Ozone (O3) is the major oxidant of photochemical smog. Its biological effect is attributed to its ability to cause oxidation or peroxidation of biomolecules directly and/or via free radical reactions. A sequence of events may include lipid peroxidation and loss of functional groups of enzymes, alteration of membrane permeability, and cell injury or death. An acute exposure to O3 causes lung injury involving the ciliated cell in the airways and the type 1 epithelial cell in the alveolar region. The effects are particularly localized at the junction of terminal bronchioles and alveolar ducts, as evident from a loss of cells and accumulation of inflammatory cells. In a typical short-term exposure the lung tissue response is biphasic: an initial injury-phase characterized by cell damage and loss of enzyme activities, followed by a repair-phase associated with increased metabolic activities, which coincide with a proliferation of metabolically active cells, for example, the alveolar type 2 cells and the bronchiolar Clara cells. A chronic exposure to O3 can cause or exacerbate lung diseases, including perhaps an increased lung tumor incidence in susceptible animal models. Ozone exposure also causes extrapulmonary effects involving the blood, spleen, central nervous system, and other organs. A combination of O3 and NO2, both of which occur in photochemical smog, can produce effects which may be additive or synergistic. A synergistic lung injury occurs possibly due to a formation of more powerful radicals and chemical intermediates. Dietary antioxidants, for example, vitamin E, vitamin C, and selenium, can offer a protection against O3 effects.