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Objective: Learn
about ultraviolet radiation and what affects its intensity.
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.)
This module will give
you some background about UV radiation and how it is measured. You will
also learn about factors that can make UV radiation more or less intense.
Electromagnetic Spectrum
[ Text
Description ]
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/
lessons/roygbiv/spectrum.html.
Back to Top
 UV
rays have wavelengths shorter than visible light. There are three types of
UV rays. They are grouped by wavelength from longest to shortest: UVA, UVB,
and UVC.
UVA
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
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
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.
Back to Top
The
UV Index
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:
- 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; or
- how much ultraviolet
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.
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/
stratosphere/uv_index/uv_meanmax.html
First take a look at
the maximum UV Index for the month of June.
- Which state has the
lowest maximum UV Index for the month of June?
- Which state has the
highest maximum UV Index for the month of June?
- What trends do you
see in the UV Index at different latitudes?
Now look at the maximum
UV Index for the month of December.
- How do these indices
compare with those of June?
- What would you
expect the maximum UV Index in March to look like? Check your answer.
Back to Top
UV Index Changes
Throughout the Day
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?
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 surface.
- Scattered clouds
allow 89 percent of UV rays to reach the surface.
- Overcast clouds
allow 32 percent of UV rays to reach the 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.
(Read module
7, "The Earth's Ozone Layer and the UV Index" to learn more
about the Earth's protective ozone layer.)
Reflective Surfaces
Make UV Exposure More Intense
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:
- 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.
Back to Top
Other Dangers of 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 (burn) ones' skin is to the sun]
- immune system
depression (This can put you at greater risk of getting sick.)
- eye damage
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.
Back to Top
Back
to Skin Cancer Home
* 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.
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