UV Radiation Types

In terms of sunbathing and the effects of sunlight on the human body, it is the ultra-violet part of the spectrum that is most important. Wavelengths above 100 nm are in turn split into three further regions, A, B, and C, that will now be described in more detail. The shorter end of the UV spectrum, below 100 nm, is often called ‘Extreme’.

UVA: 315 to 400 nm

At about 95% UVA is by far the dominant form of Solar UV that reaches the Earth’s surface. Since it is only weakly absorbed by the atmosphere, it is also present all day, all year round, at similar levels. Clouds do absorb UVA, but not completely.

UVA penetrates deeply into the skin, to the level of the underlying connective tissues, and can therefore cause serious long-term damage. It is the primary cause of light-induced damage to the immune system, and a trigger for phototoxic and photoallergenic reactions.

Although it does not damage the DNA of skin cells directly, it does generate highly reactive chemicals that can in turn damage DNA and increase the risk of cancers developing. It also causes photoaging effects, such as wrinkles and sagging through loss of skin elasticity, and blotchiness and discolouration.

UVA activates the melanin already present in the skin causing darkening, or a light tan. Although this type of tan develops quickly, within a couple of hours, it is also lost almost as quickly after exposure ceases.

UVB: 280 to 315 nm

UVB is strongly absorbed by the atmosphere, particularly the ozone layer: only about 15% of the Sun’s UVB radiation that hits the Earth reaches the surface. This atmospheric absorption also means that UVB levels at the surface depend heavily on the time of day, time of year, latitude and altitude. The higher the Sun is in the sky, the more nearly overhead it is, the less atmosphere its light has to travel through to reach the surface and the stronger the UVB levels will be. The height of the surface above sea level also has a significant effect, increasing UVB levels by 15% for each 1,000 m increase in height. Clouds and atmospheric pollution, however, reduce UVB levels.

UVB does not penetrate the skin as deeply as UVA, but even small amounts can do considerable damage, including directly damaging the DNA of skin cells, thereby increasing the risk of cancers developing. Limited amounts of damage, however, are usually repaired by the body’s natural processes. UVB exposure is also strongly linked to the development of cataracts in the eyes.

Excessive exposure to UVB is also the primary cause of sunburn – severe damage to the upper layers of the skin – which can develop with as little as 20 minutes exposure.

UVB exposure triggers the production of additional melanin in the skin, which darkens the skin and increases the absorption of UV, thereby protecting the skin against further exposure to a limited extent. This type of tan takes a couple of days to develop, but then lasts for several weeks. UVB also stimulates thickening of the epidermis, producing the leathery ‘weather-beaten’ look and increasing protection further.

UVB is also the main stimulant for the production of vitamin D in the skin. This vitamin, which is difficult to obtain from foods, is essential for the proper processing of calcium by the body, and hence for the growth and maintenance of the bones. It is also important for the muscles and immune system, and there is growing evidence that it can help prevent certain types of cancer – possibly even including skin cancers – and other serious diseases.

UVC: 100 to 280 nm

UVC is completely absorbed by the atmosphere, so that the only sources at the Earth’s surface are man-made. It is highly damaging to living tissues and organisms, to the extent that it is widely used as an effective germicide to kill bacteria and other micro-organisms, for example to sterilize work surfaces and tools and in food and drinking water production processes.

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