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Distribution of Microbes in Air

	Distribution of Microbes in Air - No microbes are indigenous to the atmosphere rather they represent allochthonous populations transported from aquatic and terrestrial habitats into the atmosphere. Microbes 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, straw 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 exposures. More microbes are found in air over land masses than far at sea. Spores of fungi, especially Alternaria, Cladosporium, Penicillium and Aspergillus are more numerous 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 Bacillus and Clostridium, ascospores of yeasts, fragments of myceilium and spores of molds and streptomycetaceae, pollen, protozoan cysts, algae, Micrococcus, Corynebacterium etc. In the dust and air of schools and hospital wards or the rooms of persons suffering from infectious diseases, microbes such as tubercle bacilli, streptococci, pneumococci and staphylococci 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 tract are also transmitted by dust and air. Pathogens in dust are primarily derived from the objects contaminated with infectious secretions that after drying become infectious dust. Droplets 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 pathogens which cause important airborne diseases such as diphtheria, meningitis, pneumonia, tuberculosis and whooping cough are described in the chapter "Bacterial Diseases of Man".

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