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Controlling Respiratory Disease Transmission in Air conditioned Spaces

In view of the pandemic COVID-19 respiratory disease, we urge you to go through the safety measures to be considered while staying indoors

Airborne Diseases

Airborne diseases are diseases caused by microbes those use air as medium of transmission to the host. There are diverse population of microorganisms in the air, but only a few of them are significant pathogens for susceptible humans. . The “Chain of infection” is a necessary requirement for a transmission to take place. .

  • Adequate number of pathogenic organisms (dose)
  • Pathogenic organisms of sufficient virulence
  • A susceptible host
  • An appropriate mode of transmission or transferal of the organism in sufficient number from source to host
  • The correct portal of entry into the host

CDC recommends Non Pharmaceutical Interventions (curbing spread of a respiratory illness without vaccination and medicines) as measure against a pandemic flu. CDC gives example of the same as below:

Personal
Staying home when sick , Covering coughs and sneezes with a tissue, Washing hands often or using hand sanitizer

Community
Creating physical distance between people in settings where they commonly come in contact with one another (like workplaces), Temporarily closing schools where students, teachers, and staff members come in close contact with one another, Modifying, postponing, or canceling large public events

Environmental
Routinely cleaning frequently touched surfaces and objects

Air conditioned spaces and Transmission of Microbes

To be able to create an artificial comfort zone in an indoor space, the air is either cooled or heated depending on the ambient temperature. A continuous air flow is also maintained to carry the heat.

A standard room air conditioner which is used for cooling the indoor space uses electric energy for cooling the air. This room air conditioner only recirculates the same air. Gradually over a period of time, the oxygen gets consumed by the occupants and Carbon dioxide is released to the room. Thus the room air oxygen level depletes. In order to make up the oxygen level, it is required to exhaust this air out and bring in fresh air. This process is termed as ventilation.

Inducing outside air adds load to the air conditioner machine as the out side air is warm and humid. Hence if more ventilation is to be provided, the air conditioners are to be sized with more capacity. Also the energy required for operation increases due to handling the extra load.

As ventilation is an expensive deal, it is often compromised. This cause air quality being reduced considerably. If an infected person enters an air conditioned space, microbes enter the air through coughing, sneezing, talking breathing etc. As the air conditioner recirculates the same air, the microbes stay in the conditioned space for longer time. This increases the risk of infecting to other occupants in the same space.

Another thing about air conditioned space is that if the humidity is reduced very low, it reduces the efficiency of the defense mechanism of human respiratory tract. This is because of the mucus layer getting dried out due to the lower humidity. Lower humidity also increases the infectivity of the virus.

Hence sufficient care is to be taken to reduce the risk of respiratory disease transmission in the air conditioned space.  

Control Measures

As for obvious reasons, air conditioning is a necessity, we have to adopt proper measures to reduce the issues discussed above. 

  • Using sufficient grade of air filters is one essential thing. In fact it is not practical to reduce the particle count considerably low as in a clean room due to the increased fan power and higher cost of HEPA filters etc. Standard Room Air conditioners are not equipped with higher grade filtration. Larger Air Handling units, packaged air conditioners etc., employ high power  fans and use filters of better grade. The floating particles act as a substratum for the microbes to stick on. Thus reduction in particles help reducing the floating microbes as well.
  • Keep sufficient ventilation rate. The more the air changes, the contaminated air gets diluted with incoming fresh air. 
  • Disinfect floors, table tops, High touch surfaces etc as frequently as possible with recommended disinfectants.
  • Masks provide both outward and inward protection. Hence wearing a mass during an outbreak shall reduce the spread.

Ventilation is an expensive deal and uncontrolled ventilation shall cause high energy cost. A qualified HVAC engineer can help optimising the ventilation rate keeping the balance of safety and cost.