General Public UV Index

UV Index: What is it?

Some exposure to sunlight can be enjoyable; however, too much could be dangerous. Overexposure to the sun's ultraviolet (UV) radiation can cause immediate effects such as sunburn and long-term problems such as skin cancer and cataracts. The UV Index is a 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. 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.

There are two prices to pay for overexposure to UV radiation: a severe sun burn following an intense short term overexposure, and the more serious skin cancers developing 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.

UV Index: How to use it!

As described earlier, the UV Index is a forecast of the probable intensity of skin damaging ultraviolet radiation reaching the surface during the solar noon hour (11:30-12:30 local standard time or 12:30-13:30 local daylight time). The greater the UV Index is the greater the amount of skin damaging UV radiation. How much UV radiation is needed to actually damage one's skin is dependant on several factors. But in general the darker one's skin is, (that is the more melanin one has in his/her skin) the longer (or the more UV radiation) it takes to cause erythema (skin reddening). For those who always burn and never tan the times to burn are relatively short compared to those who almost always tan.

Below are general guidelines on how to protect oneself from overexposure to UV radiation and the maximum recommended exposure time:

Exposure Category UV Index Maximum Exposure to Sun Protective Actions
Minimal 0, 1, 2 Less than an hour Apply skin protection factor (SPF) 15 sun screen.
Low 3, 4 Less than an hour SPF 15 & protective clothing (hat)


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