Pathology of toxic responses to the RD50 concentration of chlorine gas in the nasal passages of rats and mice.

Male Swiss-Webster mice and Fischer-344 rats were exposed to chlorine gas at their respective RD50 concentrations (ca. 9 to 11 ppm). The RD50 concentration is that concentration which reduces respiratory rate by 50%. The exposures were carried out for 6 hr per day for 1, 3, or 5 days, and the animals were killed immediately at the end of the last exposure. The nasal passages were examined by light and scanning electron microscopy. Lesions were observed in all exposed groups and were of similar severity and character in rats and mice. The most severe changes were found in the olfactory mucosa of the anterior portion of the dorsal meatus and consisted of partial to complete degeneration of olfactory sensory cells, with olfactory sustentacular cells being more resistant to chlorine exposure. Lesions in the respiratory epithelium were located primarily on the free margins of the naso- and maxilloturbinates and adjacent nasal septum. Scanning electron microscopy, using large size specimens, demonstrated loss of olfactory cilia in areas of the olfactory epithelium which appeared unaffected by light microscopy. Scanning electron microscopy was also helpful in locating areas of respiratory epithelium exhibiting loss of cilia and cellular exfoliation, which occurred primarily on naso- and maxilloturbinates. Therefore, chlorine-induced severe lesions in specific locations in both the olfactory and respiratory epithelia of the nasal passages with more widespread loss of respiratory and olfactory cilia.