Effect of mixing conditions on irritant potency of zinc oxide and sulfur dioxide.

Measurement of mechanics of respiration in guinea pigs was used to assess the irritant potency of zinc oxide and sulfur dioxide mixed under different conditions of temperature and humidity. Concentrations were 1-2 mg/m3 zinc oxide and 1 ppm sulfur dioxide. Dry conditions of mixing (Chamber RH 30%) either at 24 degrees C in the exposure chamber or at 480 degrees C in a dry furnace gave a biological response which could be completely accounted for by responses to zinc oxide and/or sulfur dioxide alone. Chemical examination of the aerosols did not indicate the formation of particulate sulfur species. Zinc oxide and sulfur dioxide mixed dry at 480 degrees C and fed into the exposure chamber at 80% RH reacted to produce an irritant aerosol as evidenced by a rapid increase to levels 29% above control; reversal was rapid when exposure ended. Chemical studies indicated the presence of sulfite on these aerosols. Addition of water vapor to the furnace during mixing at 480 degrees C produced a different irritant aerosol. The resistance rose slowly to 19% above control values and remained elevated during the post-exposure hour. Chemical studies indicated the presence of sulfate, sulfite, and adsorbed sulfur trioxide on these aerosols.