The role of ventilation in COVID-19 transmission
Researchers from the University of Cambridge and Imperial College London have used mathematical models to show how SARS-CoV-2 — the virus that causes COVID-19 — spreads in different indoor spaces, depending on the size, occupancy, ventilation and whether masks are being worn. Their results, published in the journal Proceedings of the Royal Society A, show that social distancing measures alone do not provide adequate protection from the virus, and further emphasise the vital importance of ventilation and face masks in order to slow the spread of COVID-19.
The researchers found that when two people are in a poorly ventilated space and neither is wearing a mask, prolonged talking is far more likely to spread the virus than a short cough. When speaking, we exhale smaller droplets, or aerosols, which spread easily around a room and accumulate if ventilation is not adequate. In contrast, coughing expels more large droplets, which are more likely to settle on surfaces after they are emitted.
It only takes a matter of seconds for aerosols to spread over two metres when masks are not worn, implying that physical distancing in the absence of ventilation is not sufficient to provide safety for long exposure times. Masks slow the breath’s momentum and filter a portion of the exhaled droplets, in turn reducing the amount of virus in aerosols that can spread through the space.
“We consider the wide range of respiratory droplets humans exhale to demonstrate different scenarios of airborne viral transmission — the first being the quick spread of small infectious droplets over several metres in a matter of a few seconds, which can happen both indoors and outdoors,” said first author Dr Pedro de Oliveira, from the University of Cambridge. “Then, we show how these small droplets can accumulate in indoor spaces in the long term, and how this can be mitigated with adequate ventilation.”
The researchers used mathematical models to calculate the amount of virus contained in exhaled particles, and to determine how these evaporate and settle on surfaces. In addition, they used characteristics of the virus, such as its decay rate and viral load in infected individuals, to estimate the risk of transmission in an indoor setting due to normal speech or a short cough by an infectious person. For instance, they show that the infection risk after speaking for one hour in a typical lecture room was high, but the risk could be decreased significantly with adequate ventilation.
Based on their models, the researchers built Airborne.cam — a free, open-source tool that can be used by those managing public spaces, such as shops, workplaces and classrooms, in order to determine whether ventilation is adequate. The tool is already in use at the University of Cambridge, enabling academic departments to easily identify hazards and control-measure changes needed to ensure aerosols are not allowed to become a risk to health.
“The tool can help people use fluid mechanics to make better choices and adapt their day-to-day activities and surroundings in order to suppress risk, both for themselves and for others,” said study co-author Savvas Gkantonas, who led the development of the app with Dr de Oliveira.
“We’re looking at all sides of aerosol and droplet transmission to understand, for example, the fluid mechanics involved in coughing and speaking,” said senior author Professor Epaminondas Mastorakos, also from Cambridge. “The role of turbulence and how it affects which droplets settle by gravity and which remain afloat in the air is, in particular, not well understood. We hope these and other new results will be implemented as safety factors in the app as we continue to investigate.”
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