In the first moments of the pandemic, much attention was paid to hand washing and cleaning of surfaces, but as the months went by, the airborne transmission of the coronavirus was confirmed . Infected people can spread it by coughing, talking, or even breathing, expelling larger droplets that settle on surfaces or smaller aerosols that float in the air.
In this context, the health authorities recommended maintaining a safety distance of 2 meters to reduce infections. Now, an investigation carried out by engineers from the University of Cambridge (United Kingdom) points out that this rule is arbitrary , since the term ‘risk’ is not a specific measure or percentage of safety, but rather depends on the one that each country or country wants to assume. authority.
Each cough is different, and their risk is also different, so that at 2 meters one can be perfectly safe and another very risky. An infected person can infect another at that distance, even when they are outdoorsEpaminondas Mastorakos (U. Cambridge)
The authors, who publish their work in the journal Physics of Fluids , have used computer models to simulate and quantify how droplets spread when people cough. In this way, they have shown that the airborne transmission of covid-19 is very random and have verified that, in the absence of masks, an infected person can infect another at two meters, even when they are outdoors.
“We have quantified the large variations in the drop distributions and we have seen that each cough is different,” explains Epaminondas Mastorakos , an expert in fluid mechanics who has led the research, “and, therefore, the risk corresponding to each coughing is also different, so that, at 2 meters, one can be perfectly safe, but the other can be very risky ”. The ‘safe’ distance could have been set between one and three or more meters, depending on the risk tolerance that one wants to assume, the researchers point out.
Virology and fluid mechanics united
“Part of the way this disease spreads is virology: the amount of virus you have in your body, the viral particles that you expel when you speak or cough,” says first author Shrey Trivedi , “but another part is fluid mechanics: what happens to the droplets once they are expelled. And that is where we come in, and we have seen that the duration and distance that a drop can travel depends on both its size and the surrounding conditions ”.
Trivedi indicates that the simulations and models of the past offered conclusions based on ‘averages’, but in the expansion of a highly infectious disease such as covid-19 this is not the case: “The variations in the spread are very large , so while a cough may appear ‘safe’ at 2 m, others may have a real risk of infection at much greater distances and this should be taken into account in future mathematical models on safety ”.
We have seen that the duration and distance that a drop can travel depends on both its size and the surrounding conditions.Shrey Trivedi (U. Cambridge)
Computational models and simulations helped the researchers solve turbulent flow equations , describe and visualize the motion of the droplets and their evaporation over time. Thus they discovered that there is no sharp cut in the two meters.
When a person coughs and does not wear a mask, most of the larger droplets fall on nearby surfaces. However, the smallest ones, suspended in the air, can quickly and easily spread further. The distance and the speed of propagation of these aerosols will depend on the quality of the ventilation of the room.
The authors recall that, in addition to the variables related to the use of the mask and ventilation, there is also a high degree of variability in the individual cough itself. “Every time we cough, we can emit a different amount of liquid, so if a person is infected with covid-19, they could be emitting many virus particles or very few, and due to the turbulence they spread differently in each cough, ”explains Trivedi.
The term ‘risk’ is not a specific security measure, but depends on what each country or health authority wants to assume
“But even if I expel the same number of drops each time I cough, since the flow is turbulent, fluctuations occur ”, adds Mastorakos, “and variations in speed as well as in temperature and humidity (which affect the evaporation rate, buoyancy and vertical movement of the droplets) mean that the amount someone receives at two meters can be very different each time ”.
In any case, the researchers acknowledge that the two-meter rule is an effective and easy-to-remember message for the public, although they insist that it is not a safety mark , given the large number of variables associated with a virus transmitted by it. air.
Vaccination, ventilation and masks remain essential to fight the pandemic
They also indicate that social distancing is not an effective mitigation measure on its own, underscoring the importance of continuing vaccinations, ventilation, and masks as we approach the winter months in the Northern Hemisphere. Together, they are still vital in containing the virus.
“We are all desperate to see the end of this pandemic, but we strongly recommend that people continue to wear masks in indoor spaces such as offices, classrooms and shops,” emphasizes Mastorakos, “there is no reason to expose themselves to this risk as long as the virus continues between U.S”.
Distinguish droplets from aerosols?
Regarding the limits between ‘droplets’ and ‘aerosol’, the professor points out: “Personally, I don’t like the distinction : they are all ‘drops’, the only thing that changes is the time it takes for them to fall to the ground or remain floating, which It depends on its size, but also on the movement of the air ”.
I don’t like the distinction between droplets and aerosol, they are all ‘drops’, the only thing that changes is the time it takes for them to fall to the ground or remain floating, which depends on their size, but also on the movement of the air.E. Mastorakos (U. Cambridge)
“In other words,” he continues, “if there is a windy day or a powerful ventilation system in a room, even the largest drops can remain suspended. I don’t think it is very useful to associate a fixed size to make that distinction, so we think simulations like ours that show what happens with ‘all’ the drops are useful . We hope virologists can use our results in conjunction with their studies to better understand scattering. “
Regarding the percentage of transmission of the coronavirus by the different types of drops, Mastorakos does not comment: “For a dinner with friends talking through the table, it is more likely in the nearby area through a few larger drops. However, sitting in a stuffy office for hours, the route of the spray through many small floating droplets can be important. But I do not have these data, they are only conjectures ”.
Continuing with their models and simulations, the team is currently investigating the spread of the coronavirus in spaces such as conference rooms, and they trust that their new results will help assess risk as people spend more time indoors.
Shrey Trivedi et al. “Estimates of the stochasticity of droplet dispersion by a cough.” Physics of Fluids , 2021.
Rights: Creative Commons.