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Protective Ozone
Most ozone is concentrated in the stratosphere, (27k jpeg) at
about 25 km in altitude, and is considered to be "good ozone." In this region, ozone acts as a shield to
protect Earth's surface by absorbing harmful ultraviolet radiation. Without this shield, we would be
more susceptible to skin cancer, cataracts, and impaired immune systems. A 1 % decrease in total
column ozone causes the amount of transmitted UV radiation, in the spectral region that damages
deoxyribonucleic acid (DNA), to increase by about 2 %. Although good ozone only represents a tiny
fraction of the atmosphere, it is crucial for life on Earth.
The proportion of good and bad ozone in the atmosphere depends on the balance between processes
that create ozone and those that destroy it. An upset in this balance can have serious consequences for
life on Earth, and scientists are finding evidence that the balance has changed. Concentrations within
the protective ozone shield are decreasing, while levels in the air we breathe are increasing.
Ozone amounts in the stratosphere are small, rarely exceeding 10 parts per million by volume. Ozone
is measured in Dobson Units. (40k jpeg) One Dobson Unit (DU)
corresponds to 2.69 x 10E16 molecules per square centimeter, which is equivalent to the amount of
gas in one square centimeter at 1 atmosphere of pressure. Average ozone levels are 300 DU, which
would be equivalent to a layer three millimeters thick if compressed to the planet's surface. Levels may
range from less than 100 DU to over 500 DU globally.
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next: Ozone Production and Destruction
menu: Ozone and the Atmosphere
Goddard DAAC Helpdesk: 301-286-3209 or 1-800-257-6151 -- daacuso@daac.gsfc.nasa.gov
Web Curator: Daniel Ziskin -- ziskin@daac.gsfc.nasa.gov
NASA official: Paul Chan, GDAAC Manager -- chan@daac.gsfc.nasa.gov
Last Revision Date: January 8, 1996