Ozone (O3), sometimes called "activated oxygen," contains three atoms of oxygen rather than the two atoms we normally breathe. Ozone is the second most powerful sterilant in the world and can be used to destroy bacteria, viruses and odors. Interestingly, ozone occurs quite readily in nature, most often as a result of lightning strikes that occur during thunderstorms. In fact, the fresh, clean, spring rain smell that we notice after a storm most often results from nature's creation of ozone. However, we are probably most familiar with ozone from reading about the "ozone layer" that circles the planet above the earth's atmosphere. Here ozone is created by the sun's ultraviolet rays. This serves to protect us from ultraviolet radiation.
Ozone can be either good or bad, depending on where it is found in the environment.
Bad ozone: You may have heard of ozone being in the Earth's lower atmosphere, near the surface level of the ground. One way that ozone is developed is when emissions by chemical plants, industrial boilers, power plants, vehicles and other sources react in the presence of sunlight. Thus, ozone at ground level is a pollutant, part of what is more commonly known as smog.
Good ozone: Approximately 6-30 miles above the Earth's surface, our Earth has a protective ozone layer that shields us from the sun's harmful ultraviolet rays. Manmade chemicals are known to destroy this beneficial ozone to us. Hence, in 1987, the U.S. along with over 180 countries adopted a treaty called the Montreal Protocol to phase out the production and use of substances that can deplete the ozone layer.