Glossary Ozone is one of the several gases that make up the earth's atmosphere. It is the triatomic form of oxygen and makes up approximately one part in three million of all of the gases in the atmosphere. If all the ozone contained in the atmosphere from the ground level up to a height of 60 km could be assembled at the earth's surface, it would comprise a layer of gas only about 3 millimetres thick, and weigh some 3000 million tonnes.
Ozone is toxic at high concentrations because it reacts strongly with other molecules.
Many experimental studies of plants and animals, and clinical studies of humans have shown the harmful effect of excessive exposure to UVB radiation. The total column ozone is measured with an instrument called a Dobson spectrophotometer, compares the amount of sunlight at two ultraviolet wavelengths. One wavelength is affected strongly by ozone, the other is not. The difference between the two is directly related to the total column ozone. Polar orbiting satellites are now used now give a map of total column ozone over the entire globe.