Air conditioning, aerosols and COVID-19

Greg Davies

Greg Davies
Director of Market Development, Assurity Consulting
21st July 2020

Recent research seems to be adding the possibility of very fine droplets/aerosols containing SARS-CoV-2 (that remain airborne and so can be breathed in over time), to the more local droplet and touch (surface to mouth, nose, eye,) routes of transmission. This in turn has produced a lot of column inches on “concerns” over “air conditioning” and COVID-19. So how does all this now affect what we should be doing in managing our buildings?

Except for having to handle further questions from now more concerned staff, the answer is, it changes very little to the advice or practices that have been in place since the onset of the pandemic.

The Health and Safety Executive (HSE) are still saying:

“The risk of air conditioning spreading coronavirus (COVID-19) in the workplace is extremely low, as long as there is an adequate supply of fresh air and ventilation.”

The guidance to promote well ventilated spaces still applies through opening windows, maximising fresh air supply rates and extracted air in mechanically ventilated buildings and reducing any air recirculation through central or local systems.

The HSE has provided further guidance on both general ventilation and air conditioning (forced air ventilation), as do amongst others, REHVA and CIBSE.

There is some discrepancy here though about air recirculation, for example the HSE are saying:

“You do not need to adjust air conditioning systems that mix some of the extracted air with fresh air and return it to the room as this increases the fresh air ventilation rate. Also, you do not need to adjust systems in individual rooms or portable units as these operate on 100% recirculation. You should still however maintain a good supply of fresh air ventilation in the room.”

CIBSE on the other hand say:

“However, it is also believed that airborne transmission is another possibility, so the ventilation strategy of all shared spaces needs review. CIBSE’s advice is basically to increase ventilation as much as possible, increasing the flow of outside air and preventing any pockets of stagnant air. Recirculation of air within buildings should be avoided to reduce the risk of transmission.”

On digging down into some of those column inches, this is this issue that is being highlighted. While increased fresh air supply and extract will have a diluting effect in the occupied space, localised recirculation only (with no fresh air make up), typically via split or packaged units, is doing just that, recirculating the same air. So, while this benefits temperature control, it does not improve air quality.

Because of lockdown, one of the building services trends established with the vast majority of our customers is the clean and well-maintained condition of forced air ventilation/air conditioning systems they have. The amount of uninterrupted maintenance time seems to be the culprit for this. What we are seeing as a result of this, and as you would expect, is a very good quality of air being supplied within buildings, from a dust and gaseous pollutant perspective as well as microbiologically.

The conclusion, on current guidance advice and information, where practical:

  • Increase the levels of fresh air to your buildings through opening window or greater fresh air supply;
  • Increase extract rates on central or localised systems; and
  • Avoid recirculating air, particularly on large centralised systems with multiple distribution and on localised units with no fresh air make up.

Combine this with the right levels of maintenance on your systems and the chances are your indoor air quality will improve – although of course your energy use and bills may not.