Oxygen, vital to breathing creatures, constitutes about one-fifth of
the earth's atmosphere. It occasionally occurs as a single atom in the
atmosphere at high temperature, but it usually combines with a second
oxygen atom to form molecular oxygen (O2). The oxygen in the air we
breathe consists primarily of this stable form.
Oxygen has also a third chemical form in which three oxygen atoms are
bound together in a single molecule (03), called ozone. Though less
stable and far more rare than O2, and principally confined to upper
levels of the stratosphere, both molecular oxygen and ozone play a
vital role in shielding the earth from harmful components of solar
radiation.
Most harmful radiation is in the "ultraviolet" region of the solar
spectrum, invisible to the eye at short wavelengths (under 3,000 A).
(An angstrom unit--A--is an exceedingly short unit of length--10
billionths of 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.