but includes others such as halons, methyl bromide, carbon tetrachloride, and methyl chlo-
roform. CFCs were once widely used because they are cheap, nontoxic, nonflammable, and
non-reactive. They were used as spray-can propellants, refrigerants, and in many other
products.
Once they are released into the air, CFCs float up to the stratosphere. Air currents move
them toward the poles. In the winter, they freeze onto nitric acid molecules in polar strato-
spheric clouds (PSC). PSCs form only where the stratosphere is coldest, and are most com-
mon above Antarctica in the wintertime. In the spring, the sun’s warmth starts the air
moving, and ultraviolet light breaks the CFCs apart. The chlorine atom floats away and
attaches to one of the oxygen atoms on an ozone molecule. The chlorine pulls the oxygen
atom away, leaving behind an O 2 molecule, which provides no UV protection. The chlorine
then releases the oxygen atom and moves on to destroy another ozone molecule. One CFC
molecule can destroy as many as 100,000 ozone molecules.
Ozone destruction creates the ozone holewhere the layer is dangerously thin (Figure
22.11). As air circulates over Antarctica in the spring, the ozone hole expands northward
over the southern continents, including Australia, New Zealand, southern South America,
and southern Africa. UV levels may rise as much as 20% beneath the ozone hole. The hole
was first measured in 1981 when it was 2 million square km (900,000 square miles)). The
2006 hole was the largest ever observed at 28 million square km (11.4 million square miles).
It had the lowest ozone levels ever recorded and also lasted the longest. The difference in
the size of the ozone hole each year depends on many factors, including whether conditions
are right for the formation of polar stratospheric clouds.
Ozone loss also occurs over the north polar region, but it is not enough for scientists to call
it a hole. The region of low ozone levels is small because the atmosphere is not as cold and
PSCs do not form as readily. Still, springtime ozone levels are relatively low. This low moves
south over some of the world’s most populated areas in Europe, North America, and Asia.
At 40°N, the latitude of New York City, UV-B has increased about 4% per decade since
- At 55°N, the approximate latitude of Moscow and Copenhagen, the increase has been
6.8% per decade since 1978.
Ozone losses in population centers increase sunburns, cataracts (clouding of the lens of the
eye), and skin cancers. A loss of ozone of only 1% is estimated to increase skin cancer cases
by 5 to 6%. People may also suffer from decreases in their immune system’s ability to fight
off infectious diseases. Ozone loss may reduce crop yields, since many plants are sensitive
to ultraviolet light. Excess UV appears to be decreasing the productivity of plankton in the
oceans. A decrease of 6 to 12% has been measured around Antarctica, which may be at least
partly related to the ozone hole. The effects of excess UV on other organisms is not known.
When the problem with ozone depletion was recognized, world leaders took action. CFCs
were banned in spray cans in some nations in 1978. The greatest production of CFCs was
in 1986, but has declined since then. This will be discussed more in the next lesson.