08 What is the relationship between UV and skin cancer?
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This article is from the Ozone Depletion: UV Radiation and its Effects FAQ, by Robert Parson rparson@spot.colorado.edu with numerous contributions by others.
08 What is the relationship between UV and skin cancer?
Most skin cancers fall into three classes, basal cell carcinomas.
squamous cell carcinomas, and melanomas. In the US there were
500,000 cases of the first, 100,000 of the second, and 27,600 of
the third in 1990. [Wayne] More than 90% of the skin carcinomas in
the US are attributed to UV-B exposure: their frequency varies
sharply with latitude, just as UV-B does. The mechanism by which UV-B
induces carcinomas has been identified - the pyrimidine bases
in the DNA molecule form dimers when they absorb UV-B radiation.
This causes transcription errors when the DNA replicates, giving
rise to genetic mutations.[Taylor] [Tevini] [Young et al.] [Leffell
and Brash]. Fortunately, nonmelanoma skin cancers are
relatively easy to treat if detected in time, and are rarely fatal.
Fair-skinned people of North European ancestry are particularly
susceptible; the highest rates in the world are found in Queensland,
a northerly province of Australia, where a population of largely
English and Irish extraction is exposed to very high natural UV
radiation levels.
[Madronich and de Gruijl] have estimated the expected increases in
nonmelanoma skin cancer due to ozone depletion over the period 1979-1992:
Lat. % ozone loss % increase in rate, % increase in rate,
1979-1992 basal cell carcinoma squamous cell carcinoma
55N 7.4 +-1.3 13.5 +-5.3 25.4 +-10.3
35N 4.8 +-1.4 8.6 +-4.0 16.0 +-7.6
15N 1.5 +-1.1 2.7 +-2.4 4.8 +-4.4
15S 1.9 +-1.3 3.6 +-2.6 6.5 +-4.8
35S 4.0 +-1.6 8.1 +-3.6 14.9 +-6.8
55S 9.0 +-1.5 20.4 +-7.4 39.3 +-15.1
Of course, the rates themselves are much smaller at high latitudes,
where the relative increases in rates are large. A more extensive
evaluation of the effect of ozone layer depletion upon skin cancer
rates can be found in [Slaper et al. 1996]. They estimate that if
no restrictions had been placed upon halocarbon emissions, the resulting
excess skin cancer cases in the U.S. due to ozone depletion would
total 1.5 million for the next century. With current restrictions
under the Montreal Protocol and subsequent Amendments, this number
falls to 8000. These estimates do not take expected changes in
lifestyle (i.e. people taking better care to reduce their exposure
to solar UV) into consideration.
Malignant melanoma is much more dangerous, but its connection with UV
exposure is not well understood. [van der Leun and de Gruijl] [Ley].
There seems to a correlation between melanomas and brief, intense
exposures to UV (long before the cancer appears.) Melanoma incidence
is correlated with latitude, with twice as many deaths (relative to
state population) in Florida or Texas as in Wisconsin or Montana, [Wayne]
but this correlation does not necessarily imply a causal
relationship. There is some evidence that UV-A, which is not absorbed
by ozone, may be involved. [Skolnick] [Setlow et al.] [Ley] There is
a good summary [De Gruijl 1995] in the electronic journal _Consequences_,
at http://www.gcrio.org/CONSEQUENCES/summer95/impacts.html
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