Radiation effects of Sun exposure

The sun emits electromagnetic rays comprising a continuous spectrum of short to long waves. Only a narrow range of wavelengths between 400 nm and 770 nm react with photocells in the retina and observed as the various colors of the rainbow. Beyond red (770 nm) is infrared. Heat is due to infrared radiation, which can be felt. Below violet (400 nm) are the ultraviolet (200–400 nm) and X-rays. Most short wavelengths, that can neither be seen nor felt, are filtered out by the Earth's thick atmosphere which includes ozone and water vapour. Therefore, as there is less atmosphere above mountain tops, the danger of radiation exposure is greater. The content of water vapor in the atmosphere varies, which accounts for protection from sunburn in winter, cloudy days, the early morning, or late evening sun. Glass filters out wavelengths below 320 nm, so that the closed windows of a car will protect even in a tropical desert unless one is sensitive to the longer wavelengths of ultraviolet radiation. Porphyria is, for example, a disease triggered by UVA radiation and is thus difficult to protect against by shade, cloud, or glass.
Ultraviolet radiation (UVR) is arbitrarily divided into UVC (200–280 nm), UVB (280–320), and UVA (320–400 nm). The principal effects of ultraviolet light on the skin are elastic fibre damage and cutaneous ageing, apoptosis, immunosuppression, suntan, sunburn, and carcinogenesis. While each subclass of UVR can produce all of these effects, UVC is relatively more likely to cause cancer, UVB relatively more likely to produce burning, and UVA relatively more likely to produce ageing. UVA is estimated to be 10 times less effective than UVB at producing a suntan and 100 times less effective than UVB at producing non-melanoma skin cancer. This is the basis of so-called ‘safe tanning’ using UVA light in solariums. However, high-dose UVA does cause non-melanoma skin cancer. In addition, the spectrum responsible for producing melanoma has not been established and may include UVA. UVR immunosuppression may suppress tumour surveillance and thereby enhance carcinogenesis.
The diagnosis of UVR damage is determined by recognizing the distribution of the rash as being typical of exposure. Thus the head, nose, and cheeks are principally affected, but there is often sparing below the eyebrows, under a forelock, beneath and behind the ears, and below the chin. The sides and back of the neck are picked out, but there is a sharp border to the sun damage where the collar shields the skin from sunlight. Much, of course, depends on the style of clothing as well as on the direction of irradiation. The backs of the hands and dorsum of the feet are often caught by the sun; however, there may be some tolerance of such skin previously exposed and tanned so that skin not so tolerant is clearly more prone to burning. Mediation of sunburn erythema is partly due to the generation of prostaglandins.