nt in the nasal epithelium of rats exposed to 0.25 and 0.5 ppm ozone. By 13 wk postexposure, this proliferative alteration was still evident only in t he rats exposed to 0.5 ppm ozone. Ozone-induced MCM with associated intraep ithelial mucosubstances was evident only in the nasal tissues of rats expos ed to 0.5 ppm ozone. Though attenuated, these alterations in the nasal muco us apparatus were still detectable at 13 wk after the end of the exposure. At this same time after the chronic exposure, an acute (8 h) exposure to 0. 5 ppm ozone induced an additional increase of mucosubstances in the nasal e pithelium of rats previously exposed to 0.5 ppm ozone, but not in rats chro nically exposed to 0 or 2.5 ppm ozone. The persistent nature of the ozone-i nduced MCM in rats documented in this report suggests that ozone exposure m ay have the potential to induce similar longlasting alterations in the airw ays of humans.