Ozone gas (O₃) forms a thin protective layer encircling the earth and screens out 99% of the sun’s harmful UV light. The ozone layer occupies the outer two-thirds of the stratosphere, 20 to 50 kilometers above the earth’s surface. The screening effect of the ozone layer protects all organisms from damage caused by UV light, which is known to be mutagenic and carcinogenic.
When UV light strikes ozone molecules, it causes them to split apart. The products, however, quickly reunite, reforming ozone and giving off heat. Thus, the ozone layer is a renewable layer that converts UV light into heat.
Three major human activities are believed that are destroying the ozone layer: (1) the use of spray cans and refrigerants that contain freon gas, (2) high flying supersonic and subsonic jets, and (3) the detonation of nuclear weapons.
Freons, as spray-can propellants, are in the use since 1951. Freons are also known as chlorofluorocarbons (CFCs). Two CFCs were commonly used: freon-11 (trichloromonofluoromethane), used as spray-can propellant, now banned in several countries, and freon-12 (dichlorodifluoromethane), used as coolant in refrigerators, freezers, and air conditioners, which is still used. Until the early 1970s chemists listed freons as inert (unreactive) chemicals.
The freons, or CFCs, are dissociated by UV light in the stratosphere. This produces a highly reactive chlorine free radical. The free radical can react with ozone layer, thus reducing the ozone concentration and eliminating the UV screen. A single molecule of freon gas can eliminate many thousands of molecules of ozone, because the chlorine free radical is regenerated. Chloride oxide (formed when the chloride free radical reacts with ozone) can also react with ozone.
The ozone layer may be vulnerable to other human activities as well. Supersonic and subsonic jets flying in the stratosphere may also destroy ozone through the release of nitric oxide produced by jet engines. The nitric oxide gas reacts with ozone to form nitrogen dioxide and oxygen.
The detonation of nuclear weapons in the atmosphere also produces nitric oxide. This suggests that nuclear war could cause a dangerous reduction.
Nitrogen fertilizers that farmers apply to their fields may be converted into nitric oxide gas.