r/EPA
UNITED STATES
ENVIRONMENTAL PROTECTION
AGENCY
AIR AND RADIATION
6205J
EPA430-F-95-006
APRIL1995
uv
RADIATION
UV INDEX
The sun gives out 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, stratospheric ozone screens most harmful UV radiation.
However, what gets through the ozone layer can cause a number of problems, particularly for people
who spend substantial time outdoors:
• skin cancer
• cataracts
• immune suppression
• premature aging of the skin
Because of these adverse health effects, limit your exposure to UV radiation and protect yourself
when working or recreating outdoors.
TYPES OF UV RADIATION
Scientists have classified UV radiation into three types or bands — UVA, UVB, and UVC.
The stratospheric ozone layer absorbs some but not aM of these types of UV:
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
UVA and especially UVB penetrate the surface of the skin and can cause the adverse
health effects listed above.
UV LEVELS DEPEND ON A NUMBER OF FACTORS -
STRATOSPHERIC OZONE
The ozone layer absorbs most of the sun's harmful UV rays, but varies depending on the time of
year and changing weather patterns. The ozone layer has thinned in certain areas due to emissions
of ozone-depleting chemicals widely used in industry.
TIME OF DAY
The sun is at its highest in the sky around the noon hour. 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 obliquely through the atmosphere and the intensity of UVB
is greatly reduced. UVA levels are not sensitive to ozone and vary throughout the day much like
visible sunlight does.
TIME OF YEAR
The sun's angle varies with the seasons, causing the intensity of UV rays to vary. UV intensity tends
to be highest during the summer months.
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ATITUDE
The sun's rays are strongest at the equator where the sun is most directly over head
and UV rays must travel the least distance through the atmosphere. Ozone is also nat-
Virally thinner in the tropics compared to the mid- and high-latitudes, so there is less
o'loW to absorb the UV radiation as it passes through the atmosphere. At higher lati-
tudes 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.
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 summer day even if it doesn'tieel
very warm.
ABOUT THE UV INDEX
™«" ' '"l|1111 The UV Index, developed by the National Weather Service and the Environmental
Protection Agency, provides a forecast of the expected risk of overexposure to the sun
£tid indicates the degree of caution you should take when working, playing, or exercising
outdoors. The UV Index predicts exposure levels on a 0 -10+ scale, where 0 indicates a
minimal risk of overexposure, and 10+ means a very high risk of overexposure. Calculated
^;,",QAa.nejcl-day_basis for dozens of cities across the U.S. by the National Weather Service,
mmif the UV Index takes into account clouds and other local conditions that affect the amount
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"" - - prevent ozone depletion, call EPA's Stratospheric Ozone Hotline at (800) 296-1996.
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THE STRATOSPHERIC OZONE LAYER SCREENS OUT
MUCH OF THE SUN'S HARMFUL UV RADIATION
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