The COVID-19 pandemic has delivered a ‘stress test’ like no other to all businesses operating within the UK.
As we learn to live with the ongoing threat of the virus and its variants, and prepare for the UK to unlock, how do we return in the safest way possible?
When the UK Government initially published COVID-19 guidance for workplaces and construction sites, it focused on direct contact-based infection as well as near-field transmission, between people less than two metres apart. Little was mentioned in the early days of the pandemic about the risks of far-field aerosol transmission (see first picture, below), over distances beyond two metres.
From an early stage it became apparent that the World Health Organisation (WHO) was reluctant to acknowledge advice from aerosol scientists, who were warning about the risk of far-field airborne aerosol transmission.
Some of those experts cited apparent flaws in the WHO’s understanding of the size of aerosol particles and their ability to travel distances greater than two metres. On 6 July 2020, two leading aerosol scientists, supported by an international group of 237 related specialists, published a commentary in the journal Clinical Infectious Diseases, which urged the authorities to acknowledge the potential for airborne transmission. Since then, the evidence of far-field transmission has steadily grown, and it is now widely recognised as being a significant cause of coronavirus infection.
Minimising far-field COVID risks
Concerned about the return of our own team from lockdown to our offices, we developed a COVID-19 ventilation risk assessment. This incorporated the latest guidance from UK Government and reputable industry bodies. An embedded natural ventilation calculator was used to estimate air-flow rates through windows, and we invested in air-flow measuring equipment to assess mechanical ventilation systems. This enabled us to reconfigure our offices to minimise far-field COVID risks.
Shortly after, we were approached by a client who needed advice on how to assess the adequacy of their ventilation systems, to help them provide a safe return for their workforce. Today, we are working with numerous clients to deliver COVID-19 air ventilation audits, to help businesses prepare for unlocking during 2021.
As a father of two school-age children, my thoughts recently shifted to the return of pupils to the classroom. There remains a question as to whether school classrooms may prove to be COVID-19 ‘super spreader’ environments (see picture, below). Current UK guidance from SAGE and the HSE is to prioritise improvements where room carbon-dioxide levels are greater than 1500ppm, indicating poor ventilation, and where fresh air rates are less than 5 litres per second per person.
Checking back on school design regulations, since 1997 (at least), classroom ventilation should be capable of providing 8 litres per second per person, though reduced ventilation rates are often encountered during colder weather, when windows are closed or automated ventilation systems are turned down to maintain classroom temperatures.
The Federation of European Heating, Ventilation and Air Conditioning Associations (REHVA) has published an online calculator for assessing airborne COVID-19 risk for different space types, including classrooms. The following table gives results for a typical classroom under varying ventilation rates and other variables, based on one COVID-19 infected occupant.
Scenario
Air flow (l/s/person)
Time in room (hours)
Face mask type
Speech levels (% speaking)
Virus strain
Probably of infection
A
4
1
None
5%
Wuhan
0.4%
B
4
4
None
5%
Wuhan
1.9%
C
8
4
None
5%
Wuhan
1.1%
D
8
4
None
50%
Wuhan
5%
E
8
4
Good fit, multi-layer
5%
Wuhan
0.8%
F
4
4
None
50%
UK (Kent)
13%
Source: REHVA, based on mask and strain data from MIT
Points of note from the above results:
Reducing time spent by pupils in a classroom, with gaps between to purge the air, will prevent the build-up of COVID19. Transmission risk is very low for one-hour lessons (scenarios A and B);
Well-ventilated classrooms are key to transmission-risk reduction (scenarios C-E);
The amount of speech occurring within the classroom has a significant impact on transmission risk (scenarios D and F);
Multi-layer fabric masks can significantly reduce the level of exhaled particles and therefore the transmission risk (scenario E);
A combination of factors including poor ventilation, long duration in the classroom, no masks, and noisy children will significantly increase transmission risk.
Research is ongoing into airborne COVID-19 transmission and the REHVA calculation tool recognises this and is caveated in numerous places. It also ignores near-field and contact transmission, which would both tend to increase the transmission risks stated above.
Richard Lewis is technical director at Davies Partnership. For more information about COVID-19 ventilation audits, contact him via richard.lewis@daviespartnership.co.uk

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How we’re helping unlock UK buildings through safe ventilation – Contributor – 2021-03-26 07:50:19

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