Because of its vital protective role, it was recognized that depletion of the ozone layer would have serious consequences, such as increased incidence of skin cancer and declining crop yields, and a movement to ban all CFCs began. By the 1980s, observations at Japan's Showa Station demonstrated an unusual decline in the ozone layer at the South Pole, confirmed by data from other nations' Antarctic outposts as well, which came to be called the "ozone hole." This phenomenon was observed at the North Pole as well in the 1990s. Presently the ozone hole is linked to formations called polar stratospheric clouds (PSCs), and it is believed that chlorine oxides (ClOx), a key constituent of CFCs, is the agent that destroys the ozone layer.
If the entire atmosphere were under a uniform temperature of 0℃ and pressure of 1 atmosphere, it would have a thickness of 8,000m. The thickness of ozone if gathered in a similar fashion is used to define the total quantity of ozone; this quantity is recorded in Dobson units, named after the ozone-layer researcher G.M.B. Dobson. Under normal conditions, total ozone volume is 300-500D.U. At the South Pole, however, ozone volume is observed to be only 120D.U. Observations at the North Pole indicate a quantity of 250D.U. Ultraviolet light is divided into three wavelengths. From the wavelength closest to the visible spectrum, these wavelengths are long-wave UV (UVA: 400-320nm), medium-wave UV (UVB: 320-290nm) and short-wave UV (UVC: 290-200nm) ("nm" stands for "nanometer"). Of these three classes of solar UV radiation, UVC is completely absorbed by the ozone layer and does not reach the earth's surface. In contrast, most UVA radiation escapes the ozone layer to reach the earth, while the quantity of UVB that reaches the earth varies with the expansion and contraction of the ozone layer. In one example of UVB research, the volume of ozone recorded over the Greek city of Thessaloniki was plotted against the quantity of UVB reaching the earth, or "incident UVB," there in the 1990s. A strong negative seasonal correlation was found; moreover, ozone volume steadily decreased during this time, while the quantity of incident UVB steadily increased.
Exposure to UVB radiation has been indicated as a cause not only of skin cancer and reduced crop yields as described above, but of damage to DNA as well. Research into the mechanisms behind this phenomenon, its prevention and treatment are being stepped up, and researchers are placing high priority on observing ozone volume and quantity of incident UVB in the northern hemisphere. Although it is not yet clear what the long-term effects of human activity will be on the hole in the ozone layer, researchers are determined to find a way to eliminate the hole.