This is based on New Zealand Qualification Authority Unit 12460  entitled "Demonstrate knowledge of the microbiology of air."
1.1 and 1.2
Microbes normally found in atmosphere within 300-10000 feet above from the land. Fungal spores which are found in air consist of Alternaria, Cladosporium, Penecillium and Aspergillus found above 4000 feet from the land, found in both polar and non polar air masses. Organisms found below 500 feet is mainly in overpopulated area, these include spores of Bacillus and Clostridium, ascospores of yeast and fragments of mycelium, mould, streptomycetaceae, pollen, protozoan cysts, algae, Micrococcus and corynebacterium. Air found in school and hospital or living places of the person suffered from infectious disease usually found microbes like tubercle bacilli, streptococci and pneumococci.
The characteristics of atmosphere as a habitat include extreme temperature variations, light, temperature, low amount of available water and organic water. All these characters make the atmosphere unsuitable for growth of micro organisms. Usually most of the organisms are found in the lower region of atmosphere. The air in the atmosphere is often exposed to sunlight thus it contains less moisture and higher temperature. Thus if the micro organisms are not protected from desiccation, almost most of the organisms will die.
The origin of the micro organisms takes place through various ways. Soil is one of the source to transfer micro organisms to the air. Whenever the wind blows it disturbs the micro organisms and liberate them into the air and these micro organisms remains suspended in the air for long time. Another way of transferring micro organisms to the air is by manmade actions like plugging and digging. Organisms can also be released in the form of water droplets or aerosols which are produced by wind or tidal actions. Micro organisms from plant and animal surfaces are also transferred by air currents. But the main source of micro organisms is human beings. These are discharged through human activities like coughing, sneezing, laughing and even talking. This article gives a description about the origin of micro organisms in air
This article gives a range of how microorganisms can be dispersed into the air and the different places they can come from.Sources of Microorganisms in Air
In this site, some important respiratory diseases and the role of airborne spores in crop disease caused by airborne microbes, and methods for monitoring spores in the air.
Microorganisms in the air are evaluated using tow different air samplers and plate counting.
[Power Point presentation - Microbiology of Air]
Air micro flora significance in human health: Human being inhales air every moment. Even most of the micro organisms present in air are harmless but still less than 1% of the airborne bacteria is pathogens. Outdoor air mostly does not contain disease causing pathogen whereas indoor air has more chances of infections especially in large gatherings like theaters and schools.
Food processing: Micro organisms transferred through various methods in air and are settled on various materials are involved in various fermentation products like alcoholic beverages, vinegar, dairy products etc. In Industrial processes which requires growth of any particular organisms, getting supply of sterile air free from contamination is a problem..Food products are either animal or plant origin. Some bacteria cause food poison. Clostridia cause toxins in meat and meat products. Staphylococci produce infection in fish products. Salmonella cause infection through milk, eggs and salads. All of these are happened only when the unhygenic handling of food stuffs. Microbial biofilms also act as a significant one, it appears in food and food contact surface, if micro organisms are not removed from the surface it create biofilms will be hazard to consumers. The microbiology of air is significant in many places such as hospitals, food processing, air conditioning and many other places. Air Microflora Significance in Hospitals and Source, significance, and control of indoor microbial aerosols: human health aspects.both give examples of the significance.
The bio-aerosol in hospital indoor air is highly influenced by the number of occupants, their activity and the ventilation. Since exposure levels are high, this may be an issue in the immunocompromised patients.
Significance of microbes in public health:Micro organisms produce a lot of disease, especially in hospitals nosocomial pathogens cause nosocomial pneumonia. MRSA (methicillin resistant Staphylococcus aureaus) and Gentamicin resistant gram negative bacteria are found to be serious in nowadays ,it transmitted either by contact or by airborne spread 
In air conditioning and metal systems: Bacteria like Cladosporium have enzymatic activity capable of metabolizing diesel, jet fuels, creosote and hydrocarbons, it produce a noxious smell that impact the quality of indoor air.
This is a roughly done video which accurately describes two methods of sampling air for micro-organisms Air sampling