The oxygen molecules in the air that we breathe are made up of two oxygen atoms bound together. But up in our stratosphere, there are molecules of ozone, which are three oxygen atoms bound together. This layer of ozone filters out a lot of the sun’s UV-B rays, which are harmful to life on earth.
Some UV-B light reaches us, and it is actually helpful in converting cholesterol to vitamin D in our skin. (So a reasonable amount of sunlight is good for your body!) But it also causes suntans and sunburns.
If the ozone layer gets depleted, too many UV-B rays will reach the earth’s surface, causing health problems for animals (like more skin cancer) and harming plant life.
Molecules of ozone do break apart naturally, but they also reform naturally. However, the chemicals that people have made – chlorofluorocarbons and halons, for instance – cause a massive depletion of ozone. This happens because freed chlorine or freed bromine atoms steal an oxygen from ozone, leaving a plain O2 molecule. Then they release the oxygen atom they stole. Now both the freed chlorine/bromine and freed oxygen will steal oxygen atoms from ozone molecules. Our use of these chemicals really speeds up the process of ozone destruction. There is no way the natural reformation process can keep up with our chemical destruction.
The effects of ozone depletion extend beyond cancer and killing crops. It also may cause a greenhouse effect, gradually warming the earth to the point where our polar ice caps melt and flood the coastal cities. This is a very long-term effect, though. UV-B rays also seem to have a harmful effect on amphibian eggs, which may be one cause for the recent and drastic drop in frog populations. There are likely to be many other effects of ozone depletion which we will discover in years to come.