Microbes

 
Microorganisms such as fungi and viruses may play a role in the development of asthma and allergies involving the airways. This is for instance a problem in damp buildings or indoor environments where there is mold because many fungi release substances that cause allergies. Virus infections may also be transmitted by indoor air and some of them can lead to an increase in asthma and allergies.
(GreenFacts- Facts on Health and the Environment)


What are the microorganisms found in air?
The organisms, which can be predominantly present in air, are spores of Bacillus spp., Clostridium spp., molds and some Gram-positive (e.g., Micrococcus spp., and Sarcina spp.) species as well as yeasts.
https://www.sciencedirect.com/topics/immunology-and-microbiology/airborne-microorganism


Which bacteria is transmitted in the air?
When diseases and viruses are transmitted as very small particles by air currents, it is called airborne germ transmission. Airborne infections spread when bacteria or viruses travel on dust particles or small respiratory droplets that become aerosolized when an infected person sneezes or coughs.


What are bacteria and viruses?
Bacteria and viruses are living organisms that cause diseases, like the common cold or influenza. They also can make some diseases, like asthma, worse.
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How can airborne viruses, bacteria affect health?
Bacteria and viruses can travel through the air, causing and worsening diseases. They get into the air easily. When someone sneezes or coughs, tiny water or mucous droplets filled with viruses or bacteria scatter in the air or end up in the hands where they spread on surfaces like doorknobs. Inhaling these viruses or bacteria can spread coughs, colds, influenza, tuberculosis and other infectious agents. 
Crowded conditions with poor air circulation can promote this spread. Some bacteria and viruses thrive and circulate through poorly maintained building ventilation systems, as with Legionnaires' disease. Damp, humid air can increase the survival rate of viruses indoors. 
In addition, some individuals with allergies react to endotoxins, substances that come from the broken-down cells of dead bacteria. These microscopic particles have been associated with coughing, wheezing and worsening asthma. Even so, some studies have linked them to protecting against some health threats. 
Bacteria in the soil produce endotoxins, so they are virtually everywhere outdoors. They can come indoors with pets, pests, humidifiers, kitchen compost bins and outdoor air. Walking, dry mopping and other activities can cause them to become airborne once inside.
Reducing the spread of bacteria, viruses. Most often, the human occupants of a home or workplace are the source of infectious diseases. A key step to reducing the spread of disease through indoor air is to practice healthy behavior. Precautions, like coughing or sneezing into the bend of your elbow, can curb the spread of airborne viruses and bacteria. Washing your hands and common surfaces with soap and water can keep them from spreading on indoor surfaces.
Effective ventilation may also help keep bacteria, viruses and other pollutants out of the indoor air. Research shows that air flow and ventilation can affect how diseases spread indoors. The more stagnant the air is, the more likely diseases are to spread. 
Ventilation can also limit moisture. Damp indoor spaces foster the growth and transmission of viruses and bacteria. Controlling moisture indoors can limit the spread of these infectious diseases and also limit mold, dust mite and cockroach growth. 
https://www.lung.org/our-initiatives/healthy-air/indoor/indoor-air-pollutants/bacteria-and-viruses.html


Bacteria in HVAC
Contaminated central air handling systems can become breeding grounds for mold, mildew, and other sources of biological contaminants and can then distribute these contaminants through the home.’ (The Inside Story-A Guide to Indoor Air Quality published April 1995 by the U. S. Environmental Protection Agency). HVAC system components can act as direct or indirect sources of particles and/or volatile organic chemicals (VOC’s). Of greatest concern is biological growth and bioaerosol generation, most of which are potential allergens. Airborne allergens in the home constitute one of the hardest types of allergens to avoid. Unfortunately, heating and air conditioning systems are excellent harbors for molds, and with vents in every room this same system is an efficient distribution system.
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