Combined exposure to ozone and nitrogen dioxide.

The combined effects of ozone and nitrogen dioxide were assessed in an in vitro test system utilizing human red cells. In general, these two pollutants had additive effects on the parameters measured which included osmotic fragility, acetylcholinesterase activity, lipid peroxidation, reduced glutathione and methemoglobin levels. However, at lower pollutant doses a synergistic increase in lipid peroxides was noted while at higher doses the effect became less than additive. Further studies of this observation suggested that ferrous hemoglobin potentiates ozone-induced lipid peroxidation while methemoglobin, resulting primarily from nitrogen dioxide, inhibits this process. Ozone was also found to potentiate the methemoglobinemic effects of nitrogen dioxide, particularly in sequential studies in which ozone exposure preceded nitrogen dioxide. Inasmuch as the effects of these two pollutants vary from protective to synergistic depending on the pollutant concentration, duration and sequence of exposure, as well as on the parameter assayed, it would appear that the approach to in vivo study of the combined effects of ozone and nitrogen dioxide should be aimed at simulating ambient conditions as closely as possible.