An unstable, poisonous allotrope of oxygen having the chemical formula O3 . Ozone forms in the atmosphere through the process of photolysis, when ultraviolet radiation from the Sun strikes oxygen molecules (O2 ), causing them to split apart. When freed oxygen atoms bump into and join other O2 molecules, they form ozone. Although ozone is broken down naturally in the atmosphere through chemical reactions with other atmospheric gases (such as nitrogen, hydrogen, and chlorine), in an unpolluted atmosphere the formation and breakdown of ozone is generally balanced, and the total concentration of ozone is relatively constant. The formation and destruction rates of ozone vary with altitude in the atmosphere, and with latitude. Most ozone forms in the 15 to 30 km (10 to 19 mi) altitude range and in latitudes closest to the equator where sunshine strikes the Earth the most. The ozone is then transported northward and southward by wind and is generally most concentrated in areas above the Canadian Arctic and Siberia and above Antarctica. Ozone is used commercially in water purification, in air conditioning, and as a bleach.