The composition and viability of microbial ecology in Nature, are determined by the dynamic and close interactions with buildings they inhabit, people they inhabit with, and surrounding outdoor environment. The primary reservoir of microbes is air, water and surfaces.

Microbes are a biological and ecological important component of nature interaction, but little is known about their concentrations, sources, transformation, and the fate of microbes in the atmosphere, especially in Air Conditioning (HVAC) systems. A high concentration of these microbes can cause health disturbance characterized by reduced quality of Indoor air. Condensed water from cooling coils and water in drain pans enables the growth and proliferation of bacteria and fungi.

Microbial growth and exposure are caused by how buildings exchange air with the environment that surrounds them through infiltration and other forms of ventilation, including natural, mechanical (heating, ventilation and air conditioning [HVAC] systems) and also human activity.


Increased risk of infection in closed environments



Microorganisms distribution

In single-family residential buildings, there is control over sources of microorganisms as indoor microbiome tends to obey the action and inaction of occupants. Air conditioning systems here rarely incorporate and circulate outdoor air into indoor environment but instead recirculate indoor air.

High rise buildings often have mechanical outdoor air intake system that incorporates air which is then circulated by air conditioning systems indoors.

Due to the higher number of occupants in commercial and public buildings, there is a higher density of contact with surfaces through which microorganisms transfer, grow and proliferate. Presence of condensed water with microorganisms, rooftop air conditioning system components (air intakes and cooling tower), intentional outdoor air intake increase entry of outdoor air and subsequently, microorganisms. Cooling towers for air conditioning systems are recorded source of Legionnaires' disease outbreaks. Legionella pneumophila have been identified as the main microbial risk in cooling towers where they often grow and proliferate in the water evaporated from air conditioning systems. Cooling towers are located on rooftops and thus release L. pneumophila into atmospheric air taken in by air conditioning systems, leading to sickness in users and people living close to the source with higher risk occuring closer to the source.

Poor air conditioning system and poor maintenance arising from inadequate slopes in drain pan, frequently wet cooling coil and humidifiers, accumulated dust and dirt raises the concentration of bacterial and viral particles and may lead to microbiologic exposures and consequent illness.

According to research, the average number of viral particles were 4.7±2.5x107/litre, and 1.2±0.7x108/litre for air sample collected indoor and outdoor, respectively. Their geometric means and geometric standard deviations were 5.6±0.3 and 6.0±0.3 for indoor and outdoor air respectively.

The average number of bacterial particles were 5.4±2.6x106/litre and 8.4±4.4x107/litre of air samples collected from the indoor and outdoor environment. Their geometric means and geometric standard deviations were 5.7±0.2 and 5.9±0.2 for indoor and outdoor air respectively.

The virus-to-bacteria ratio (VBR) for indoor air sample is 0.9±0.1 and ranged between 0.7 and 1.1, indicating that more bacteria particles were typically present in indoor air. The VBR for the outdoor air sample is 1.4, indicating that there are approximately 40% more viruses than bacteria in outdoor air. All values are subjected to slight changes depending on location and time and also, the source strength and climatic conditions.



Other factors that affect the distribution of microorganisms in air and air conditioning systems

  1. Presence of virus-like bacteriophages: bacteriophages constitute a large percentage of viral air particles. If the fractions of surface bacteria responsible for replicating and releasing bacteriophages into the air are dormant, there is the corresponding decrease in total viral particles population, lowering the VBR.
  2. Particulate Nature and Transport Mechanism: Viral and bacterial particles in outdoor air contribute significantly to their concentration indoors, as particulate matter has been shown to penetrate effectively from outdoor environment to indoor air and circulated through air conditioning systems. In many cases, variation in particulate outdoor matter explains the majority of variation in indoor microorganism concentration.
  3. Climate. Microorganisms depend strongly on climate. Climatic conditions have a severe impact on the viability and growth of microorganisms, and different climate has a different impact on how water behaves in cooling systems. In cold climates, there is little or no water vapour, relative humidity declines and so, growth of bacteria recedes, and viral particles suspension in the air becomes higher, leading to various respiratory tract infections in HVAC users. Indoor humidity influences microorganism survival and virulence of infectious microorganisms.
  4. Outdoor and Indoor air circulation: Although, outdoor air circulation into the indoor environment is a significant factor affecting indoor viral and bacterial concentrations of air and air conditioning system, human activities indoor can also contribute to microorganism population.
  5. Human activities affect and elevate indoor microbial distribution in conditioning system by shedding microorganisms on surfaces through shared contact with surfaces, sneezing, coughing or re-suspension during walking and cleaning. Depending on their size and diameter, bacterial and viral particles can be deposited on surfaces as a result of Brownian motion (below 0.1 μm); accumulate in the air, circulated through air conditioning system and inhaled directly (0.1–1 μm).



Virus-to-bacteria ratio of indoor air is lower than outdoor air, indicating an enhanced source of growth and proliferation of bacteria as air circulation occurs. Also, virus and bacterial particle population are higher in residential houses compared to most of the public places monitored in research studies. Residential houses are enclosed places, higher temperature and humidity contributes to this increase in microorganism population. Also, higher ventilation rate in public places may contribute to this finding. Indoor VBR, viral and bacterial population can be affected by the presence of an air conditioning system and a filter, although, the removal efficiency of filters used in air conditioning systems varies with particle size.

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