Oxidation of fatty acid methyl esters was markedly increased by trace quantities (1.5 ppm) of nitrogen dioxide or ozone. Nitrogen dioxide-catalyzed oxidation occurred through conventional peroxidation mechanisms. Ozone reacted in thin films by directly attacking the double bond. Phenolic antioxidants retarded NO2—but not O3—oxidation. In aqueous media, O3-oxidation occurred by direct attack and peroxidation; NO2-oxidation by conventional peroxidation. Both were retarded by phenolic antioxidants. Exposure of rats to 10 ppm NO2 for four weeks resulted in reduction of polyunsaturated fatty acids in the vitamin E-depleted group. Exposure to 1.0 ppm ozone resulted in death from pulmonary edema of all animals within 22 days. Lethal time for 50% of the population (LT50) was 8.2 days for vitamin E-depleted animals, and 18.5 days for those receiving vitamin E. Vitamin E appears to protect against the biological effects of photochemical air pollutants.