Microbiology of Air: Sources of Micro-organisms in Air

  Air is the mixture of gases that makes up the Earth's atmosphere. It is a mixture of about
- 78% Nitrogen,
- 21% Oxygen,
- 0.9% Argon,
- 0.04% Carbon dioxide and very small amounts of other gases.
Air also contains the variable amount of water vapours.

  Air microbiology is concerned with the study of living microbes in the air. They are usually referred to as bioaerosols. Atmosphere is actually unsuitable for growth of microorganisms due to extreme temperature variations, solar radiations, lack of nutrients, low amount of available water etc.

  Therefore, the number of microorganisms in air is less than in the other natural environments. However, there is still a large enough number that they can affect the atmosphere.

Microbes in air have an opportunity to travel long distances with the help of wind. They are ecologically significant because they can be associated with disease in humans, animals and plants.

  Most of the organisms are found in the lower region of atmosphere. Atmosphere is usually occupied by those forms of microorganisms that are resistant to adverse conditions in the air. They can be bacterial spores or cysts, capsulated bacteria, fungal spores, enveloped viruses etc.

Sources of Micro-organisms in Air

  • Entry of microorganisms in air takes place through various ways.   
  • Natural sources such as soil, lakes, oceans, animals and humans.
  • Unnatural sources such as sewage treatment plants, animal rendering, fermentation processes and agricultural activities.
  • Soil is one of the source.
  • Wind blow disturbs soil surface and liberates the soil microorganisms into the air.
  • These microorganisms remain suspended in air for long time.
  • Manmade actions like plugging and digging.
  • Plant and animal surfaces.
  • Large water bodies like oceans and bays.
  • Water droplets or aerosols produced by wind or tidal actions at surface of these water bodies.
  •  Most droplets are produced from top 0.1 mm water surfaces called as 'micro layer'.
  •  It consists of many more micro-organisms than from deep layers.
  • Bubbles from surface add thousands of microbes in air.
  • Most significant source is the man himself. Organisms from oral, nasal, rectal passages of man and animals come in air.
  • Human activities like coughing, sneezing, laughing and even talking.
  • Such biological aerosoles may spread bacteria upto distance of about 15 feet.
  • Infected persons release pathogens.
  • Respiratory pathogens are drought resistant and can survive in air for long period.
  • Air in the hospital is a site where microbial load is always very high.
  • Many human infections are caused by microorganisms from atmosphere.
  • For example - Epidemics of Legionnaires disease (a severe form of pneumonia caused by bacterium Legionella) were due to aerosols generated from contaminated water of air conditioning equipments.
  • Aerosols formed by high speed drills in dental clinics add a large population of bacteria.
  • Various industrial, agricultural, municipal are responsible for the generation of bioareosols.
  • For example - Sprinkler irrigation of crops and forest land with effluent from sewage treatment plants, trickling filter beds in sewage treatment plants, slaughter houses, spray washing processes etc.

Infectious dust, Droplets & Droplet Nuclei

(i) Infectious dust:

  • Dust in air arises from sand, soil, ash, lint from bedding, clothing, carpets.
  • It usually contains all saprophytic organisms.
  • Dust loaded with saprophytic organisms is harmless for human. But if it is loaded with air borne pathogens, it is called as 'infectious dust.
  • Air borne pathogens are added to the dust by body secretions.
  • When secretions are dried , pathogens remain in dust and form infectious dust.
  • Nasal or throat secretions on the handkerchief eventually dry and leave some residual material.
  • During sneezing or coughing, large aerosol droplets get expelled and settle on floor or bed clothes.
  • Moisture content of these droplets evaporates leaving behind residue.
  • These residues are then disturbed during the handling of handkerchiefs, sweeping of floor, bed making This makes the dust infectious.
  • If it is inhaled by healthy persons, they may get infected.

(ii) Droplets:

  • Sneezing or coughing activity of respiratory tract infection persons expels (exhales) millions of micro droplets of saliva and mucus along with varying number of micro-organisms.
  •  Such particles are called as droplets. Thus, droplets contain thousands of living micro- organisms along with saliva and mucus.
  • Most of the droplets are larger in size (0.1 mm in diameter).
  •  Such droplets settle rapidly, within shorter distance from their source of origin before drying.
  • Even if such droplets are inhaled by a healthy person before their settling, they are immediately trapped in nasal baffles and in the nasopharynx material and are prevented from reaching upto lungs.
  • Thus, droplets are not that much dangerous as far as the spread of infection is concerned.

(iii) Droplet Nuclei :

  •  In warm and dry atmosphere, small droplets are dried due to evaporation of moisture content leaving behind dried mucous with live bacteria.
  • They are called as 'droplet nuclei'. They are very much small in size with diameter less than 0.1 mm.
  • Such droplet nuclei can remain in air for longer period of time (upto hours to days).
  •  The dried mucus protects the organisms present at the center.
  • They are also carried away to a longer distance.
  • If these are inhaled by health persons, they directly enter into the lungs, without capturing in mechanical traps of nasal baffles and mucus of nasopharynx.
  •  The organisms from droplet nuclei directly settle in the alveoli of lungs.
  • Chances of spread of infection via droplet nuclei increases when people are crowded together.
  • Frequency of air borne infections is more in winter when people prefer to live in crowd places.

   In modem days, the chances of spreading infection are increased because of modern urban areas like buses, aero planes, restaurants and bars which are always crowded.
 The diseases caused by Klebsiella pneumoniae, Streptococcus pneumoniae and Mycobacterium tuberculosis may spread through droplet nuclei.

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