Tim Schrunk, age 12, of Sioux City, Iowa, for his question:

HOW MUCH OZONE IS IN THE AIR?

Ozone is a form of oxygen. Ozone molecules have two atoms of oxygen tightly joined together with a third oxygen atom loosely attached.

The third atom can easily separate from the molecule and combine with other substances. As a result, ozone is a chemically active gas. Every flash of lightning converts some oxygen into ozone. It is also formed by electric sparks.

On the average, air at ground level contains less than one part of ozone per million parts of air. But 15 miles above the ground there is a more concentrated layer of ozone. Here there are about six parts of ozone per million parts of air.

The ozone layer above the earth shields the earth from much of the sun's ultraviolet light. Without this protective layer, many plants and animals probably could not live on earth.