Nitrate formation in rats exposed to nitrogen dioxide.

Sprague-Dawley rats exposed to atmospheres containing low levels of nitrogen dioxide (NO2) for 24 hr had increased levels of nitrate in their urine on the day of exposure and on the 3 subsequent days. The total increase in urinary nitrate was linearly related to the nitrogen dioxide concentration administered. We recovered in urine 8.4 +/- 1.1 mumol nitrate/ppm NO2/24-hr exposure (slope +/- 95% confidence limits) for 185-g rats. Both the linearity and magnitude of this effect imply that reaction with respiratory tract water is not a major pathway of NO2 absorption in the lung. Instead, our observations support the hypothesis that the major interaction of NO2 in the lung is with readily oxidizable tissue components to form nitrite. We estimate that 9.6 mumol of nitrite is formed in the respiratory tract of the rat per ppm NO2 per 24-hr exposure. We also estimate that humans breathing air containing 0.1 ppm NO2 have about 3.6 mg of nitrite formed in their respiratory tract per day.