Cambridge scientists have developed a website that enables users to calculate the risk of coronavirus transmission in a given room.
Officials and doctors alike have long stressed the risk of catching the infection is significantly lower outside, hence why outdoor exercise with one other person is permitted amid England’s ongoing lockdown.
This is affected by many factors, including the room’s size, ceiling height and occupancy.
Perhaps surprisingly, the scientists also found the risk from prolonged talking is far higher than coughing.
The public can use these same models via the free tool Airborne.cam, which calculates the transmission risk over time.
“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 co-author Savvas Gkantonas.
Mathematical models were used to calculate the amount of coronavirus in exhaled particles, as well as how these evaporate and settle on surfaces.
Known characteristics of the virus, like its “decay rate”, helped the scientists estimate the transmission risk indoors after an infected person spoke normally or coughed.
“Our knowledge of airborne transmission of [the coronavirus] has evolved at an incredible pace, when you consider it’s been just a year since the virus was identified,” said study author Dr Pedro de Oliveira.
“In our work, 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.
“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.”
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Results – published in the journal Proceedings of the Royal Society A – suggest that when two people are in a poorly-ventilated space with neither wearing a mask, prolonged talking is far more likely to spread the virus than a short cough.
Speaking causes a person to exhale smaller droplets, with these aerosols spreading easily around a poorly-ventilated room.
The heavy droplets expelled during a cough are more subject to gravity, causing them to settle on surfaces.
The results suggest it takes just a few seconds for aerosols to spread over 2m when masks are not worn, implying social distancing in the absence of ventilation is insufficient.
“The time-of-flight to reach 2 m is only a few seconds resulting in a viral dose above the minimum required for infection, implying physical distancing in the absence of ventilation is not sufficient to provide safety for long exposure times,” wrote the scientists.
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While the benefits of masks have been debated throughout the pandemic, any form of face covering is said to slow the breath’s momentum, while also filtering some of the exhaled droplets.
The vast majority of coronavirus cases are thought to arise indoors, which may explain why patient numbers have skyrocketed during the UK’s winter as people huddle out of the cold.
This is not the first time the importance of ventilation has been flagged.
Welsh GP Dr Eilir Hughes has previously said the government slogan “hands, space, face” does not go far enough.
Dr Hughes – who has become known as “Dr Fresh Air” – believes it should be “hands, space, face, replace”, in terms of swapping stale indoor air for that from the outdoors.
Professor Shaun Fitzgerald from Cambridge University previously told the BBC he “refuses to be in a place that is not well-ventilated”.
Amid the cold weather, the solution is simple – open windows just a crack and put on a jumper.
“That’s what we should be doing anyway, to save on heating bills and reduce our energy demand as we all do our bit to tackle climate change,” added Prof Fitzgerald.
The Cambridge scientists hope Airborne.cam will be used by people who manage public spaces, like shops and classrooms.
It is already being used in several academic departments at the university and is even a requirement for its higher-risk spaces.
“We’re looking at all sides of aerosol and droplet transmission to understand, for example, the fluid mechanics involved in coughing and speaking,” said co-author Professor Epaminondas Mastorakos.
“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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