Total column ozone is measured in Dobson Units (DU), the thickness the total ozone would have at atmospheric pressure and 0° C, measured in 10^-5m. For example 350 DU corresponds to a 3.5 mm thick pure ozone layer under the specified conditions. About 90% of the atmospheric ozone is actually located in the stratosphere, where it reaches its highest concentration between 19 and 23 km in altitude.