Particles in the Atmosphere

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 frequentlyfound 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 exhaledagain – although some of this matter is deposited in the minuteair 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.

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 humidityin the air, many desiccate and die. Short exposureto 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.