A single atom can make a big difference in the way a molecule works. So can its location. The everyday air we breathe has two atoms of oxygen (O2) in it. Ozone is a gas made up of three atoms of oxygen (O3). We need oxygen to survive - but ozone can either help us or hurt us, depending on where it is.
Helpful ozone is high above the Earth’s surface and protects us from the sun’s ultraviolet rays. Harmful ozone is close to the Earth. It comes from human activities that release chemicals into the air, which react with sunlight to create ground-level ozone.
Human-made air pollutants create the harmful closer ozone and damage the helpful higher ozone. Either way, the result can be harmful to our health and damage crops and other plants.
At this Web site, find out what people all over the world are doing to prevent problems with ozone and how you can limit your exposure.
Ozone pollution near the ground is the most wide-spread air quality problem in the United States. - Texas Commission on Environmental Quality