Environmental Engineering Reference
In-Depth Information
sun and oxygen in the atmosphere. UV radiation splits the two-atom
oxygen molecule into separate atoms, a small percentage of which recom-
bines with available two-oxygen molecules to form the unstable and
short-lived three-oxygen molecule called ozone. The balance between the
rate of formation and the rate of decay of ozone results in a constant
percentage of ozone in the layer. The amount of ozone is small—only
about 1 molecule of ozone among 10 million molecules of normal oxygen.
One molecule in 10 million seems insignifi cant, but without it, life on
earth would probably cease to exist. The ozone molecule absorbs 93 to
99 percent of the destructive and lethal UV radiation from the sun; only
a few percent of the UV radiation emitted by the sun reaches the earth's
surface. Ultraviolet radiation damages the DNA in skin cells, and the
effect is cumulative over a person's lifetime. If not for the ozone layer,
humans would have greatly increased rates of skin cancer, damaged
immune systems, increased numbers of eye cataracts at earlier ages,
genetic mutations, and lesser amounts of edible crops.
The importance of the ozone layer is clearly seen by considering the
occurrence of skin cancer before the layer was damaged by chlorofl uo-
rocarbons (CFCs). The sun is the cause of at least 90 percent of skin
cancers, so in a normal ozone layer, we would expect this malady to be
more common at low latitudes, where the sun's intensity is higher, than
at high latitudes. This is evident in the United States, where skin cancer
is highly correlated with latitude (fi gure 10.3). It increases by 10 percent
for each three degrees of latitude. For example, in Montana and Wash-
ington, annual deaths from skin cancer are 1.1 to 1.2 per 100,000
people; in Florida and Texas, it is twice as high.
The thinner the ozone layer is, the more UV radiation we receive and
the greater is the likelihood of skin cancer. For each 1 percent loss of
ozone molecules, skin cancer is expected to increase about 4 percent. If
you lived near the South Pole, where ozone concentrations have decreased
75 percent, your skin cancer risk would increase by 300 percent.
Higher UV radiation also affects the reproduction and growth of the
phytoplankton (one-celled plants) that live and fl oat on the ocean surface
and are at the base of the entire food chain in the ocean. Their abundance
has declined over the past century. It is hard to imagine life in the sea
without plankton. Increased UV radiation also decreases the survival rate
of shrimp and fi sh larvae.
The Cause
The reason for the serious amount of loss of our protective
ozone layer is the production of artifi cial chemicals called chlorofl uoro-
