By Ali Shaw, Gwilym Still & Bill Watts

11 June 2020

Having looked at the processes of infection and their direct influence on building design in Part 1, this second part of our blog series examines the different types of ventilation systems within buildings and how they can potentially impact the spread of COVID-19.

As far as we are aware, the number of viral particles required to cause an infection of COVID-19 is not yet known, but, based on other influenza type viruses, experts suggest it could be as few as 10,000, 1,000, or even fewer viral particles. That is not many given that infected people could be carrying billions of viral particles and exhaling millions in the liquid droplets from their nose and mouth. While normal breathing will release a stream of droplets without much velocity, speaking, coughing and sneezing (without a mouth covering) will produce increasing numbers and volume of droplets and project them further. While we would hope that people who are showing symptoms of COVID-19 would not be in a public space, we know that people can be asymptomatic (i.e. infected and spreading the virus without symptoms). Managing the risk of transmission within buildings from people who don't know they are infectious is the biggest challenge.

The general advice is to educate people not to cough or sneeze into the open but instead wear a mask, which will reduce the volume of the droplets and the distance they will travel. It is suggested that wearing masks at all times reduces the risks. But what else can you do?

Research suggests that the droplets form an aerosol that come in a range of microscopic diameters. The larger the droplets, the faster they fall to the surfaces around the person. These surfaces then become infectious to anyone touching them and then their face. However, smaller droplets (the size of dust particles) will be carried in air currents and will hang around until they are extracted or the viral particles deactivate over time. The humidity and temperature of the air do seem to influence this process. Drier air will encourage water evaporation and turn larger droplets into smaller ones that will hang in the air currents, while it has been suggested that cold temperatures also seem to preserve the viability of the virus.

There is a risk that an individual will receive the crucial number of viral particles from just being in the same air as an infected person. Studies show that the chances will depend on both the density of viral particles in the air and the length of time a person is in the space. The question for building managers is how to control the concentration. In particular, there are implications on the ventilation systems that can either be beneficial (by potentially diluting and removing the particles) or be detrimental to the situation (by mixing and spreading them around the spaces or even to other areas).

This blog is an attempt to assess the risks and mitigation measures for the ventilation and engineering systems. Some of the measures will have a significant impact on the energy consumption of the building, such as leaving the windows wide open in the winter. The origin of the risk, however, is not the systems but the number of people who are either transmitting or dangerously susceptible to the infection. One can imagine, and hope, that this will reduce over time.

As the number of infected people decreases, so does the risk. This makes the mitigation measures redundant, but the chances of them being left in place are high. It is important to consider potential system changes alongside the other possible steps that could be taken to manage the risk. This is to say that any building or system modifications should be done in conjunction with the overall risk management plan for the building, rather than just getting something done because it can be.

 

VENTILATION MODES

Local mixing in room
Any air movement with any velocity will create some local air recirculation that will serve to mix the air and any airborne pollutants in a space. Such air movement can be created by an air inlet grille, a fan, a fan-coil unit, a window or indeed a radiator.

How the air in the space is mixed within the room is determined by how forceful the air jets and convection cells are. It is fair to say that ventilation systems are generally designed to mix the air thoroughly. However, a person will get a much higher dose of aerosol if they are downwind of an infected person in a jet or flow of air.

Much industry guidance refers to “recirculation”. Where industry guidance refers to “local recirculation”, this is typically fan coil units, but also includes things like chilled beams.

Fan coils
Fan coils force the circulation of the air in a space to mix the air. The example of a restaurant in Guangzhou at the start of the pandemic is an example of how potent the system is at mixing the air and indeed localising the spread. It is likely that the “air conditioner” was a DX fan coil unit and the people infected were in both the forced air and the recirculating air currents it created.

 

Natural ventilation systems
How good natural ventilation is at controlling the concentration of the virus is highly variable and depends on how the open windows are managed and positioned along with the geometry of the building.

Some building's natural ventilation design involves air movement between spaces. This is often to maximise air movement in hot weather, for example the warm air rising through an atrium that draws cooler air in through surrounding spaces.

Some naturally ventilated spaces do enable high volumes of outside air which would dilute the virus effectively. However, some naturally ventilated spaces can have poor air quality, particularly if densely occupied with limited windows. The range is huge and the actual airflow depends on the weather as well as the building.

 

Central recirculation
This involves recirculating air centrally at some mechanical plant. Some air that is extracted from a space is tempered and then returned. Sometimes a system serves several spaces, so air from one space could be returned to another space.

Where central recirculation happens in the UK, it is usually for a system serving a single space. The reason is usually to maintain climatic conditions with minimal energy consumption. Examples include galleries (where conservation requirements go along with the need for stable temperature and humidity) and swimming pool halls (which are kept at high temperatures).

Some very large individual spaces may have central recirculation, such as open plan offices linked by an atrium over many floors. This is similar to central recirculation between different spaces.

Given that infection by aerosolised particles appears to be a mechanism for the spread of COVID-19, central recirculation is likely higher risk for systems serving multiple or very large spaces. This is because any particles that are aerosolised can be distributed to other parts of a building, further from the initial infected person. While the concentration of viral particles in recirculated air may be low in large or distributed buildings, exposure times may be long.

This risk should be quantified, as adopting full-fresh air for spaces such as pool halls and galleries would involve massively increased energy consumption. The same is true to some extent for large offices with few people in them.

 

Full fresh air
This involves extracting air from spaces and exhausting it to outside. Fresh air is brought into the building from outside, which would tend to dilute viral particles within a building.

In isolation, this is fairly low risk (although different heat exchanger technologies have different characteristics). However, the risks of a full fresh air system depend on how it has been set up – the system introduces forced air movement after all.

Some energy efficient designs supply air to occupied spaces and extract from transient spaces such as WCs and corridors. This can lead to particles being moved between spaces by the ventilation system. We suspect that this is not particularly high risk, because the sorts of spaces which receive second-hand air are not typically occupied for very long, but this is not conclusive.

 

Hybrid ventilation systems
These systems mix fresh outside air in with recirculated air with the aim of reducing draughts. Air is recirculated, albeit generally within a single space. We think these generally carry a similar risk to local recirculation systems such as fan coil units.

However, in some cases risks could be greater, because hybrid ventilation systems are often designed to throw mixed air to the opposite side of the room to the ventilation unit.

 

POTENTIAL RISKS WITH ANY SYSTEM
The risk of any system will depend as much on a space’s occupancy as the system itself. Even in spaces with high airflow rates, with air being both supplied and extracted, it is conceivable for a system to be high risk. Where systems are designed to create a net flow of air across a room, some people might find themselves “downwind” of infected people, ie “sources” of aerosol particles.

 

WHAT THE INDUSTRY SAYS
The guidance from the WHO includes a prompt in specific measures for workplaces: "Increased ventilation rate, through natural aeration or artificial ventilation, preferably without re-circulation of the air.” In a separate document they suggest to “Open windows and doors whenever possible to make sure the venue is well ventilated.” The WHO also propose a number of other measures.

In addition to this guidance, various industry bodies have produced advice on building systems and how they might be used to help lower the internal spread of COVID-19. We have summarised and compared these in the table (which can be clicked on to see it full size).

Apart from the need to choose the appropriate ventilation system, COVID-19 will also have an impact on a range of other things. These include future HVAC design and energy consumption, both within buildings and the wider environment, as buildings operate at lower density, in a different configuration and people potentially continue to work from home.

We’ll venture a glimpse into this uncertain future in Part 3 of our series, outlining our thoughts on the wider implications of living and working in socially safe cities.