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The importance of ventilation in managing airborne transmission of COVID-19

The risk of airborne transmission of COVID-19 was first recognised by the World Health Organisation (WHO), back in June 2020.

As the return to the office gathers pace this autumn, we examine both the current focus of the Health & Safety Executive (HSE) on ventilation and recent research which suggests monitoring carbon dioxide levels can be an effective guide to levels of airborne transmission risk.

The risk of airborne transmission of COVID-19 was first recognised by the World Health Organisation (WHO), back in June 2020, with the concomitant risk that COVID-19 can be spread by those who are either pre or asymptomatic and amongst those working in an office or school environment.

The research

A recent study published in Sage Journals, “Predictive and Retrospective modelling of airborne infection risk using monitored CO2, Burridge, Fan & Others”, 28 September 2021, examined the risks associated with airborne transmission of COVID-19, concluding that four main variables determine transmissibility, being; the nature of the variant (how infectious it is), occupancy levels within the workplace, the rate of ventilation and finally ‘the behaviour’ of the occupants.

The researchers concluded that those engaged in more vocal activity – for example, call centre employees or sales office workers - posed a higher risk of transmissibility than those undertaking quieter, desk-based roles and that those workplaces could “significantly contribute to the spread of COVID via the airborne route”.

The authors also concluded that by reducing occupancy levels by half (for example, adopting a ‘week in, week out’ model), this reduced transmissibility by a factor of four – given the halving of both occupancy and those potentially infected. The study recommended that carbon dioxide levels are assessed regularly given that human breathing is the dominant source of CO2 in indoor spaces, to permit the effectiveness of current ventilation to be monitored.

Although the authors felt that open plan offices, regularly attended by the same people with ventilation in line with UK guidance doing quiet desk- based work, was unlikely to be a significant contributor to the spread of airborne COVID-19, they did however point out that for poorly-ventilated call centres, a “single infector” or employee with pre or asymptomatic COVID-19 risked infecting between two and four other people.

The HSE approach

The HSE, in conjunction with the British Occupational Hygienists’ Society, have designed a “ventilation tool” to allow employers to measure how effective their ventilation is, which uses a basic scoring system. The tool enables organisations to look at a number of key variables to include floor space, ceiling height, number of people, their distance apart, windows and types of ventilation systems.

Professor A Curran, the HSE Chief Scientific Advisor, commented upon both Burridge’s research and ventilation as follows;

“This research demonstrates airborne transmission can be a significant contributor to
COVID-19 infection risk in offices and schools………ensuring adequate ventilation is a
key element alongside carbon dioxide monitoring, Building Managers can gain a better
understanding of their own ventilation systems and how they are performing”.

As office occupancy increases alongside falls in ambient outside temperatures, the temptation for many will be to prioritise comfort over ventilation. Both the study and the recommendations of the HSE suggest that unless ventilation is properly managed and assessed – however broadly - the risk of airborne transmission remains, particularly in high vocal activity workspaces such as call centres or sales offices.

For the majority of workplaces where quiet, desk-based activity is undertaken and where employers have adopted hybrid home/office working, reducing occupancy, the risk of airborne transmissibility of COVID-19 will be low.

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