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Talking to a friend when he is infected with the coronavirus could be as dangerous as coughing near them thanks to persistent particles, research shows.
Covid can be spread through a number of routes, including droplets containing a virus emitted when an infected person breathes, talks or coughs – a factor experts say could help explain why Covid seems to spread more easily indoors .
While large droplets fall to the ground over short distances, tiny droplets called aerosols can carry the virus over distances of over two meters and persist.
Now, experts have developed models to explore the risk posed by large droplets and aerosols and explore ways to mitigate it. Their results suggest that it only takes a few seconds for the expelled particles to travel beyond two meters.
“You need masks, you need distance and you need good ventilation so that these particles do not accumulate in an indoor space and are safely removed,” said Professor Pedro Magalhães de Oliveira , an expert in fluid mechanics at the University of Cambridge and co-author of the study.
In the journal Proceedings of the Royal Society A, de Oliveira and colleagues reported how they built models that take into account the size of droplets emitted by infected people when they speak or cough, as well as factors such as composition of the droplets and the time it takes for them to settle.
The team also looked at the risk of infection, taking into account the viral load of people with Covid, and the estimated dose needed to cause an infection – the latter was based on studies of a different coronavirus.
The team concluded that it was dangerous to stand without a mask two meters from an infected person who is talking or coughing, both of which pose a risk of infection.
The team adds that an hour after an infected person speaks for 30 seconds, the remaining total aerosol contains much more viral mass than after a cough – adding that in small spaces and without ventilation, that might be enough to provoke Covid.
“Speech is a very important issue that must be taken into account because it produces much finer particles [than coughing] and these particles, or aerosols, can be suspended for more than an hour in amounts sufficient to cause disease, ”said de Oliveira.
But whether people would catch Covid, he said, depends on how much aerosol they inhale – which is influenced by factors such as wearing masks, whether the situation is indoors , ventilation levels and distance between people involved.
The team used their work to develop an online calculator, called Airborne.cam, that allowed users to explore their risk of getting infected indoors from airborne particles only.
Depending on the tool, spend an hour in a 250m2 store – assumed to have a maximum capacity of 50 people and ventilation comparable to offices – gives an individual an estimated 8% chance of being infected with the coronavirus, assuming there are five infected people in the store and that no one wears a mask.
If ventilation is improved so that the air is changed five times per hour rather than three times, this risk can be reduced to less than 2%; a similar drop can be achieved if everyone wears three-layer masks.
Although the risks of infection are only theoretical estimates and are not specific to the new variant of the British coronavirus, the team said the tool could help users explore ways to stay safe in different scenarios. .
“The idea is not to get absolute risk numbers from the tool, but to use it to see how mitigation strategies impact the risk of infection. It can be used to rank these strategies, for example, ”said de Oliveira.
Prof Catherine Noakes, a member of the Scientific Advisory Group for Emergencies and an expert in airborne infections at the University of Leeds, praised the study, but warned that the results are based on a number of assumptions.
“The results are likely to represent realistic worst-case scenarios, as the model uses a fairly high viral load as one of the assumptions, and this has a significant influence on the predicted risk,” she said, adding that viral load varies between people and throughout the course of the disease.
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