Sunlight, an essential prerequisite for life, may be extremely dangerous to human health. Excessive exposure to the sun is known to be associated with increased risks of various skin cancers, cataracts and other eye diseases, as well as accelerated skin ageing. It may also adversely affect people's ability to resist infectious diseases, and compromise the effectiveness of vaccination programmes. Sunlight is electromagnetic energy, which is propagated by electromagnetic waves. Healthwise, the most important parts of the sunlight electromagnetic spectrum are:ultraviolet radiation (UV), invisible to the eye; visible light that allows us to see; and infrared radiation, which is our main source of heat but is also invisible. Excessive exposures to them poses particular risks to health. Skin: Excessive UV exposure results in a number of chronic skin changes. These include various skin cancers of which melanoma is the most life-threatening; an increased number of moles (benign abnormalities of melanocytes) and a range of other alterations arising from UV damage to keratinocytes and blood vessels. UV damage to fibrous tissue is often described as "photoageing". Photoageing makes people look older because their skin loses its tightness and so sags or wrinkles.
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