a centimeter, or about 4 billionths of an inch.) Unlike X-rays, ultraviolet
photons are not "hard" enough to ionize atoms, but pack enough energy to
break down the chemical bonds of molecules in living cells and produce a
variety of biological and genetic abnormalities, including tumors and
cancers.

Fortunately, because of the earth's atmosphere, only a trace of this
dangerous ultraviolet radiation actually reaches the earth. By the time
sunlight reaches the top of the stratosphere, at about 30 miles altitude,
almost all the radiation shorter than 1,900 A has been absorbed by
molecules of nitrogen and oxygen. Within the stratosphere itself,
molecular oxygen (02) absorbs the longer wavelengths of ultraviolet, up to
2,420 A; and ozone (O3) is formed as a result of this absorption process.
It is this ozone then which absorbs almost all of the remaining ultraviolet
wavelengths up to about 3,000 A, so that almost all of the dangerous solar
radiation is cut off before it reaches the earth's surface.