Nucleic Acids in Chemistry and Biology

8.8.4 DNA and the Ozone Barrier

Life is shielded from DNA-damaging solar radiation by the ozone layer.77,78The amount of ozone involved
is very small indeed. It is formed photochemically in the stratosphere by UVC radiation and is distributed
throughout the atmosphere. Its maximum density is at some 20–25km above the earth’s surface, where its
pressure is about 130nbar. Were all the ozone to be concentrated at sea level it would form a layer only
3mm thick!
The absorption spectrum of ozone is very similar to that of DNA (Figure 8.34), so it generally serves to
prevent short-wavelength UVC radiation from reaching the earth’s surface and causing DNA damage. To
put that in perspective, it has been estimated that a 1-h exposure of DNA to ozone-filtered sunlight at sea
level would generate about seven cis-syn thymine dimers per 1000 reactive sites. Were the ozone barrier
level to be halved, the time would be reduced to 10min. In the complete absence of ozone, the same DNA
damage would occur in 10s. Other calculations have estimated that for every 1% decrease in the ozone
column there could be a 4% increase in the incidence of skin cancer.
One of the major causes for concern is the depletion of ozone resulting from the photochemical behav-
iour of chlorofluorocarbons (CFCs)in the upper atmosphere.58,78This interferes with the formation of
ozone through the capture of oxygen atoms by chlorine atoms, as shown by the following equations.

Ozone formation O 2 h →O•O•
O•O 2 N 2 →O 3 N 2
Ozone depletion Cl•O 2 →ClO
ClOO 3 →Cl•O 2 O 2
A more cautious view is that the two dominant variables that determine the UV incidence at ground
level integrated over 24h are (i) the elevation of the sun above the horizon and (ii) the duration of daylight.
These two factors, which ultimately relate to the earth’s solar orbit, lead to variations that far exceed the
changes predicted to occur at any middle latitude location as a consequence of any decline in the column
of ozone since 1970. The situation over the Antarctic is, however, a special case. The large depletion in
ozone over that area in springtime has resulted in UVB irradiances that are substantially larger than existed
in that part of the world before the 1980s.
In 2003, the Director General of WHO Lee-Jong Wook said: “As ozone depletion becomes more marked
and as people around the world engage more in sun-seeking behaviour, the risk of health complications
from overexposure to ultraviolet radiation is becoming a substantial health concern.”^79 The International

Covalent Interactions of Nucleic Acids with Small Molecules and Their Repair 321

240 280

Wavelength (nm)

320 360

UVCUVBUVA

Relative absorption or intensity

O 3

DNA

solar

sea-level

(6-4)

Dewar

T<>T

cys-syn

Figure 8.34 Absorption spectra for DNA and ozone (red) and the various photoproducts (broken lines) compared to
the incidence of sunlight at sea level and above the atmosphere