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 highest 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.
[September 1999; EPA430-F-99-024]
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