Energy from the sun reaches the earth as visible, infrared, and ultraviolet (UV) rays.
Ultraviolet A (UVA) is made up of wavelengths 320 to 400 nanometers (nm) in length.
Ultraviolet B (UVB) wavelengths are 280 to 320 nm in length.
Ultraviolet C (UVC) wavelengths are 100 to 280 nm in length.
Only UVA and UVB ultraviolet rays reach the earth's surface. The earth's atmosphere absorbs UVC wavelengths.
UVB rays cause a much greater risk of skin cancer than UVA. However, UVA rays cause aging, wrinkling, and loss of elasticity.
UVA also increases the damaging effects of UVB, including skin cancer and cataracts (an eye disorder characterized by a change in the structure of the crystalline lens that causes blurred vision).
In most cases, ultraviolet rays react with a chemical called melanin, that is found in the skin. This is the first defense against the sun, as it is the melanin that absorbs the dangerous UV rays that can do serious skin damage. A sunburn develops when the amount of UV damage exceeds the protection that the skin's melanin can provide. While a small amount of exposure to sunlight is healthy and pleasurable, too much can be dangerous. Measures should be taken to prevent overexposure to sunlight in order to reduce the risks of cancers, premature aging of the skin, the development of cataracts, and other harmful effects.
What is the Ultraviolet (UV) Index?
In response to the increasing incidence of skin cancer, cataracts, and other effects from exposure to the sun's harmful rays, the National Weather Service (NWS), the US Environmental Protection Agency (EPA), and the Centers for Disease Control and Prevention (CDC) collaborated on a sun-awareness information program. An important part of this program is the Ultraviolet (UV) Index, developed by the National Meteorological Center of the National Weather Service.
The Index is a next-day forecast that estimates the amount of ultraviolet radiation that will reach the earth's surface—providing important information to help you prevent overexposure to the sun's rays. The Index also includes the effects of cloud cover on the anticipated UV exposure level for the next day.
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