A triatomic molecule of oxygen; a natural constituent of the atmosphere, with the highest concentrations in the ozone layer or stratosphere; it is found at a level between 15 and 30 km above the Earth, which prevents harmful ultraviolet B radiation, which causes skin cancer and threatens plant life, from reaching the ground. The fragile shield is being damaged by chemicals released on Earth. The main chemicals that are depleting stratospheric ozone are chlorofluorocarbons (CFCs), which are used in refrigerators, aerosols and as cleaners in many industries and halons, which are used in fire extinguishers. The damage is caused when these chemicals release highly reactive forms of chlorine and bromine. (Source: GEMET/GILP96 / WRIGHT)