Environmental Engineering Reference
In-Depth Information
UV RADIATION, DNA DAMAGE, MUTATIONS AND SKIN CANCER
FRANK R. DE GRUIJL
Department of Dermatology, Leiden University Medical Centre,
NL-2333 AL Leiden, The Netherlands.
1. Introduction
About UV radiation, ozone and life
More than blue light, UV radiation is scattered in the earth's atmosphere: going
from a wavelength (O) of 400 down to 340 nm, the scattering goes up almost 2 fold
(Rayleigh scattering vO
-4
). Although we do not see it, the sky is more violet-ultraviolet
than it is blue. Blocking the UV radiation from the direction of the solar disc - e.g. by a
small parasol - would still leave a substantial part of the UV radiation, i.e. the part
reaches us through scattering from the blue sky (and possibly by reflection from sand or
snow). UV radiation can initiate a great variety of photochemical reactions. We are all
familiar with examples such as “photochemical smog”, photodegradation of plastics and
bleaching of pigments in paints. Although high-energy UV radiation form the sun may
have contributed to the early synthesis of organic molecules in the primitive anoxic
atmosphere (wavelengths below 200 nm are absorbed by CH
4
, H
2
O and NH
3
), it needed
to be blocked out in early evolution because of its very detrimental effects on highly
evolved organic molecules such as proteins and DNA, essential building blocks of
living organisms. Most of the sun's harmful UV radiation could pass through the earth's
primordial anoxic atmosphere, and life could not sustain itself on the earth's bare
surface. Early life on earth must have evolved shielded from the UV radiation by
barriers, e.g., in oceans below a layer of dissolved organic compounds
1
or very deep
near hot vents and/or in the earth's crust, and/or a few millimeters deep in soil
2
. By a
very fortunate evolutionary event plant-like organisms started to produce oxygen from
photosynthesis in substantial amounts over 2 billion years ago, and this oxygen
provided protection against UV radiation from the sun.
Up in the stratosphere, oxygen (O
2
) is bombarded by high-energy UV radiation,
absorbs this radiation, splits up (in O) and recombines with oxygen to form ozone (O
3
).
This stratospheric layer of ozone is spread over a wide range in altitude (from 10 to
50 km), but it is extremely rarified and would amount to a sheath of only about 3 mm
thick if it were compressed at ground level conditions. This modest amount of ozone
shields off most of the harmful UV radiation in the UVB band that is not absorbed by
oxygen. Life on earth has thus generated its own “stratospheric sunscreen”. However,
the UVB radiation is only partially absorbed, and the fraction that reaches ground level
is still capable of damaging and killing unprotected cells. To cope with this residual UV
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