<Dd> O + O ↔ O </Dd> <P> About 90 percent of the ozone in the atmosphere is contained in the stratosphere . Ozone concentrations are greatest between about 20 and 40 kilometres (66,000 and 131,000 ft), where they range from about 2 to 8 parts per million . If all of the ozone were compressed to the pressure of the air at sea level, it would be only 3 millimetres (⁄ inch) thick . </P> <P> Although the concentration of the ozone in the ozone layer is very small, it is vitally important to life because it absorbs biologically harmful ultraviolet (UV) radiation coming from the sun . Extremely short or vacuum UV (10--100 nm) is screened out by nitrogen . UV radiation capable of penetrating nitrogen is divided into three categories, based on its wavelength; these are referred to as UV - A (400--315 nm), UV - B (315--280 nm), and UV - C (280--100 nm). </P> <P> UV - C, which is very harmful to all living things, is entirely screened out by a combination of dioxygen (<200 nm) and ozone (> about 200 nm) by around 35 kilometres (115,000 ft) altitude . UV - B radiation can be harmful to the skin and is the main cause of sunburn; excessive exposure can also cause cataracts, immune system suppression, and genetic damage, resulting in problems such as skin cancer . The ozone layer (which absorbs from about 200 nm to 310 nm with a maximal absorption at about 250 nm) is very effective at screening out UV - B; for radiation with a wavelength of 290 nm, the intensity at the top of the atmosphere is 350 million times stronger than at the Earth's surface . Nevertheless, some UV - B, particularly at its longest wavelengths, reaches the surface, and is important for the skin's production of vitamin D . </P>

Where is ozone layer found in atmosphere what is its importance