UVB and UVA Radiation

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Things to know:

  • 50 % of the sun’s daily dose of UV radiation occurs between 11 a.m. and 2.00 p.m.
  • 75 % of the UV radiation is able to penetrate through thick clouds.
  • When the sky is slightly overcast, the UV radiation intensity may even be temporarily increased through refraction of the rays.
  • As a result of reflection, up to 50 % of the UV radiation even reaches us in the shade.
  • 60 % of the UV rays can penetrate water to a depth of 50 cm (swimming depth).
  • For each increase in altitude of 1000 metres, the UV radiation increases by 15 %.
  • Through reflection, snow intensifies radiation by an additional 50%
  • Clothing and umbrellas do not protect us completely from ultraviolet radiation.
  • UVA and UVB rays account for only 5 % of the sun’s radiation that reaches the earth, thanks to the ozone layer.

Infra-red light is long-wave radiation that is low in energy. It penetrates the epidermis and reaches the dermis and deeper layers of tissue, generating a feeling of warmth. As warming increases, the blood vessels in the dermis dilate and cause temporary reddening of the skin (“flush”). The body seeks to get rid of the heat through the dilated blood vessels in the skin. If the body becomes overheated under the influence of sunlight and the blood vessels have dilated to their maximum, increasing quantities of fluid enters the tissue from the capillaries. The lymph vessels may not be able to carry away this fluid all at once, and the result is swelling of the skin, known medically as oedema. Infra-red radiation is able to penetrate to a depth of about 1 cm.

The skin has various mechanisms to provide protection against the harmful effects of the sun. One is through the activation of pigment formation in the melanocytes (pigment cells) of the epidermis, the other through thickening of the horny layer of the skin under exposure to the sun. UVA rays induce immediate pigmentation, UVB rays delayed pigmentation.

Melanin, the pigment of the melanocytes, is what mainly determines the skin colour of humans. Eumelanin produces a black-brown colouring, phaeomelanin a reddish-brown colouring. In addition to melanin, the normal skin colour is also influenced by haemoglobin (blood pigment) and the carotinoids (e.g. the substance that gives carrots their colour). The melanocytes release their pigment through long cell processes (i.e. cell extensions) in the form of pigment particles to the surrounding cells of the epidermis. The result is a protective barrier of pigment around the cell nucleus, as damage to the nucleus of the epidermis cells can be the first step towards the formation of skin cancer.

The differences in human skin colour from fair, yellow to dark depends on the size, density, distribution and accumulation of pigment particles in the skin cells. Dark-skinned people do not have more melanocytes than pale-skinned people, but their melanocytes are more active and produce greater quantities of pigment.