Colorless (or faintly blue), unstable (reactive), and water soluble gas having chlorine-like odor, and formed in the upper atmosphere by the action of solar radiation on oxygen. Its presence as a layer in stratosphere serves as a screen (called ozone shield) to block harmful ultraviolet radiation from reaching the earth's surface. At ground level it is formed by the combination of hydrocarbons and nitrogen oxides in the presence of sunlight and is the main ingredient of smog. In high-enough concentrations it can reduce lung function, inflame lung tissue, and cause coughing, shortness of breath, and other respiratory problems. Excessive exposure can result in permanent lung damage. Used commercially for bleaching, deodorizing, and disinfecting. Also called iriatomic oxygen or trioxygen. See also ozone layer.
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