Ozone as concentrations in μg/m. The study of ozone concentration in the atmosphere started in the 1920s. The highest levels of ozone in the atmosphere are in the stratosphere, in a region also known as the ozone layer between about 10 km and 50 km above the surface (or between about 6 and 31 miles). However, even in this "layer", the ozone concentrations are only two to eight parts per million, so most of the oxygen there is dioxygen, O, at about 210,000 parts per million by volume. Ozone in the stratosphere is mostly produced from short-wave ultraviolet rays between 240
Tropospheric ozone is the reaction of •OH with NO to form nitric acid at high NOx levels. If nitrogen monoxide (NO) is instead present at very low levels in the atmosphere (less than 10 approximately ppt), the of peroxy radicals (HO• ) formed from the oxidation will instead react with themselves to form peroxides, and not produce ozone. Health effects depend on ozone precursors, which is a group of pollutants, primarily generated during the combustion of fossil fuels. Reaction with daylight ultraviolet (UV) rays and these precursors create ground-level ozone pollution (Tropospheric Ozone). Ozone is known to have the following health effects