Time--dose response for nitrogen dioxide exposure in an infectivity model system.

The concentration of NO2 in polluted atmosphere is subject to wide variation, according to peak traffic load, industrial productivity, intensity of sunlight and meteorological conditions. Normally NO2 has a low basal concentration with superimposed spikes when the above conditions are optimal for its production. Thus, it is important to determine the relative importance of a short-term, relatively high concentration of NO2 versus exposure for longer periods of minimal dose levels. This problem was approached experimentally by measuring the effect of NO2 on an animal's resistance to the induction of bacterial pneumonia. The data collected indicate that: (1) in short-term dose-response studies using the same Ct (concentration x time) product of 7, the actual concentration exerts a greater influence on NO2 effect than does the duration of exposure; (2) when concentration is held constant and the time increased, the average difference in mortality from controls can be seen after only 1 hr exposure to 3.5 ppm and after 3 weeks of exposure to 0.5 ppm; and (3) the relative mean survival time at 3.5 ppm for 1 hr was 18--36 hr less than that of the control animals.