By Ali Shaw, Gwilym Still & Bill Watts

29 May 2020

We now share a world with COVID-19. We also spend much of our time on this world inside buildings. Recently, the majority of people have mostly been in their homes, in line with rules on social distancing, but that will not always be the case. So the question is... How can we spend time in other buildings as safely as possible?

In the coming weeks, we will examine this question through a series of blog posts, of which this is the first part.

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The virus is tiny: one thousandth the size of a fine grain of sand, one hundredth the size of dust particles and one tenth the size of the smallest bacteria. It’s too small to be seen with a light microscope. They don’t live on their own but hijack cells in the body turning them in to factories to reproduce copies of themselves by the billion. Estimates based on other diseases suggest that perhaps 1,000 to 10,000 viral particles could be enough to infect a person if inhaled or transferred onto part of your face; in an enclosed space this risk intensifies even more, as the number of particles will build up through an infected person constantly adding more into the air.  

To understand how buildings can be made safer, the processes involved in infection need to be understood. Many experts have already written knowledgeably and eloquently about this and further research is ongoing. In summary, it is suggested there are four main mechanisms of transmission:

• Direct - this is where an infected person touches someone else, spreading the infection
• Indirect - this is where an infected person touches something else, someone else then touches that, spreading the infection
• Large droplets - this is when an infected person produces larger droplets, which follow a ballistic trajectory under gravity
• Small droplets - this is when an infected person produces small droplets, which form an aerosol and remain suspended in the air for much longer

All of these relate to building design in some way. It is widely thought that direct transmission can be largely eliminated through social distancing. This reduces the density of the buildings we inhabit so may require more buildings or affect how they are designed and operated. Social distancing also reduces indirect transmission, as does the regular hand washing that’s been recommended. Other measures, such as replacing taps and switches with PIR sensors reduce physical contact, so can also help to reduce indirect transmission, as can enhanced cleaning regimes.

Large droplet transmission is affected by a room’s temperature and humidity. This affects how quickly a large droplet evaporates. Sometimes a large droplet evaporates to become small droplets and form an aerosol.

The spread of particles as an aerosol is perhaps the most pertinent to the operation of a building and its systems, so that’s what we’ll be focusing on. However, it isn’t yet clear how much the aerosol mechanism contributes to the overall spread of COVID-19. 

In buildings, it is typically the ventilation systems that affect the spread of aerosol the most. Systems that remove particles from buildings quickly may reduce the risk, while systems that are more likely to transfer particles between rooms will keep them within the building for longer. In Part 2 of our series on COVID-19: Operating Buildings and The Future Design of the Built Environment we will be looking in more detail at buildings to examine the role of ventilation and how different systems can potentially impact the spread of COVID-19.