The sun gives us light
in the form of visible radiation. It gives us warmth in the form of infrared radiation.
It also gives us a third type of radiation that we can neither see nor feel: ultraviolet radiation.
Ultraviolet (UV) radiation can seriously threaten our health. UV rays can cause cancer by damaging cells' genetic material. The damage allows cells to form cancerous tumors. (See module 3, "Cancer Biology," for review.)
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To learn more about the electromagnetic spectrum, click on http://www.lbl.gov/MicroWorlds/teachers/emspectrum.pdf. (PDF–70K)
Visible, infrared, and ultraviolet radiation are all forms of electromagnetic radiation. Electromagnetic radiation moves through the air as a waveform. We measure its wavelength as the distance between two successive peaks or valleys of the waveform.
Wavelengths are measured in nanometers. A nanometer is one billionth of a meter. The short form of nanometer is "nm."
Our eyes sense only a
very small part of the electromagnetic spectrum. We only see the
wavelengths of visible light, from red to violet. To find out more about
the electromagnetic spectrum go to http://imagine.gsfc.nasa.gov/docs/teachers/
UVA is ultraviolet radiation with wavelengths from 320-400 nm. It passes right through the Earth's ozone layer. UVA can cause early aging of the skin.
UVB is ultraviolet radiation with wavelengths of 280-320nm. It does not go as deeply into the skin as UVA does. UVB causes skin cancer. It might also be involved with cataracts. (Cataracts are 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 is ultraviolet radiation with wavelengths shorter than
280 nm. It is also dangerous to people. But it is completely absorbed by the Earth's ozone layer.
The National Weather Service and the Environmental Protection Agency (EPA) developed the UV Index. The UV Index is meant to help people understand and protect themselves from the harmful effects of UV rays. The UV Index forecasts daily the intensity of the sun's rays when it is at its highest. (This is called "solar noon.")
The UV Index ranges from 0 to 15. It tells how much ultraviolet 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:
Each of these factors affects either:
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. Thus June would have the highest UV Index for the Northern Hemisphere, but the lowest for the Southern Hemisphere.
The Climate Prediction
Center tracks and predicts the UV Index throughout the year in various
cities in the United States. Go to their site: http://www.cpc.ncep.noaa.gov/products/
First take a look at the maximum UV Index for the month of June.
Now look at the maximum UV Index for the month of December.
Time of day has a very
important effect on UV Index. Remember the concept of "solar
noon?" Click on http://www.cpc.ncep.noaa.gov/products/
stratosphere/uv_index/uv_diurnal.html to learn about how the UV Index changes throughout the day. Compare Figure 1 and Figure 2. Many experts recommend that you avoid sun exposure between 10 am and 4 pm. Based on the data in Figure 1, do you think this is a good way to protect yourself from the sun?
Clouds also affect the amount of UV radiation that reaches the Earth's surface.
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.
(Read module 7, "The Earth's Ozone Layer and the UV Index" to learn more about the Earth's protective ozone layer.)
At a given UV Index, will your exposure be the same if you are playing soccer on a grassy field or skiing down a snow-covered mountain? No way! Reflective surfaces intensify UV exposure to varying degrees. Water, snow, and sand reflect the most.
Consider these facts:
Skin cancer is the most dangerous and deadly risk of UV radiation. But it is not the only one. Other risks include:
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.
Check out http://www.dermnet.org.nz/index.html* (go to information on 250 skin conditions and treatment and then click on aging skin) and http://www.centerforderm.com/articles/aging.html.* These sites show images of this kind of early aging from the sun. This is called "photoaging." ("Photo" means "light.") Photoaging can be caused by UV rays from the sun or from tanning salons.
* Links to
non-Federal organizations are provided solely as a service to our users. Links
do not constitute an endorsement of any organization by CDC or the Federal
Government, and none should be inferred. The CDC is not responsible for the
content of the individual organization Web pages found at these links.
This page last reviewed April 24, 2007
United States Department of Health Human Services