Since the air does not, under normal conditions, contains the nutrients and moisture for growth, maintenance and multiplication of microorganisms, it could not be considered their natural environment. Nevertheless, air normally abounds in their numbers as microorganisms gain entry into it from soil and other dry decomposed material including excrete exposed to the action of wind. Air-borne microorganisms becomes an important source of contamination in laboratories, hospitals, industries, and of exposed food material and drinks. Depending upon the nature of microorganisms, some contaminations may cause spoilage of contaminated products and diseases when ingested. By mere sneeze and cough, infection from mouth and lungs may be discharged into the air around. In view of this, knowledge of quantity and quality of air microorganisms seems essential because we need pure air for respiration.
As stated earlier, air is not a medium for microorganisms but is a carrier of particulate matter, dust, and droplets which remain generally laden with microorganisms. These carrier transport microorganisms and the ultimate fate of such microorganisms is governed by a complex set of conditions such a sunlight, temperature, humidity, size of microbe laden particulates, degree of susceptibility or resistance of a particular microbe to the new physical environment, and the ability of microbe to form resistant spores or cysts.
In still air, the microorganisms tend to settle down quickly with their carriers leaving the air fairly free of them. The air after heavy rains is fairly free of microorganisms. Development even of slightest current can keep the microorganisms suspended in air for protracted period of time. In general, air above the warmer regions of earth harbours greater number of microorganisms