Aeromicrobiology is the study of living microbes which are suspended in the air. These microbes are referred to as bioaerosols.

Dispersal of disease causing microbes through air microbial population in air, their quality and quantity in air comes under the perview of this branch. Though there are significantly less atmospheric microorganisms than there are in oceans and in soil, there is still a large enough number that they can affect the atmosphere. Once suspended in the air column, these microbes have the opportunity to travel long distances with the help of wind and precipitation, increasing the occurrence of widespread disease by these microorganisms. These aerosols are ecologically significant because they can be associated with disease in humans, animals and plants. Typically microbes will be suspended in clouds, where they are able to perform processes that alter the chemical composition of the cloud, and may even induce precipitation

Physical Environment

There are many factors within the physical environment that affect the launching, transport and deposition of bioaerosols. Particles which become suspended in the air column arise mainly from terrestrial and aquatic environments and are typically launched by air turbulence . Winds are the primary means of transport for bioaerosols. Bioaerosols can be deposited by a number of mechanisms, including gravity pulling them down, making contact with surfaces, or combining with rain which pulls the particles back down to earth's surface

Atmosphere

Along with water droplets, dust particles and other matter, air contains microbes .  Microbes follow a particular pathway in which they are suspended into the atmosphere. First they are launched into the air. The source of the launching of airborne microbes stems from humans, animals and vegetation. then they are transported (by various methods including winds, machinery and people) and finally are deposited somewhere new. The atmosphere can have a variety of physical characteristics, and can be very extreme in terms of the relative humidity, temperature and radiation. These factors play a huge role in what kinds of microbes will survive in the atmosphere and how long they will stay alive.

Clouds

One area that bioaerosols can be found in is within clouds. Cloud water is a mixture of organic and inorganic compounds suspended within moisture (contribution of microbial activity of clouds). The conditions in clouds are not conducive to much life, as microbes present there must withstand freezing temperatures, the threat of desiccation, and extreme UV rays. Clouds are also an acidic environment, with a pH ranging from 3 to 7. Nevertheless, there are extremophile microbes which can withstand all of these environmental pressures. Clouds serve as a transport for these microbes, dispersing them over long distances

Physical Environment Stresses

The atmosphere is a difficult place for a microbe to survive. Dessication is the primary stress that aeromicrobes face, and it limits the amount of time that they can survive while suspended in the air . Humidity within the air is a second factor which can affect the survival of organisms. Certain bacteria, including Gram bacteria, are more tolerant of high humidity in the air, while others are more tolerant of dessication and dry conditions, such as Gram . Temperature must be in an intermediate range, as too hot of temperatures can denature proteins, and too cold of temperatures can cause ice crystal formation Finally, radiation poses a potential hazard for aeromicrobes, as it can damage DNA within the cells.

Microbial Communities

Many different microorganisms can be in aerosol form in the atmosphere, including viruses, bacteria, fungi, yeasts and protozoans. In order to survive in the atmosphere, it is important that these microbes adapt to some of the harsh climatic characteristics of the exterior world, including temperature, gasses and humidity. Many of the microbes that are capable of surviving harsh conditions can readily form endospores, which can withstand extreme conditions

Many of these microorganisms can be associated with specific and commonly known diseases.

Some important diseases of humans transmitted from person to person by inhaled airborne particles
Virus diseases
(virus type in brackets)
Bacterial diseases
(bacterial name in brackets)
Chickenpox (Varicella) Whooping cough (Bordetella pertussis)
Flu (Influenza) Meningitis (Neisseria species)
Measles (Rubeola) Diphtheria (Corynebacterium diphtheriae)
German measles (Rubella) Pneumonia (Mycoplasma pneumoniae, Streptococcus species)
Mumps (Mumps) Tuberculosis (Mycobacterium tuberculosis)
Smallpox (Variola)

