Ozone is one of the naturally occurring trace gases that make up our atmosphere.
The atmosphere serves three critical functions: it provides life-giving oxygen, keeps the earth warm, and protects us from deadly ultraviolet (UV) radiation from the sun. Most of the atmosphere consists of nitrogen and oxygen, the air we breathe. These gases do not hold heat so they do not keep us warm. They also do not protect the earth from UV rays.
For those functions you have to turn to the trace gases found in the atmosphere, commonly referred to as greenhouse gases. They are: water vapor, carbon dioxide, methane, ozone, and nitrous oxide.
These trace gases function like the transparent covering of a greenhouse, allowing sunlight to filter through to the earth's surface, then trapping the heat. Without the greenhouse effect the earth's temperature would plummet far below zero each night.
Ozone is a particularly critical trace gas because it plays two roles. In the lower atmosphere it adds to the greenhouse gases, keeping the earth warm. But it serves a more critical function in the upper atmosphere where it blocks nearly all of the sun's deadly UV rays from reaching the earth.
UV rays are associated with skin cancer. The "UV index" is used in summer months to let people know how long it is safe to stay in the sun. A decrease in the ozone correlates to an increase in skin cancer. This is important because the ozone has been in a steady rate of depletion, creating holes in the upper ozone layer.
The holes were first discovered in 1985 over Antarctica where atmospheric circulation, temperature, and other factors "draw" holes to that region. Discovery of the holes created worldwide concern.
The culprit was human-produced chlorofluorocarbons (CFC's). CFC's have been released into the atmosphere for years. They are emitted in part from aerosols made with CFC propellant, refrigeration units and air conditioners. As the CFC's reach the upper stratosphere UV rays cause the gas to release free chlorine atoms. It only takes a single chlorine molecule to cause tens of thousands of ozone molecules to break down into simple oxygen. And again, oxygen does not filter UV rays. The danger then, is that very small amounts of CFC gas destroy enormous amounts of ozone.
The move to ban CFC's was slow but all major countries producing them phased them out by the year 2000. The CFC's already released will take another estimated 50 years to break down, and CFC's will continue to be released by old products still in use. Consequently, ozone levels and the hole over Antarctica continue to be monitored closely.
In addition to being a naturally occurring gas, ozone is also created in the burning of fossil fuels as one component of smog. Burning fossil fuels also releases carbon dioxide into the air, thickening greenhouse gases, adding to the greenhouse effect and global warming.