Total column ozone is the total amount of ozone in a column extending vertically from the earth’s surface to the top of the atmosphere. It is measured using ground-based stations and satellites and is reported in Dobson units (DU). The ozone hole is defined in terms of reduced total column ozone—less than 220 DU.
Stratospheric ozone is beneficial for humans and other life forms as it absorbs harmful ultraviolet (UV) radiation emitted by the sun and stops it from reaching the surface of the earth. As solar radiation passes through the atmosphere, all UV-C and approximately 90 per cent of ozone, water vapour, oxygen and carbon dioxide absorb UV-B radiation. The atmosphere doesn’t affect UV-A much. Thus, the UV radiation reaching earth is mostly UV-A with a small amount of UV-B. The increased exposure to UV-B radiation enhances the risk of skin cancer, cataracts and suppressed immune system in humans. Excessive UV-B exposure can also harm fauna, single-cell organisms and aquatic ecosystems.