Several other diseases, below, are acquired by inhaling particles from environmental sources, not directly from an infected person.
Disease Source
Psittacosis (Chlamydia psittaci) Dried, powdery droppings from infected birds (parrots, pigeons, etc.)
Legionnaire's disease (Legionella pneumophila) Droplets from air-conditioning systems, water storage tanks, etc., where the bacterium grows.
Acute allergic alveolitis (various fungal and actinomycete spores) Fungal or actinomycete spores from decomposing organic matter (composts, grain stores, hay, etc.)
Aspergillosis (Aspergillus fumigatus, A. flavus, A. niger) Fungal spores inhaled from decomposing organic matter
Histoplasmosis (Histoplasma capsulatum) Spores of the fungus, in old, weathered bat or bird droppings
Coccidioidomycosis (Coccidioides immitis) Spores in air-blown dust in desert regions (Central, South and North America) where the fungus grows in the soil

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Bacterial

One such bacterial microorganism that can resist environmental stresses is Bacillus anthracis. It is a gram positive rod shaped bacteria that utilizes spore formation to resist environmental stresses. The spore is a dehydrated cell with extremely thick cell walls which can remain inactive for many years. This spore makes Bacillus anthracis a highly resilient bacteria, allowing it can survive extreme temperatures, chemical contamination, and low nutrient environments). This bacteria is associated with Anthrax, which is a severe respiratory disease that infects humans.

Fungal

Another such microorganism that can resist environmental stresses is Aspergillus fumigatus, which is a major airborne fungal pathogen . This pathogen is capable of causing many human diseases when conidia are inhaled into the lungs. While A. fumigatus lacks virulence traits, it is very adaptable to changing environmental conditions and therefore is still capable of mass infection.

Viral

An example of a viral airborne pathogen is the Avian Influenza Virus, which is a single stranded RNA virus that can infect a broad range of animal species as well as humans and cause the Avian Influenza.

Microbial Processes

The microbes undergo the emission process, in which they are emitted from surfaces such as water, soil or vegetation and become airborne and transported into the airstream. The red boxes indicate some of the harsh environmental conditions that the microbes must withstand while airborne. The microbes that are able to withstand and survive these environmental pressures are the more resistant varieties. The microbes make it into clouds, where they can begin the breakdown of organic compounds. Finally, the microbes are "rained" out of the clouds through wet deposition, and they begin colonization of their new location

Droplet Formation

The emission process mentioned above, in which microbes are lifted in the air often involves microbes being suspended in droplets, which are large enough to keep the microbes hydrated and large enough to maintain a virulent amount of pathogen, but are still small enough to stay suspended in the air.

Psittacosis is a serious disease acquired by handling birds or by inhaling dust from bird faeces. It is caused by the bacterium Chlamydia psittaci, an obligate intracellular parasite. After entering the respiratory tract, the cells are transported to the liver and spleen, multiply there and then invade the lungs, causing inflammation, haemorrhage and pneumonia.

Legionnaire's disease is a fairly common form of pneumonia in older or immunocompromised people. It is seldom transmitted directly from person to person. The bacterium is an aquatic rod-shaped species with a temperature optimum of about 36oC, and is a common inhabitant of warm-water systems in buildings. Infection occurs when people inhale aerosol droplets containing the bacteria.

Extrinsic allergic alveolitis is a serious hypersensitive response, usually associated with repeated exposure to airborne spores in the work environment. A classic example is the condition termed farmer's lung, caused by exposure to spores of thermophilic actinomycetes.

AspergillosisHistoplasmosis and Coccidioidomycosis are examples of serious fungal infections of humans, initiated by spores deposited in the alveoli. They can be life-threatening diseases of immunocompromised people, when the fungi disseminate from the lungs to major organs of the body. However, in all cases the infection of humans is incidental to the fungus, playing no part in its normal biology. These are fungi that grow naturally as decomposer organisms in soil, bird faeces or other organic substrates.

-Microbe Wiki