By Barney Richardson

A new document that came to me last month from the European Centre For Desease Prevention and Control deals with evidence on transmission in closed spaces.

The writers consider the role of air conditioning and ventilation systems in the spread of Covid-19. Transmissions of respiratory infections in confined spaces with poor fresh air ventilation are associated although the roles have not been well defined.

It is thought that Covid-19 is primarily transmitted by droplets. This has been emphasised by our Minister of Health Dr Mkhize in insisting on everyone wearing face masks. There are now reports that aerosols can be implicated in the transmission of Covid-19.

An aerosol is a suspension of fine liquid droplets and droplet nuclei not only in the air, but also deposited onto surfaces. This is possibly where air conditioning and ventilation plays a part. Respiratory infections like Covid-19 can be transmitted through these droplets.

When the droplet particles are >5-10 μm in diameter they are respiratory droplets, and when then are <5μm in diameter, they are referred to as droplet nuclei. There is still a vast amount of research needed to be able to draw a firm conclusion on this.

According to current evidence, Covid-19 virus is primarily transmitted between people through respiratory droplets and contact routes such as on surfaces. Some studies of outbreaks have shown that crowded indoor spaces like office and workshop workplaces and churches, restaurants, shopping centres and supermarkets are places of concern.

Do you remember the cruise ships in Japan and California with large numbers of infections, and the airline industry which has virtually closed down? Building maintenance personnel should be very aware of the risks and requirements of Covid-19 transmission.

Air conditioning and ventilation systems may have a role to play in reducing the transmission of the virus in closed spaces by ensuring careful and comprehensive maintenance of the air handling units. This would include cleaning of the filters and the cooling evaporator coils or replacement of filters that are old and no longer efficient.

The increase of the fresh air introduced into the system and reducing the return air should be an option that be considered. The addition of an air exhaust may also be a consideration.

Not many commercial systems use High Efficiency Particulate Air (HEPA) filters, but in health care facilities and testing laboratories these must be carefully handled and maintained and risk-assessed regularly.

These are the only filters that are capable of filtering droplets at >5μm and aerosols at <5μm. In most central systems 80% nominal efficient filters are used which will handle the filtering of the >5μm droplets.

Of biggest concern to the building owners and the occupiers are the room air conditioning units installed in offices and small shops where no fresh outside air is introduced. These units do not have filters which may or may not filter out the >5μm droplets. The room air conditioner recirculates the air without any fresh air if windows and doors are closed especially on cold winter days.

The recommendation should be to have a dedicated fresh air supply ventilation system with filters if possible. Alternatively, a window should be opened slightly to let air in. ASHRAE have stated that ventilation and filtration provided through ventilation and air conditioning can reduce airborne concentration of SARS-CoV-2, the virus that causes COVID-19, and thus the transmission through the air. In general therefore, disabling heating, ventilation and air conditioning systems is not recommended to reduce transmission of the virus.

Thank you to ECDC and ASHRAE for the information used in this article.

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