Preventing Covid-19 spreading in buildings

In response to the coronavirus pandemic, REHVA experts have published a guidance document on how to operate and use building services to minimise the spread of Covid-19. Alex Smith provides a summary of their findings

Credit: – Smartboy10/Lisa-Blue 

REHVA, the Federation of European Heating, Ventilation and Air Conditioning Associations, has produced interim guidance on the operation and use of building services in areas with a coronavirus disease (Covid-19) outbreak.

Based on a survey of recent academic literature, the guidance aims to prevent the spread of coronavirus through HVAC or plumbing systems, and is targeted primarily at HVAC professionals and facility managers.

Its scope is limited to commercial and public buildings, such as offices, schools, shopping areas and sports premises, where only occasional occupancy by infected people is expected. Information and research on the disease and virus responsible for it (SARS-CoV-2) is very limited, so REHVA says best-practice recommendations have been made based on evidence from SARS-CoV-1, which occurred in 2003-04.

The guidance is focused on ‘temporary, easy-to-organise measures that can be implemented in existing buildings which are still in use with normal occupancy rates’ says REHVA. It intends the advice to be for a short period depending on how long local outbreaks last.

The document will be updated with more information and evidence when it becomes available.

Transmission routes

The guidance document states that there are two dominant transmission routes: via large droplets (droplets/particles emitted when sneezing, coughing or talking); and via surface contact (hand-to-hand, hand-to-surface, and so on).

However, it says the World Health Organization (WHO) also recognises a faecal-oral transmission route for SARS-CoV-2. In a technical briefing on 2 March, WHO recommended closing toilet lids when flushing, and avoiding dried-out drains in floors and other sanitary devices by regularly adding water (every three weeks, depending on climate).

In the SARS 2003-04 outbreak, open connections with sewage systems appeared to represent the primary transmission route in the Amoy Gardens apartment building in Hong Kong. Transmission was probably because of a dried-out floor drain and airborne dissemination by the toilet exhaust fan and winds.

Professor Catherine Noakes, professor of environmental engineering for buildings at Leeds University, says aerosolisation from water systems may be important. ‘The REHVA guidance recommends checking floor-drain traps, but drain traps in high-rise buildings can be susceptible to being blown out by wind pressure. So even traps in drains that are used more regularly are important to watch, too.’

Air transmission

There are two exposure mechanisms, which the guidance document describes as follows:

Large droplets (> 10 microns)

Airborne transmission through large droplets that are released and fall to surfaces no further than 1-2 metres from the infected person. Droplets are formed from coughing and sneezing (the latter typically forms more particles).

Most of these fall on surfaces such as desks and tables. People could catch the infection by touching contaminated surfaces and objects and then their eyes, nose or mouth. People standing 1-2 metres from an infected person could catch it directly in droplets sneezed or coughed out.

Noakes believes that drops greater than 10 microns can travel further than two metres. ‘Some of those very big droplets will fly ballistically, but even particles up to 20 microns can be carried further than we might expect because of airflows in the room,’ she says. ‘It doesn’t necessarily mean there’s huge additional risk, because there’s probably a small concentration of virus, but we should be aware of where surfaces might be contaminated.’

Small particles (< 5 microns)

These may stay airborne for hours and can be transported long distances. They are generated through coughing, sneezing or talking. Small particles (droplet nuclei or residue) form from droplets that evaporate (usually within milliseconds) and desiccate.

The coronavirus particle is 80-160 nanometres (1 micron = 1,000 nanometres) and remains active in common indoor air conditions for up to three hours and two to three days on room surfaces. These small particles can stay airborne and travel long distances by airflows in the room or via air ducts of ventilation systems.

REHVA says there is no evidence yet for Covid-19 infection via this route, but it noted that there were no studies that ruled it out. It also referred to a case where coronavirus SARS-Cov-2 was isolated from swabs taken from exhaust vents in rooms occupied by infected patients.

