(Chem; Symb: 03) A triatomic molecule of oxygen, bp -11(1)9ºC, mp -192. 5ºC, that occurs in small concentrations in the atmosphere and is produced by electric sparks in air. Ozone is a highly active element and is a powerful oxidizer. Prolonged exposure to concentrations of 0. 1 to 1 ppm are considered hazardous