The Ultrviolet Index is a forecast of the amount of skin damaging radiation expected to reach the surface of the earth at noon local time. The UV Index can range from 0 at nighttime to as high as 15 in the Tropics under clear skies.
Here's the key to the UV Index. The higher the UV Index the greater the threat is to one's skin and eye's. At the same time the higher the UV Index, the lesser the time it will take to damage human skin.
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). Attached is a look up chart where by one can cross check his/her propensity to burn versus the UV Index. For those who always burn and never tan the times to burn are relatively short compared to those who almost always tan.
The EPA has devised general guidelines as far as what to do to protect oneself from overexposure to UV radiation. These are shown in the table below.
||0, 1, 2
||Apply skin protection factor (SPF) 15 sun screen.
||SPF 15 & protective clothing (hat)
||SPF 15, protective clothing, and UV-A&B (sun glasses).
||7, 8, 9
||SPF 15, protective clothing, sun glasses and make attempts to avoid the sun between 10am to 4pm.
||SPF 15, protective clothing, sun glasses and avoid being in the sun between 10am to 4pm.
Ultraviolet (UV) radiation is a specific part of the sun's entire spectrum of wavelengths. UV radiation can be further sub-divided into three parts. UV-C is characterized by wavelengths less than 280 nm. Although highly dangerous to plants and animals, this part of the UV spectrum is completely absorbed by stratospheric ozone and does not reach the earth's surface. UV-B ranges in wavelengths between 280 and 320 nm. Ozone absorbs much of the shorter wavelength radiation, but this absorption weakens as 320 nm is approached. Plants and animals are particularly affected by this part of the UV spectrum. UV-B effects to humans are reddening of the skin (erythema) and reduction of vitamin-D synthesis in the short term, development of skin cancer, cataracts and suppression of the immune system in the long term. The wavelengths of UV-A radiation range between 320 and 400 nm. Ozone absorbs very little of this part of the UV spectrum. UV-A radiation is needed by humans for the synthesis of vitamin-D; however, too much UV-A causes photoaging (toughening of the skin), suppression of the immune system and, to a lesser degree, reddening of the skin, and cataract formation.