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University of California, San Francisco |  |
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UV Radiation
 UV Radiation Some exposure to sunlight can be enjoyable, but too much sunlight can 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. Sunlight consists of two types of ultraviolet (UV) radiation, UVB and UVA. Both UVB and UVA radiation contribute to freckling, skin wrinkling and the development of skin cancer. UVB radiation (290-320nm) has the most energy and causes the most damage. UVB is only partially blocked by clouds or fog; therefore, it is important to wear sunblock even on cloudy days. This type of radiation intensifies during the summer and with higher elevations . UVB can do more damage more quickly than UVA rays. Because of its damaging affect to the DNA of skin cells, UVB radiation is the main cause of sunburn and skin cancer. Over the past 25 years, the thinning ozone means more UVB penetrates the atmosphere, increasing the risk for UVB-related sun damage. UVA radiation (320-400nm) is less powerful than UVB, but it penetrates deeper into the skin. Small daily doses of UVA causes long-term skin injury, even without signs of sunburn. UVA light is used in tanning booths. Tanning booths not only cause the same type of skin and eye damage as natural sunlight, they may also be as much as 20 times stronger. When the sun's ultraviolet radiation reaches the surface of the skin, the skin reacts by producing melanin, a skin pigment that has a protective effect on the skin. Therefore, tanning after sun exposure is your body's way response to sun damage. Having a tan provides minimal protection against sun overexposure and is not a substitute for good sun protective measures. 
The UV Index: Predicting when to cover up from the sun The UV Index, which was developed by the National Weather Service and the U.S. Environmental Protection Agency (EPA), provides important information to help you plan your outdoor activities to prevent overexposure to the sun's rays. The UV Index predicts the risk of overexposure to the sun for the following day. The Index predicts UV intensity levels on a scale of 1 to 11+, where a low number indicates a minimal risk of overexposure and a high number (e.g.11+) means an extreme risk. The UV Index takes into account the day, weather condition, time of year, elevation, latitude, amount of ozone coverage, as well as other local conditions that affect the amount of UV radiation reaching the ground.
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