Ozone is a form of oxygen.
It generally forms when oxygen (O2) from
near Earth's surface drifts high up into the atmosphere. Ultraviolet
radiation from the sun hits the oxygen molecules.
Some molecules split apart, creating single
atoms.
These atoms sometimes link up with O2
molecules, forming a new molecule with three oxygen atoms. That's
ozone (O3).
Ozone can be helpful
or harmful, depending on where it's located. If it's high up,
it's good ozone. Ozone in the stratosphere
protects people, animals, and plants from harmful ultraviolet
rays.
In 1985 scientists found a large ozone hole in the stratosphere
over Antarctica.
Each year the hole became bigger, letting more ultraviolet light
through to the Earth's surface. This situation is called stratospheric
ozone depletion (SOD). Without the protection provided by
ozone, we face a number of health problems.
THE OTHER FACE OF OZONE
Ozone high in the stratosphere is good, but
ozone near the ground is not. Do you ever hear "ozone warnings"
on the weather report? Ozone can be very unhealthy.
Inhaling ozone can damage the respiratory
tract, which enables us to breathe. Ozone also stunts
plant growth. That hurts farmers, agriculture, and our economy.
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Troposphere: This thick, 
