Ultraviolet
Radiation
All
energies that move at the speed of light are collectivelly referred
to as electromagnetic radiation or 'light'. Various types of light
differ in their wavelength, frequency and energy; higher energy
waves have higher frequencies and shorter wavelengths. Pigments
inside the retina of our eyes absorb wavelengths of light between
400nm-700nm, collectively referred to as 'visible light'. A "nm''
is a nanometer which is one billionth, or 10e-9, meters.
Stratospheric Oxygen and Ozone molecules absorb 97-99% of the sun's
high freguency Ultraviolet light, light with wavelengths between
150 and 300nm. Ultraviolet-B(UV-B) is a section of the UV spectrum,
with wavelengths between 270 and 320nm.
The amount of UV-B light recieved by a location is strongly dependent
on:
-
latitude and elevation of the location. At the high-latitude
polar regions the sun is always low in the sky; because the sunlight
passes through more atmosphere so more of ithe UV-B is absorbed.
For this reason average UV-B exposure at the poles is over a thousand
times lower than at the equator.
- cloud
cover ; the reduction in UV-B exposure depends the cover's
thickness.
- proximity
to an industrial area because of the protection offered
by photochemical smog. Industrial processes produce ozone, one
of the more irritaiting components of smog, which aborbs UV-B.
This is thought to be one of the main reasons that significant
ozone losses in the southern hemisphere have not been mirrored
in the northern hemisphere.
Health
effects of UV-B light
Genetic
damage DNA absorbs UV-B light and the absorbed energy can
break bonds in the DNA. Most of the DNA breakages are repaired by
proteins present in the cells nucleus but unrepaired genetic
damage of the DNA can lead to skin cancers. In fact one method
that scientists use to analyze amounts of 'genetically-damaging UV-B
is to expose samples of DNA to the light and then count the number
of breaks in the DNA. For example J.Regan's work at the Florida Institute
of Technology used human DNA to find that genetically significant
doses of solar radiation could penetrate as far as 9 feet into non-turbulant
ocean water.
The
Cancer link The principle danger of skin cancer is to light-skinned
peoples. A 1%decrease in the ozone layer will cause a estimated
2%increase in UV-B irradiation; it is estimated that this will lead
to a 4%increase in basal carcinomas and 6%increase in squamous-cell
carcinomas.[Graedel&Crutzen]. 90% of the skin carcinomas are attributed
to UV-B exposure [Wayne] and the chemical mechanism by which it
causes skin cancer has been identified [Tevini]. The above named
carcinomas are relatively easy to treat, if detected in time, and
are rarely fatal. But the much more dangerous malignant melanoma
is not as well understood. There appears to be a correlation between
brief, high intensity exposures to UV and eventual appearance (as
long as 10-20yrs!) of melanoma. Twice as many deaths due to melanomas
are seen in the southern states of Texas and Florida, as in the
northern states of Wisconsin and Montana, but there could be many
other factors involved. One undisputed effect of long-term sun exposure
is the premature aging of the skin due to both UV-A, UV-B and UV-C.
Even careful tanning kills skin cells, damages DNA and causes permanent
changes in skin connective tissue which leads to wrinkle formation
in later life. There is no such thing as a safe tan.
Possible
eye damage can result from high doses of UV light,
particularly to the cornea which is a good absorber of UV light.
High doses of UV light can causes a temporary clouding of the cornea,
called 'snow-blindness', and chronic doses has been tenitively linked
to the formation of cataracts. Higher incidences of cataracts are
found at high elevations,Tibet and Bolivia; and higher incidences
are seen at lower latitudes(approaching the equator).
Damage to marine life The penetration of increased
amounts of UV-B light has caused great concern over the health of
marine plankton that densly populate the
top 2 meters of ocean water. The natural protective-responce of
most chlorophyll containing cells to increased light-radiation is
to produce more light-absorbing pigments but this protective responce
is not triggered by UV-B light. Another possible responce of plankton
is to sink deeper into the water but this reduces the amount of
visible light they need for photosynthesis, and thereby reduces
their growth and reproduction rate. In other words, the amount of
food and oxygen produced by plankton could be reduced by UV exposure
without killing individual organisms. There are several other considerations:
- Ultraviolet
levels are over 1,000 times higher at the equator than at the
polar regions so it is presumed that marine life at the equator
is much better adapted to the higher enviromental UV light than
organisms in the polar regions. The current concern of marine
biologists is mostly over the more sensitive antarctic phytoplankton
which normally would recieve very low doses of UV. Only one large-scale
field survey of Anarctic phytoplankton has been carried out so
far [Smith et.al _Science_1992] ; they found a 6-12% drop in phytoplankton
productivity once their ship entered the area of the spring-time
ozone hole. Since the hole only lasts from 10-12weeks this translates
into a 2-4%loss overall, a measurable but not yet catastrophic
loss.
- Both
plants and phytoplankton vary widely in their sensitivity to UV-B.
When over 200 agricultural plants were tested, more than half
showed sensitivity to UV-B light. Other plants showed neglible
effects or even a small increase in vigor. Even within a species
there were marked differences; for example one variety of soybean
showed a 16% decrease in growth while another variety of the same
soybean showed no effect [R.Parson]. An increase in UV-B could
cause a shift in population rather than a large die-off of plants
- An
increase in UV-B will cause increased amounts of Ozone to be produced
at lower levels in the atmosphere. While some have hailed the
protection offered by this 'pollution-sheild' many plants have
shown themselves to be very sensitive to photochemical smog.
References:
- R.Parson
FAQ 111 ,UV and biological effects of UV
- FDA
Consumer Magazine and publications: FDA#87-8272, #81-8149 and
#92-1146
- M.Tevini,
ed. UV-B Radiation and Ozone Depletion: Effects on humans,
animals, plants, microorganisms and materials Lewis Pub.
Boca Raton, 1993.
- R.P.Wayne,
Chemistry of the Atmospheres 2nd ed. Oxford 1991
- R.Smith
et al. "Ozone depletion: Ultraviolet radiation and phytoplankton
biolgy in antarctic waters"' Science , 255, 952.
(1992)
Author: Brien Sparling
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| Curator:
Jill Dunbar |
Last Update:
May 30, 2001
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| NASA Official: Walt Brooks |
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