Effects of repeated exposures to peak concentrations of nitrogen dioxide and ozone on resistance to streptococcal pneumonia

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Abstract
Exposures to various mixtures of nitrogen dioxide (NO 1 ) and ozone (O 3 ) reduced the resistance of mice to streptococcal pneumonia as evidenced by increased mortality rates and shortened survival time. Daily 3‐h exposures (5 d/wk) for 2–6 mo to an air pollutant mixture consisting of 940 μg/m 3 (0.5 ppm) NO 2 and 196 μg/m 3 (0.1 ppm) O 3 were most effective in reducing the resistance to infection. The decrease in resistance to the infection occurred sooner when the mice continued to be exposed to the air pollutants instead of clean air for 14 d after the respiratory challenge with Streptococcus pyogenes aerosol. After 3 mo of exposure to the pollutant mixture, there was some decrease in the ability of mice to clear inhaled streptococci from their lungs. At the same time the total cell count in the fluid lavaged from the lungs of mice was markedly reduced, as were the viability and phagocytic activity of the alveolar macrophages. Exposure to the pollutants combined with challenge with Streptococcus aerosol resulted in marked morphological changes in lung tissues as seen by scanning electron microscopy.