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Ultraviolet Radiation

The sun gives us light in the form of visible radiation. The sun gives us warmth in the form of infrared radiation. The sun gives us a third type of radiation that we can neither see nor feel - ultraviolet (UV) radiation.

Ultraviolet radiation or UV radiation is the sun's harmful rays, the "invisible rays" that are part of the energy that comes from the sun. Ultraviolet (UV) radiation can seriously threaten your health. UV rays can cause cancer by damaging cells' genetic material. The damage allows cells to form cancerous tumors.

Visible, infrared, and ultraviolet radiation are all forms of electromagnetic radiation. Electromagnetic radiation moves through the air as a waveform. Wavelengths are measured as the distance between two successive peaks or valleys of the waveform. Wavelengths are measured in nanometers (nm). A nanometer is one billionth of a meter.

Our eyes sense only a very small part of the electromagnetic spectrum. We only see the wavelengths of visible light, from red to violet.

Wavelengths of UV Radiation

UV rays have wavelengths shorter than visible light. UV radiation is made up of three parts, grouped by wavelength from longest to shortest, UVA, UVB, and UVC:

• UVA: Wavelengths from 320 - 400 nanometers (nm). It passes right through the Earth's ozone layer. UVA can cause early aging of the skin. Originally believed to be harmless but now believed to cause skin damage.
• UVB: Wavelengths of 280 - 320 nanometers (nm). It does not go as deeply into the skin as UVA does. Causes skin damage, sunburn, and skin cancer. It might also be involved with cataracts - a clouding of the lens of the eye that can lead to blindness. The ozone layer absorbs most of the sun's UVB, but even then the small amount of UVB rays can do substantial damage. Also, with the possibility of the thinning of the ozone layer, more UVB rays might result in more damage.
• UVC: Wavelengths shorter than 280 nanometers (nm). It is dangerous to people, but none reaches the Earth's surface - it is all absorbed by the ozone layer.

UV Index

The UV Index forecasts daily the intensity of the sun's rays when it is at its highest. This is called "solar noon." Usually, this is not the same time as "clock noon". The relationship between clock noon and solar noon depends on your location within your time zone and the time of year. To find the time of solar noon at your location, use the sunrise sunset solar noon calculator at the NOAA (National Oceanic and Atmospheric Administration) Web site at www.srrb.noaa.gov/highlights/sunrise/sunrise.html

The UV Index ranges from 0 to 15. It tells how much ultraviolet (UV) radiation reaches the Earth's surface over the one hour period around noon. Index numbers are low when the sun is low in the sky and when it is overcast. The Index is higher when the sun is high in the sky and during clear or partly cloudy conditions.

The UV Index is based on several things:

• Latitude
• Elevation
• Day of year
• Time of day
• Total ozone in the atmosphere
• Predicted cloud conditions

Each of these factors affects either:

• How far rays must travel from the sun to a given point on Earth.
• How much ultraviolet (UV) radiation is absorbed, transmitted, or scattered on its way from the sun to the Earth.

Latitude and Season Change UV Index
Changes in latitude and day of year alter the distance between the sun and a person on Earth. The UV Index is higher closer to the equator and during the summer months. June would have the highest UV Index for the Northern Hemisphere, but the lowest for the Southern Hemisphere.

UV Index Changes Throughout the Day
The time of day has a very important effect on UV Index. Remember the concept of "solar noon?" Many experts recommend that you avoid sun exposure between 10 am and 4 pm. The sun is at its highest in the sky around noon. At this time, the sun's rays have the least distance to travel through the atmosphere and UVB levels are at their highest. In the early morning and late afternoon, the sun's rays pass through the atmosphere at an angle and their intensity is greatly reduced.

Cloud Conditions Affect UV Index
Clouds also affect the amount of UV radiation that reaches the Earth's surface.

• Clear skies allow 100 percent of UV rays to reach the Earth's surface.
• Scattered clouds allow 89 percent of UV rays to reach the Earth's surface.
• Overcast clouds allow 32 percent of UV rays to reach the Earth's surface.

The Ozone Layer Lowers UV Index
The ozone layer absorbs UV rays. But it is not the same over the entire surface of the Earth. Satellites measure the total ozone. Total ozone is then inserted with the other variables into a model. This model calculates the level of UV radiation reaching the Earth at a given time and place.

• The farther UV rays must travel from the sun to the Earth's surface and the more ozone present, the less UV radiation reaches the surface.
• The shorter the distance from the sun and the less ozone present, the more UV radiation reaches the surface.

Reflective Surfaces Make Ultraviolet (UV) Exposure More Intense

At a given UV Index, your exposure will not be the same if you are playing soccer on a grassy field or skiing down a snow-covered mountain. Reflective surfaces intensify UV exposure to varying degrees. Water, snow, and sand reflect the most. Consider these facts:

• Grass reflects from 2.5 to 3 percent of UV rays hitting its surface.
• Sand reflects 20 to 30 percent of UV rays.
• Snow and ice can reflect 80 to 90 percent of UV rays.
• Depending on the angle of reflection, water can reflect up to a full 100 percent of UV rays striking the surface.

Other Risks of Ultraviolet (UV) Rays

Skin cancer is the most dangerous and deadly risk of UV radiation. But it is not the only one. Other risks include:

• Sunburn
• Premature (early) wrinkling and sagging of skin.
• Sun sensitivity - how sensitive ones' skin is to the sun (burn).
• Immune system depression - this can put you at greater risk of getting sick.
• Eye damage, cataracts.

Tanning without burning can still damage skin tissue and cause premature aging of the skin - wrinkling, sagging, uneven texture and coloring.

All sources of UV rays can cause damage, not just natural ones. Most tanning beds have more UVA than UVB rays. But they can still damage your skin. UVA rays go deeper into the skin. They are the rays that cause premature aging of the skin.

References:
EXCITE
Centers for Disease Control and Prevention (CDC)
August 2004
www.cdc.gov/excite