Earth's sunscreen – the ozone layer
Box 1 | Meet the ultraviolet family
Electromagnetic radiation is divided into different types according to its wavelength. Visible light is just a small part of the whole spectrum. Ultraviolet (UV) radiation, as you can tell from its name, lies beyond the violet end of visible light and has shorter wavelengths.
Ultraviolet is normally used to describe radiation with wavelengths between about 100 and 400 nanometres. (A nanometre is one-millionth of a millimetre.) As with all electromagnetic radiation, the shorter the wavelength the greater the energy carried.
The ultraviolet family can be divided into three parts:
UV-A (315-400 nanometres) has the longest wavelengths of the family and is the least damaging. However, it does cause sunburn and has been implicated in causing sun-induced premature ageing of skin and some cancers. This is the form of ultraviolet produced in most solariums.
UV-B (280-315 nanometres) can cause skin cancer and eye damage. It also causes sunburn. Radiation with a wavelength close to 280 nanometres is strongly absorbed by proteins, altering and often damaging their function. In this way, UV-B can reduce the immune response and it also interferes with photosynthesis in some crop plants. A very small amount of exposure to UV-B is necessary to produce vitamin D in human skin.
UV-C (100-280 nanometres) is the most dangerous member of the family. Wavelengths around 260 nanometres are absorbed by DNA and so nearly all life forms are irreparably damaged by this radiation.
The good news is that the stratospheric ozone layer absorbs all UV-C, the most deadly form, and even a thinned ozone layer is unlikely to let much through.
The intact ozone layer does, however, let through some UV-A, especially when the sun is high in the sky, and a very small amount of UV-B. The proportion of both of these reaching ground level will increase with ozone loss.
Many species have some natural protection against UV-A. For example, we can produce melanin, a dark pigment, in the outer layer of our skin. However, pale-skinned people can't produce enough melanin to protect against the amount of ultraviolet radiation that occurs across all of Australia for most of the year (even on cloudy days). Even dark-skinned people, who naturally have high melanin concentrations in their skin, can suffer sunburn after long periods of exposure.
Boxes
Box 2. Can plants get sunburn?
Box 3. How ozone is lost
Box 4. Australia finds a replacement for methyl bromide
Posted February 1997.