Upper airway irritation of terpene/ozone oxidation products (TOPS). Dependence on reaction time, relative humidity and initial ozone concentration.

Recently, we reported the formation of unidentified strong upper airway irritants in reaction mixtures of terpenes and ozone. The identified products included aldehydes, ketones and carboxylic acids. Here we report the effects of variation of reaction time, relative humidity and initial ozone concentration on irritant formation in a flow reaction system using R-(+)-limonene and isoprene. Upper airway irritation was measured in mice as reduction of the respiratory rate. For both substances maximum irritation was observed for low humidity (<2% RH)/short time (16-30 s) reaction mixtures, and both moderate humidity ( approximately 32% RH) and longer reaction times (60-90 s) resulted in significantly less irritation. These results suggest that some unidentified intermediates react with water vapor to give less irritating products. Irritation measured at four ozone concentrations (0.5, 1, 2 and 3.5 ppm) using low humidity/short time reaction conditions for limonene (50 ppm) and isoprene (500 ppm) revealed that at 0.5 ppm, irritation was at the same level as that for the pure terpenes, indicating that at 0.5 ppm ozone the combined irritant effect was near the no effect level for the product mixture.