&EF9V
United States
Environmental Protection
Agency
Air and Radiation
6205J
EPA430-F-99-024
September 1999
www.epa.gov/sunwise
UV Radiation
The sun radiates energy over a broad spectrum of wavelengths. Ultraviolet (UV)
radiation, which has a shorter wavelength than either visible blue or violet light,
is responsible for sunburn and other adverse health effects. Fortunately for life
on Earth, our atmosphere's stratospheric ozone layer shields us from most UV
radiation. What gets through the ozone layer, however, can cause the following
problems, particularly for people who spend substantial time outdoors:
• Skin cancer • Suppression of the immune system
• Cataracts • Premature aging of the skin
Because of these serious health effects, you should limit your exposure to UV
radiation and protect yourself when outdoors.
Types of UV Radiation
Scientists classify UV radiation into three types or bands—UVA, UVB, and
UVC. The stratospheric ozone layer absorbs some, but not all, of these types of
UV radiation:
UVA: Not absorbed by the ozone layer.
UVB: Mostly absorbed by the ozone layer, but some does reach
the Earth's surface.
UVC: Completely absorbed by the ozone layer and oxygen.
UVA and UVB that reach the Earth's surface contribute to the serious health
effects listed above.
UV Levels Depend on a Number of Factors
The level of UV radiation that reaches the Earth's surface can vary, depending
on a variety of factors. Each of the following factors can increase your risk of
UV radiation overexposure and its consequent health effects.
Stratospheric Ozone
The ozone layer absorbs most of the sun's UV rays, but the amount of absorp-
tion varies depending on the time of year and other natural phenomena. That
absorption also has decreased, as the ozone layer has thinned due to the release
of ozone-depleting substances that have been widely used in industry.
Time of Day
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.
-------
Time of Year
The sun's angle varies with the seasons, causing the intensity of UV rays to change. UV intensity tends to be high-
est during the summer months.
Latitude
The sun's rays are strongest at the equator, where the sun is most directly overhead and UV rays must travel the
least distance through the atmosphere. Ozone also is naturally thinner in the tropics compared to the mid- and
high-latitudes, so there is less ozone to absorb the UV radiation as it passes through the atmosphere. At higher
latitudes the sun is lower in the sky, so UV rays must travel a greater distance through ozone-rich portions of the
atmosphere and, in turn, expose those latitudes to less UV radiation.
Altitude
UV intensity increases with altitude because there is less atmosphere
to absorb the damaging rays. Thus, when you go to higher altitudes,
your risk of overexposure increases.
Weather Conditions
Cloud cover reduces UV levels, but not completely. Depending on
the thickness of the cloud cover, it is possible to burn—and increase
your risk of long-term skin and eye damage—on a cloudy summer
day, even if it does not feel very warm.
Reflection
Some surfaces, such as snow, sand, grass, or water can reflect much of
the UV radiation that reaches them. Because of this reflection, UV
intensity can be deceptively high even in shaded areas.
EPA's SunWise School Program
In response to the serious public health threat posed by
exposure to increased UV levels, the U.S. Environmental
Protection Agency (EPA) is working with schools and
communities across the nation through the SunWise School Program.
SunWise aims to teach children in elementary school and their care-
givers how to protect themselves from overexposure to the sun.
For More Information
To learn more about UV radiation, the SunWise School Program,
and actions being taken to prevent ozone depletion, call EPA's
Stratospheric Ozone Information Hotline at 800 296-1996 or visit
our Web site at .
The stratospheric ozone layer screens
out much of the sun's harmful UV
radiation.
-X) Printed on paper that contains at least 30 percent postconsumer fiber.
------- |