Although good ozone only represents a tiny fraction of the atmosphere, it is crucial for life on Earth. The proportion of good and bad ozone in the atmosphere depends on the balance between processes that create ozone and those that destroy it. An upset in this balance can have serious consequences for life on Earth, and scientists are finding evidence that the balance has changed. Concentrations within the protective ozone shield are decreasing, while levels in the air we breathe are increasing. Ozone amounts in the stratosphere are small, rarely exceeding 10 parts per million by volume.
Ozone is measured in Dobson Units .
Average ozone levels are 300 DU, which would be equivalent to a layer three millimeters thick if compressed to the planet's surface. Levels may range from less than 100 DU to over 500 DU globally.