&EPA
United States
Environmental Protection
Agency
United States
Environmental Protection
Agency
Air and Radiation
6205J
EPA 430-F-10-025
June 2010
www.epa.gov/ozone/strathome.html
UV Radiation
Did You
Know?
Ultraviolet (UV)
radiation, from the
sun and from
tanning beds, is
classified as a
human carcinogen,
according to the
U.S. Department
of Health and
Human Services
and the World
Health
Organization.
This fact sheet explains the types of ultraviolet radiation and the various factors that can affect
the levels reaching the Earth's surface. The sun emits energy over a broad spectrum of
wavelengths: visible light that you see, infrared radiation that you feel as heat, and ultraviolet
(UV) radiation that you can't see or feel. UV radiation has a shorter wavelength and higher
energy than visible light. It affects human health both positively and negatively. Short
exposure to UVB radiation generates vitamin D, but can also lead to sunburn depending on an
individual's skin type. Fortunately for life on Earth, our atmosphere's stratospheric ozone layer
shields us from most UV radiation. What does get through the ozone layer, however, can
cause the following problems, particularly for people who spend unprotected time outdoors:
Skin cancer Suppression of the immune system
Cataracts Premature aging of the skin
Since the benefits of sunlight cannot be separated from its damaging effects, it is important to
understand the risks of overexposure, and take simple precautions to protect yourself.
,v
UVB, and UVC. The ozone
'
iver, but some does reach
Types of UV Radiation
Scientists classify UV radiation into three types or bandsU
layer absorbs some, but not all, of these types of UV radiation:
UVA: Wavelength: 320-400 nm. Not absorbed by the ozone layer.
UVB: Wavelength: 290-320 nm. Mostly absorbed by the ozone lay<
the Earth's surface.
UVC: Wavelength: 100-290 nm. Completely absorbed by the ozone layer and
atmosphere.
UVA and UVB radiation that reaches the Earth's surface contributes to the serious health
effects listed above; it also contributes to environmental impacts. Levels of UVA radiation are
more constant than UVB, reaching the Earth's surface without variations due to the time of day
or year. In addition, UVA radiation is not filtered by glass.
UV Levels Depend on a Number of Factors
The level of UV radiation reaching the Earth's surface can vary. Each of the following factors
can increase your risk of UV radiation overexposure and consequent health effects.
Stratospheric Ozone Layer
The amount of UV rays the ozone layer absorbs varies depending on the time of year and other
natural events. Additionally, the ozone layer is thinner than it used to be due to ozone-
depleting chemicals used in industry and consumer products. These chemicals are being
phased out, but the ozone layer is not predicted to heal to pre-1980 levels until mid- to late-
century.
Time of Day
The sun is 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.
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Time of Year
The sun's angle varies with the seasons, causing the intensity of UV
rays to change. UV intensity tends to be highest in the summer.
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. As a result, your chance of damaging
your eyes and skin increases at higher altitudes.
Weather Conditions
Cloud cover reduces UV levels, but not completely. Depending on the
thickness of the cloud cover, it is possible to burn on a cloudy day,
even if it does not feel warm,
Reflection
Surfaces like snow, sand, pavement, and water 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 Sun Wise Program
In response to the serious public health threat posed by exposure to
UV rays, EPA works with schools and communities across the nation
through the SunWise Program, SunWise teaches children how to
protect themselves from overexposure to the sun. For more
information, please visit www.epa.gov/sunwise.
The UV Index
The UV Index forecasts the strength of the sun's harmful rays.
The higher the number, the greater the chance of sun damage.
1 2 3 4 5 6 7 8 9 10 11+
Visitwww.epa.gov/sunwise/uvindex.html
Be SunWise
Do Not Burn
Avoid Sun Tanning and
Tanning Beds
Use Sunscreen
Cover Up
Seek Shade
Watch for the UV Index
Use Extra Caution Near
Water, Snow, and Sand
Get Vitamin D Safely
The stratospheric ozone layer
screens out much of the sun's
harmful UV rays.
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