The earth still seems pretty big to most people. It measures some 25,000 miles at its circumference and its atmosphere stretches out some 600 miles into space.

Aside from a few floating spores and bacteria, life exists only within the first five miles of earth’s six-hundred-mile atmosphere. Actually, the far greater number of air-breathing things​—humans, animals, birds and plant life—​live within just the first ten thousand feet above sea level.

WHY worry about the air? When we look into the sky, it seems without limit, does it not?

“It may seem that way. But, remember, the astronauts had to take their own air supply when they took off from the earth. When you take a jet plane, the cabin has to have its air level artificially maintained.

This tells us something. What? That there is no useful air supply a few miles off the earth. The air you can breathe is found only in a relatively narrow band directly above the earth. It contains the oxygen that is vital for all human and animal life. That narrow band of usable air is now in serious danger.

The Air’s Self-cleaning Process

True, our earth’s atmosphere has a wonderful self-cleaning system built into it. The air is like an ocean with tides and currents in the form of winds and shifting air masses. The smoke from a few wood fires, for example, is quickly dispersed and dissipated. Floating solid particles from the smoke in time are washed out of the air by rain and snow. What about the gases?

Our planet’s air itself is, of course, a mixture of gases. Nitrogen forms about 78 percent and oxygen about 21 percent, the rest being minute quantities of argon, carbon dioxide, helium, and so forth. Remarkable processes work to keep that mixture free from change.

As Time magazine says: “With uncanny precision, the mixture [is] maintained by plants, animals and bacteria,” which use and return the gases at equal rates. “The result is a closed system, a balanced cycle in which nothing is wasted and everything counts.”

The precision is, indeed, amazing. Carbon dioxide, for example, forms only about one part in every three thousand parts of air by volume. When men and animals breathe air they use the oxygen and exhale carbon dioxide. But plants do the opposite. They take in carbon dioxide and give off oxygen, maintaining the balance.

Lightning shoots through the air and causes nitrogen to form a compound that the raindrops carry down to the earth. There plants use it to grow. The plants, in turn, are eaten by animals or they die and decay. Bacteria acting on decaying plants and animal manure release nitrogen back into the air. The cycle is complete.

Some gases released naturally can be dangerous in sufficient quantity​—like the ozone you smell after a thunderstorm. But the air’s self-cleaning system takes care of them, often within a few hours or days. They are flushed out through rain and snow, by their being extracted from the air by vegetation, or simply by settling slowly to the earth.