Talking to a friend when infected with the coronavirus can be as dangerous as coughing close to him thanks to persistent particles, research suggested.
Covid can spread via a variety of pathways, including droplets containing viruses emitted when an infected person breathes, speaks or coughs – a factor that experts said could help explain why Covid seems to spread more easily indoors.
While large droplets fall to the ground over short distances, small droplets known as aerosols can carry the virus over distances greater than two meters and remain.
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 takes just a few seconds for the expelled particles to travel beyond two meters.
“You need masks, distance and good ventilation so that these particles do not accumulate in an internal space and are safely removed,” said Prof Pedro Magalhães de Oliveira, specialist in fluid mechanics at the University of Cambridge and co-author of the study.
Writing in the Proceedings of the Royal Society A, de Oliveira and colleagues reported how they built models that take into account the size of the droplets emitted by infected individuals when they speak or cough, as well as factors such as the composition of the droplets and the time it takes to establish.
The team also analyzed the risk of infection, taking into account the viral load of individuals 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 is not safe to be without a mask two meters from an infected person who is talking or coughing, with both situations presenting a risk of infection.
The team adds that an hour after an infected person spoke for 30 seconds, the total remaining aerosol contains much more viral mass than after a cough – adding that in small, unventilated spaces this may be enough to cause Covid.
“Speaking is a very important issue that must be considered because it produces much finer particles [than coughing] and these particles, or aerosols, can be suspended for more than an hour in sufficient quantities to cause the disease, ”said Oliveira.
But whether people would take Covid, he said, depends on the amount of aerosol they breathe – which is influenced by factors such as wearing masks, whether the situation is indoors, ventilation levels and the distance between the people involved.
The team used their work to develop an online calculator, called Airborne.cam, for users to explore the risk of being infected indoors using only airborne particles.
According to the tool, spend an hour in 250mtwo store – presumed to have a maximum capacity of 50 people and ventilation equivalent to offices – results in an individual with an estimated 8% chance of becoming infected with coronavirus, assuming there are five people infected in the store and no one is using one mask.
If ventilation is improved so that the air is renewed five times an hour instead of three times, that risk can be reduced to less than 2%; a similar drop can be obtained if everyone wears three-layer face masks.
Although the risks of infection are only theoretical estimates and are not specific to the new variant of the UK coronavirus, the team said the tool could help users explore ways to stay safe in different settings.
“The idea is not to obtain absolute risk figures from the tool, but to use it to see how mitigation strategies impact the risk of infection. It can be used to classify these strategies, for example, ”said de Oliveira.
Prof Catherine Noakes, a member of the Scientific Advisory Group for Emergencies and an expert on airborne infections at the University of Leeds, welcomed 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 very high viral load as one of the premises, and this has a significant influence on the predicted risk,” she said, adding that the viral load varies between people. and over the course of the disease.