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UV Index: What is it?

The UV Index is a next day forecast of the amount of skin damaging UV radiation expected to reach the earth's surface at the time when the sun is highest in the sky (solar noon). The amount of UV radiation reaching the surface is primarily related to the elevation of the sun in the sky, the amount of ozone in the stratosphere, and the amount of clouds present. The UV Index can range from 0 (when it is night time) to 15 or 16 (in the tropics at high elevations under clear skies). UV radiation is greatest when the sun is highest in the sky and rapidly decreases as the sun approaches the horizon (Figure 1). The higher the UV Index, the greater the dose rate of skin damaging (and eye damaging) UV radiation. Consequently, the higher the UV Index, the smaller the time it takes before skin damage occurs (Figure 2).

There are two prices to pay for overexposure to UV radiation: a severe sun burn following an intense short term overexposure, and skin cancers developing as a result of frequent burns or after long term overexposure. Melanoma, the more deadly of the two types of skin cancer occurs when the patient has been subjected to several intense short term overexposures. Non-melanoma skin cancers, which are almost 100% curable, occur in people who are overexposed for very long periods of time, like construction workers, farmers, or fishermen. Long term overexposure to UV radiation has been linked to the formation of cataracts in the eyes as well.

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