Even when the air seems to be completely clear, it is full of atmospheric particles - invisible solid and semisolid bits of matter, including dust, smoke, pollen, spores, bacteria and viruses.

Some atmospheric particles are so large that you will feel them if they strike you. However, particles this large rarely travel far before they fall to the ground.

Finer particles may be carried many miles before settling during a lull in the wind, while still tinier specks may remain suspended in the air indefinitely. The finest particles are jostled this way and that by moving air molecules and drift with the slightest currents. Only rain and snow can wash them out of the atmosphere.

These tiny particles are so small that scientists measure their dimensions in microns - a micron is about one 25-thousandth of an inch. They include pollen grains, whose diameters are sometimes less than 25 microns; bacteria, which range from about 2 to 30 microns across; individual virus particles, measuring a very small fraction of a micron; and carbon smoke particles, which may be as tiny as two hundredths of a micron.

Particles We Breathe

Particles are frequently found in concentrations of more than a million per cubic inch of air.

A human being's daily intake of air is about 450,000 cubic inches. This means that we inhale an astronomical numbers of foreign bodies.

Particles larger than about 5 microns are generally filtered from the air in the nasal passages. Other large particles are caught by hairlike protuberances in the air passages leading to the lungs and are swept back toward the mouth.

Most of the extremely fine particles that do reach the lungs are exhaled again - although some of this matter is deposited in the minute air sacs within the lungs.

From these air sacs, particles may go into solution and pass through the lung walls into the bloodstream.

If the material is toxic, harmful reactions may occur when it enters the blood.

Fine particles retained in the lungs can cause permanent tissue damage, as with Coalworkers' pneumoconiosis (black lung disease), caused by buildup of coal dust in the lungs, and with silicosis, which is caused by the buildup of silicon dust.

Dust

If the air is still, given sufficient time, all but the smallest airborne particles will settle to the ground under their own weight.

Their rate of fall is closely proportional to particle size and density.

For example, vast amounts of fine volcanic ash were thrown into the air by the eruption of the Indonesian volcano Krakatoa, in 1883, and again by the Alaskan volcano Katmai, in 1912. In both instances, the finer dust reached the stratosphere and spread around the world high above the rains and storms that tend to cleanse the lower atmosphere.

In fact, many years elapsed before these volcanic dusts entirely disappeared from the atmosphere.

Since a two-micron dust particle may require about four years to fall 17 miles in the atmosphere, the lingering effect is not in the least surprising.

Dust storms are also prolific producers of airborne debris. Europe is sometimes showered with dust originating in the Sahara.

In March 1901, for instance, an estimated total of two million tons of Sahara dust fell on North Africa and the Europe.

Two years later, in February 1903, Britain received a deposit estimated at ten million tons.

On many occasions, Sahara dust has fallen in muddy rain and reddish snow over much of southwestern Europe.

During North America's droughts of the 1930s, dust storms blew ten million tons of dust at a time aloft in the heart of the continent. Occasionally, high winds swept the dust eastward 1800 miles to darken skies along the continent's Atlantic coast.

Salt

When the wind strikes the crest of an ocean wave, or a calm sea is agitated by rain or by air bubbles bursting at the surface, the finer droplets that enter the air quickly evaporate, leaving tiny salt crystals suspended in the air.

Winds carry these salt crystals over all the Earth

Normally, airborne salt particles from the sea are less than a micron in diameter. It would take a million of them to weigh a pound.

Salt particles play an important part in weather processes because they are hygroscopic - they absorb water.

Raindrops usually form around tiny particles that act as nuclei for condensation. Generally, each fog and cloud droplet also collects around a particle of some type at its center.

Tiny crystals of sea salt make better condensation nuclei than other natural particles found in the air. Thus, salt particles in the air help make rain.

Meteor Dust

Dust from meteor showers may occasionally affect world rainfall.

When the Earth encounters a swarm of meteors, those meteors that get to the upper reaches of the Earth's atmosphere are vaporized by heat from friction.

The resulting debris is a fine smoke or powder.

This fine dust then floats down into the cloud system of the lower atmosphere, where it can readily serve as nuclei around which ice crystals or raindrops can form.

Increases in world rainfall come about a month after the Earth encounters meteor systems in space. The delay of a month allows sufficient time for the meteoric dust to fall through the upper atmosphere.

Occasionally, large meteors leave visible trains of dust. Most often their trails disappear rapidly, but in a few witnessed cases a wake of dust has remained visible for an hour or so.

In one extreme instance-a great meteor that broke up in the sky over Siberia in 1908-the dust cloud traveled all the way around the world before it dissipated.

Forest Fires

Large forest fires are among the more spectacular producers of foreign particles in the atmosphere.

Because these fires create violent updrafts, smoke particles are carried to great heights, and, being small, are spread over vast distances by high altitude winds.

In the autumn of 1950, forest fires in Alberta, Canada produced smoke that drifted east over North America on the prevailing wind and crossed the North Atlantic, reaching Britain and continental Europe.

The light-scattering properties of this dense smoke made the Sun look indigo and the Moon blue to observers in Scotland and other northern lands.

Pollen

Wind-pollinated plants are the most prolific sources of foreign particles in the air. This is a problem for people with allergies.

Spores

Spores are closely related to pollens. Spores are the reproductive bodies of fungi, which include molds, yeasts, rusts, mildews, puffballs and mushrooms. Tiny spores are adrift everywhere in the air, even over the oceans.

Although they resemble pollens in general appearance, spores are not fertilizing agents. Instead, they are like seeds, and give rise to new organisms wherever they take hold.

Spores have been found as high as 14 miles in the air over the entire globe.

Most fungi depend on the wind for spore dissemination.

Once airborne, spores are carried easily by the slightest air currents.

Microorganisms

Once, physicians were taught that infectious microorganisms quickly settle out of the air and die. Today, the droplets ejected, say, by a sneeze, are known to evaporate almost immediately, leaving whatever microorganisms they contain to drift through the air.

Only a relatively small fraction of the microorganisms that human beings breathe cause disease. In fact, most bacteria are actually helpful. Some, for example, convert atmospheric nitrogen into usable plant food.

Pathogenic, or disease-producing, microorganisms, however, can be very dangerous. Most propagate by subdivision-each living cell splits into two cells. Each of the new cells then grows and divides again into two more cells. Provided with ideal conditions, populations multiply quickly.

Fortunately microorganisms do not thrive very well in the air. Unless there is enough humidity in the air, many desiccate and die.

Short exposure to the ultraviolet radiation of the Sun also kills most microorganisms. Low temperatures greatly decrease their activity, and elevated temperatures destroy them rapidly.
Still, many microorganisms survive in the air, despite these hazards.

Among the tiniest of airborne particles are viruses, which are on the borderline between living matter and lifeless chemical substances.