Air Conditioning (MVAC/HVAC) Considerations

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Recently the WHO has acknowledged that airborne transmission of the SARS-CoV-2 virus by droplet nuclei <5 µm diameter is possible, confirming the risk that air conditioning systems can contribute to the viral load in occupied spaces.

Further research is now showing that the risk of being infected by SARS-CoV-2 goes up as temperature and humidity decrease. This appears to be the result of drying out of the mucus in the respiratory tract which otherwise acts as a barrier to the virus reaching the lungs. While this won’t help to reduce the risk of intestinal infection, mortality from SARS-CoV-2 is related primarily to respiratory infection.


Step 1. Optimise what you have. The very first step to take as soon as possible, is to ensure that the Air Conditioning System in your building / workspace is working effectively and that it is adjusted to maximise outdoor air and minimise the risk of retransmission of viruses. We recommend an audit focused on filter/filter carrier condition, cleaning of the coils and maximising outdoor air within the limits of the system capacity.


Step 2. Improve the system. After optimising what you have, there is a need to review and make changes to improve the system to minimise the air borne viral load.


Air Conditioning system improvements focus on improved filtration, inclusion of Ultraviolet Germicidal Irradiation (UVGI) for central Air Handling Plant cooling coils and increased outdoor air supply.  Addition of ionisation also improves filtration effectiveness.


Where possible, we recommend the use of Displacement AC systems, particularly Underfloor Air Distribution (UFAD), rather than Ceiling Air Distribution (CAD) systems. CAD systems create a continuous mixing of the air in the occupied space relying on diffusion to dilute contaminants. Displacement systems reduce the level of airborne transmission through the upward movement of air which continually clears the contaminants from the occupied space.


In low humidity environments we now recommend adding humidification to maintain humidity above 40% Rh. This is particularly important in northern climates which are now moving into the seasons of reduced temperature and humidity.