This implies that keeping 1-2 metre from an infected person might not be enough, concluded REHVA, and that increases in ventilation may be useful, as it would remove more particles. It recommends taking a series of measures that help control the airborne route in buildings as follows:

Increase air supply and exhaust ventilation

The general advice is to supply as much outside air as possible. Expanded operation times are recommended for buildings with mechanical ventilation. Consider keeping the ventilation on 24/7 with lower ventilation rates when people are absent.

If employee numbers reduce, do not place remaining staff in smaller areas. Exhaust ventilation systems of toilets should always be left on 24/7, and relatively negative pressure must be maintained in the room air to help avoid faecal-oral transmission.

Use more window-driven natural ventilation

In buildings without mechanical ventilation, the use of openable windows is recommended, even if this causes thermal discomfort. Even in buildings with mechanical ventilation, open windows can be used to boost ventilation.

Open windows in toilets with passive stack or mechanical exhaust systems may cause contaminated airflow from the toilet to other rooms so, in these circumstances, it is recommended that toilet windows remain shut. If there is no adequate exhaust ventilation from toilets, and window airflow cannot be avoided, keep windows open in other spaces to achieve crossflows through buildings.

Humidification has no practical effect

Covid-19 is resistant to environmental changes and is susceptible only to a very high relative humidity (RH) above 80% and a temperature above 30°C, which is not acceptable for reasons of thermal comfort.

The reason humidification is suggested in winter (up to a level of 30%) is because nasal systems and mucous membranes are more susceptible to infections at very low RH of 10-20%. However, from March, climatic conditions will see RH higher than 30% in all European climates, without humidification.

Safe use of heat-recovery devices

Virus particles in extract air can re-enter the building. Heat-recovery devices may carry over the virus attached to particles from the exhaust airside to the supply airside via leaks. In rotary heat exchangers (including enthalpy wheels) particles deposit on the return airside of the heat exchanger surface, after which they might be resuspended when the heat exchanger turns to the supply airside.

Based on current evidence, REHVA recommends turning off rotary heat exchangers temporarily during SARS-CoV-2 episodes. Its document goes on to state: if leaks are suspected in the heat-recovery sections, pressure adjustment or bypassing can be an option to avoid a situation where higher pressure on the extract side causes air leakages to the supply side.

Transmission via heat-recovery devices is not an issue when a HVAC system is equipped with a twin-coil (‘run around’ coil) or other heat-recovery device that guarantees air separation between return and supply side.

No use of recirculation

The guidance document says virus particles in return ducts can re-enter a building if centralised air handling units have recirculation. It recommends avoiding central recirculation during SARS CoV-2 episodes and closing the recirculation dampers, even if there are return air filters, as the guidance says these don’t normally filter out viruses.

It also advises that decentralised systems, such as fan coil units that use local circulation, should be turned off to avoid resuspension of particles at room level. If they can’t be turned off, they should be cleaned regularly. 

Duct cleaning has no practical effect

Virus particles will not deposit easily in ventilation ducts and will normally be carried away by the airflow, says REHVA. No changes are needed to normal duct cleaning and maintenance procedures. Increasing the fresh-air supply and avoiding recirculation are more important.

Change of outdoor air filters not necessary

In rare cases of virus-contaminated outdoor air, fine outdoor air filters provide reasonable protection for a low concentration, but occasionally spread viruses from outdoor air, according to the guidance. Clogged filters are not a contamination source, but should continue to be changed as part of any good-practice maintenance regime.

Room-air cleaners can be useful

Particles can be removed from the air, but air cleaners must have at least HEPA filter efficiency. ‘Attractively priced’ room-air cleaners are not effective enough, says REHVA. As the airflow through air cleaners is limited, the floor area they can serve is normally quite small, typically less than 10m2. If used, they should be placed close to the ‘breathing zone’.

Special UV cleaning equipment for supply-air or room-air treatment is effective at killing bacteria and viruses, but the guidance document says this is normally only suitable for healthcare facilities.

The guidance is a live document. REHVA invites specialists to respond at info@rehva.ue A bibliography is available here See CIBSE’s guidance at