You will need to understand Newton's Laws of motion. At the low levels, ozone is harmful to humans because it inhibits the functions of the lungs and is a primary component of smog. This cuts down on the amount of ground level ozone that is produced. The other location for ozone, in the stratosphere, is very important for human existance. These rays would be very harmful to humans, animals, and plants if they were allowed to penetrate to the surface.
Skin cancer is one of the results of humans being exposed to uv radiation.
There has been much concern and discussion about a depletion of ozone in the ozone layer in the Southern Hemisphere. As a result, CFC's have become banned in many countries.