1.0                                                       INTRODUCTION
Gases, dust particles, water vapour and air contain microorganisms. There are vegetable cells and spores of bacteria, fungi and algae, viruses and protozoa cysts. Since air is often exposed to sunlight, it has a higher temperature and less moisture. So, if not protected from desiccation. Most of these microbial forms will die. Air serves as transport or dispersal medium for microorganism they occur in relatively small number in air when compared with soil or water. The microflora of air can be studied under two headings outdoor and indoor microflora.
Outdoor 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 cladosporiul and sporobolomyces, besides this two general, under general found in air are Aspergillus, Alternaria, Phytophthora and Erysiphe. The outdoor air also contains besidispores, ascopres of yeast, fragments of mycelium and canidia of molds. Among the bacterial genera Bacillus and clostridium, sarcina, mirococcus, corynebacterium and Achromobacter are widely found in the outside air, the number and kind of microorganism may very from place to place, depending upon the human population densities.

Indoor Microflora: - The 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. In case of occupants being infected, the composition shows slight variations with latitude and to a lesser extent with attitude. The ozone owes its existence in the atmosphere to photosynthesis from oxygen under the influence of solar ultraviolet radiations. (Dr. Shiva, 2009).      
There is no microbes are native to the atmosphere rather they represent allochthonous populations transperted from aquatic and terrestrial habits into the atmosphere. Microbe of air within 300 – 1,000 or more feet of the Earth’s surface are the organisms of soil that have become attached to fragments of dried leaves, strain or dust particles, being blown away by the wind. Species vary greatly in their sensitivity to a given value of relative humidity, temperature and radiation exposure.
More microbes are found in air over land masses than far at sea. Spores of fungi especially Alterneria, Cladosporium, Penicillium and Aspergillus are more numberous than other forms over sea within about 400 miles of land in both polar and tropical air masses at all altitudes up to about 10,000 feet.
Microbes found in air over populated land areas below altitude of 500 feet in clear weather include spores of Becillus and Clostridium ascos-pores of yeasts, fragments of mycelium and spores of molds and streptomycetaceae, pollen pootozoan cysts, algae, micrococcus, corynebacterium etc.
In the dust and air of schools and hospital wards or the rooms of persons suffering from infectious disease, microbe such as tubercle bacilli, streptococci, pneumococci and staphylocci have been demonstrated. These respiratory bacteria are dispersed in air in the droplets of saliva and mucus produced by coughing, sneezing, talking and laughing.
Viruses of respiratory tract and some enteric are also transmitted from the objects contaminated with infectious secretions that after drying become infectiou dust. Droplet are usually formed by sneezing, coughing and talking. Each droplet consists of saliva and mucus and each may contain thousands of microbes. It has been estimated that the number of bacteria in a single sneeze may be between 10,000 and 100,000. Small droplets in a warm dry atmosphere are dry before they reach the floor and thus quickly become droplet nuclei.
Many plant pathogens are also transported from one field to another through air and the spread of many fungal diseases of plants can be predicted by measuring the concentration of airborne fungal spores. Human bacterial pathogen which cause important airborne disease such as diphtheria, meningitis, pneumonia, tuberculosis, and whcoping cough are described in the chapter “Bacterial Disease man”
Air does not have an indigenous and flora, though a number of micro-organism are present in the air. Air is not a natural environment for microorganisms as it doesn’t contain enough moisture and nutrients to support their growth and reproduction. Quite a number of sources have been studied in this connection and almost all of them have been found to be responsible for the air microflora. One of the most common sources of air microflora is the soil. Soil microorganisms when distributed by the wind blow librated into the air and remain suspended therefore along period of time. Man made actions like digging or ploughing the soil may be release soil born microbes into the air. Similarly microorganisms found in water may also be released into the air in the form of water droplets or aerosols, splashing of water by wind action a tidal action may also produce droplets or aerosols?  

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