The air contains gases, dust particles, dried vapour droplets, in addition to these, air also contains more number of microorganisms. The air has vegetative cells and spores of bacteria, fungi and algae and protozoan cyst. In the atmosphere, air mainly acts as dispersal or transport medium for microorganisms. When compared to soil or water, air contains less numbers of microorganisms. The microbiology of air can be studied under two headings such as outdoor and indoor microflora.
The air in the atmosphere, which is found outside the buildings, is referred to as outside air. The dominant microflora of outside air are fungi. The two common genera of fungi are Cladosporium and Sporobolomyces. Besides these two genera, other genera found in air are Aspergillus, Alternaria, Phytophthora and Erysiphe. The outdoor air also contains basidispores, ascopores of yeast, fragments of mycelium and conidia of molds. Among the bacterial genera Bacillus and Clostridium, Sarcina, Micrococcus, Corynebacterium and Achromobacter are widely found in the outside air. The number and kind of microorganisms may vary from place to place, depending upon the human population densities.
air found inside the building is referred to as Indoor air. The commonest genera of fungi in indoor air are Penicillium, Aspergillus. The commonest genera of bacteria found in indoor air are Staphylococci, Bacillus and Clostridium.
ENUMERATION AND ASSESSMENT OF MICROORGANISMS IN AIR
There are several methods adopted to enumerate microorganisms in air. They require special devices and design. The most important methods are solid and liquid impingement devices, filteration, sedimentation, centrifugation, electrostatic precipitation. However, none of these devices collects and counts all the microorganisms in the air samples. In general, to assess the microorganisms in air, following methods are adopted.
Impingement in liquids
In this method, the air drawn is through a very small openings or a capillary tube bubbled through the liquid. The organisms get trapped in the liquid medium. Aliquots of the liquid are then plated with medium to determine its microbial content. In this method, living cells which are present in air can be enumerated.
Impingement on solids
In this method, microorganisms are collected or impinged directly on the solid surface of agar medium by gravitational force. Colonies develop on the surface of the medium after few days of incubation. Several devices are used; of which settling plate technique is the simplest. In this method, cover of the petridish containing agar medium is removed, and the agar surface is exposed to the air sample for several minutes. A certain number of colonies develop on incubation of the petridish. Since the technique does not record the actual volume of sampled, it gives only approximate estimate of microorganisms. However, it gives information about the kind and number of organisms in a particular area.
Sources of contamination
The major sources of contamination in the air are, automobile exhausts like incomplete combustion of fuel, agricultural sources like spraying of pesticides and insecticides, hospitals, industries like tannery industries, distilleries, nuclear power plants and chemical industries. All these industries provide smoke into the air. The smoke comprises of dust particles from the combustion of coal and oil. Automobile engines driven by petrol, produce many different substances in its exhaust. Some of these substances are serious air pollutants. Air pollutants are Co, CO2, unburnt hydrocarbon NO2, SO2 and lead from leaded petrol.