Sensory irritation of select experimental photochemical oxidants

Groups of male Swiss-Webster mice were exposed to photochemical oxidant mixtures generated by reacting various hydrocarbons with nitrogen dioxide in the presence of ultraviolet light while their respiratory rates were monitored. The hydrocarbons used were 1,3-butadiene, 1-butene, cis-2-butene, ethylene, propylene, n-butane, and ethane. The initial hydrocarbon concentrations ranged from .4 to 18 ppm, with the initial nitrogen dioxide concentration being one-third that of the initial hydrocarbon concentration. New groups (four mice per group) were exposed for 5 min at 0, 1, 2, 3, and 4 hr of ultraviolet irradiation. Dose-response curves for each hydrocarbon were developed by plotting the maximum percent decrease in respiratory rate observed during the 4 hr of irradiation of each mixture as a function of the initial hydrocarbon concentration present. The percent decrease in respiratory rate in mice was chosen as an index of the sensory irritation of the upper respiratory tract for each mixture. The results showed that the potency of the photochemical oxidant mixtures generated from the hydrocarbons to be propylene greater than 1,3,-butadiene = 1-butene=cis-2-butene greater than ethylene. When ethane or n-butane was used, no decrease in respiratory rate was observed. These results are compatible with the data obtained in human volunteer studies. Guidelines for relating the percent decrease in respiratory rate in mice to human reactions are presented.