This is the ninth ‘opinion’ article about the coronavirus pandemic. The previous article looked at what the R-number is and why it matters. This article looks at protective measures to minimize coronavirus spread in the ‘as built’ environment.
My own view, in relation to the call to open a wide range of shops, being investigated in many countries, including the UK’s Johnson-led Conservative administration, is coming in too early.
These are important considerations because, after direct transmission from coughing or sneezing (via large respiratory droplets, which fall close to where they are expired, which is why 2 metres matters) or touching a contaminated surface and then the face, transmission via droplets is the third vector of infection.
How do we know this? There have been at least four studies, using air sampling, that have found several positive samples for the SARS-CoV-2 genome (that is, RNA) as demonstrated through the application of polymerase chain reaction (or PCR) testing.
With this in mind, what are the appropriate measures that can be taken to minimize transmission?
We need to consider effective engineering controls to provide effective ventilation, particle filtration and air disinfection.
Ventilation controls how quickly room air is removed and replaced over a period of time. In terms of improving an indoor space, the placement of supply and exhaust vents ensures that adequate dilution is achieved and hence avoiding the build-up of viral contamination. Here we need to aim for a high level (20 or more) air changes per hour.
While most hospital wards, and cleanrooms, have adequate ventilation, most shared spaces, such as shops, offices, schools, may not have purpose-built mechanical ventilation and instead rely on open doors or windows. Here the ventilation rates are relatively low. Also, even where mechanical ventilation is in place, this often not operated optimally in order to lower the energy demand and to save on costs.
Hence, the possibility of infected persons sharing air with susceptible occupants is high in such settings, posing an infection risk contributing to the spread of the infectious disease.
This rests on how infectious the viral particles are in terms of time. Current data suggests at least one hour. For example, see a paper by van Doremalen. The paper is titled “Aerosol and surface stability of SARS-CoV-2 as compared with SARS-CoV-1” and it is published in the New England Journal of Medicine.
Other measures include avoiding air circulation and minimizing the number of people gathering in the same indoor space.
With the re-circulation of air, this is done to save on energy. However, this process can transport airborne contaminants (including infectious viruses) from one space and distribute them to other spaces connected to the same system. If particulate filters and disinfection equipment are in place for recirculated air stream, then this can reduce this risk – but these need to have been purposely designed and be regularly maintained. The best strategy is not to recirculate air and to instead rely upon 100 percent outdoor air.
In terms of air disinfection, there are some studies that indicate germicidal ultraviolet can be effective. This includes UV-C like. Raising the humidity can also help. While germicidal lights can be placed in rooms (especially where air stagnation is likely), the effectivity is difficult to determine. Placing units in ducts is likely to be more effective.
A useful air considerations is presented in “How can airborne transmission of COVID-19 indoors be minimised?” by Morawska and colleagues at the University of Colarado. It’s published in the journal Environmental International.
The following video expands upon some of the points discussed:
Of course, we need to consider building controls in conjunction with other established controls, such as isolation, physical distancing, and regular hand sanitization.
Putting these factors together, opening shops too early presents a new level of risk.
