In the early stages of the pandemic, a lot of attention was paid to hand washing and surface cleaning, but as the months went by, the air transmission of the coronavirus. Infected people can spread by coughing, talking or even breathing when they expel larger droplets that settle on surfaces or smaller aerosols that float in the air.
In this context, health authorities recommend the maintenance of a 2 meters of safety distance to reduce infections. Now, an investigation carried out by engineers at the University of Cambridge (United Kingdom) points out that this rule is arbitrary, since the term ‘risk’ is not a specific safety measure, but depends on which country or authority one wishes to assume.
Every cough is different, and its risk is also different, so at 2 meters one can be perfectly safe and the other very risky
Epaminondas Mastorakos (U. Cambridge)
The authors, who publish their work in the journal Fluid Physics, used computer models to simulate and quantify how the droplets spread when people cough. In this way, they showed that the airborne transmission of covid-19 is very random and found that, in the absence of masks, an infected person can infect another at two meters, even when outdoors.
“We quantified the large variations in the distribution of drops and saw that each cough is different”, he explains to SINC. Epaminondas Mastorakos, a fluid mechanics specialist who conducted the research, “and therefore the risk corresponding to each cough is also different, so that at 2 meters one can be perfectly safe, but the other can be very risky.”
For researchers, the ‘safe’ distance could be defined as between one and three meters or more, depending on the risk tolerance of a given public health authority. They also indicate that social distancing is not an effective mitigation measure per se, underscoring the importance of continuous vaccinations, ventilation and masks as we approach the winter months in the Northern Hemisphere.
Virology and fluid mechanics united
“Part of how this disease spreads is virology: the amount of virus you have in your body, the amount of viral particles you expel when you talk or cough,” says the first author, Shrey Trivedi, “But another part is fluid mechanics: what happens to the drops after they are expelled. And that’s where we come in, and we’ve seen that the duration and distance a drop can travel depends on its size and the surrounding conditions.”
Trivedi points out that past simulations and models offer conclusions based on ‘averages’, but have shown that, for the spread of a highly infectious disease like covid-19, this is not the case: ” variations in propagation are very largeTherefore, while a cough may appear ‘safe’ at 2 m, some implementations may have a real risk of infection at much greater distances and these variations should be taken into account in future mathematical safety models”.
We’ve seen that the duration and distance a drop can travel depends on its size and surrounding conditions.
Shrey Trivedi (U. Cambridge)
Computer models and simulations helped the researchers to solve turbulent flow equations, describe and visualize the movement of droplets and their evaporation over time. So they found that there is no sharp cut when the droplets extend beyond two meters.
When a person coughs and is not wearing a mask, most larger drops fall onto nearby surfaces. However, smaller ones, suspended in the air, can spread more quickly and easily. The distance and speed of propagation of these aerosols will depend on the quality of ventilation in the environment.
The researchers point out that, in addition to the variables related to the use of masks and ventilation, there is also a high degree of variability in the individual’s own cough. “Each time we cough, we may emit a different amount of fluid, so if a person is infected with covid-19, they may be emitting too many or too few virus particles and, due to turbulence, they spread differently each time. cough,” he said. explains Trivedi.
The term ‘risk’ is not a specific safety measure, but depends on what each country or health authority wishes to take.
“But even if I expel the same number of drops each time I cough, as the flow is turbulent, there are fluctuations,” adds Mastorakos, “both variations in velocity and temperature and humidity (which affect evaporation rate, buoyancy and vertical movement of the drops) means that the amount one receives at two meters can be very different each time”.
In any case, the researchers recognize 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 the air. Vaccination, ventilation and masks – even if they are not 100% effective – are still vital to containing the virus.
Vaccination, ventilation and masks remain essential to fight the pandemic
“We are all desperate to see an end to this pandemic, but we strongly recommend that people continue to wear masks indoors such as offices, classrooms and stores,” emphasizes Mastorakos. “There is no good reason to expose yourself to that risk as long as the virus remains with us.”
Distinguish aerosol droplets?
About the limits between ‘droplets’ and ‘aerosol’, the teacher points out: “I personally don’t like the distinction: they are all ‘droplets’, the only thing that changes is the time they take to fall to the ground or remain floating, which 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 they take to fall to the ground or float, 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 biggest drops can remain suspended. I don’t think it’s very useful to associate a fixed size to make this distinction, so we think simulations like ours that show what happens to ‘all’ drops are useful. We hope virologists can use our results in conjunction with their studies to better understand dispersion. “
On the percentage of coronavirus transmission by different types of drops, Mastorakos does not comment: “For a dinner among friends chatting at the table, the near field is more likely through a few larger drops. However, sitting in a stuffy office for hours, the spray’s path through many small floating droplets can be important. But I don’t have these data, they are just conjectures”.
Continuing with their models and simulations, the team is currently investigating the spread of the coronavirus in spaces such as conference rooms and are confident that the new results will help assess risk as people spend more time indoors.
Shrey Trivedi et al. “Estimates of the Stochasticity of Droplet Scattering by Cough.” Fluid Physics, 2021.