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A month ago, the World Health Organization admitted that it is very likely that you are despising the transmission through the air of the new coronavirus, the Covid-19 . Be reached to confirm what many experts believe, the SARS-CoV-2 would have an incidence higher than thought through aerosols, which we would have to carry invitablemente to rethink the measures of social distancing , the effectiveness of the systems of ventilation and our presence in shared spaces in general.

But for the measures taken are effective, one must first understand the background of the case: in this case, the physics behind the behavior of the drops that our respiratory system throws to the outside. This is the aim of a new study published in the journal “Physics of Fluids”, raised by a group of researchers of the Heriot-Watt University and the University of Edinburgh in the United Kingdom, who have created a mathematical model delimiting, on the one hand, the size of the drops (which is classified in small, intermediate and large) and its scatter .

“the physics of The flow of someone who is coughing is complex, involving turbulent jets and evaporation of drops,” explains Cathal Cummins , of the Heriot-Watt University and the study’s lead author. “And the increase of Covid-19 has revealed the gaps in our knowledge about their physical performance and how it is applied in the strategies of transmission and mitigation”.

simple mathematical Model

One of those gaps is the description clear and simple where you will drop every once ejected . “Our intention was to develop a mathematical model based on the breath of a person who in addition could be explored analytically in order to observe the physics behind this gesture,” says Cummins. Also, we took into account that depending on the manner of breathing of a person, you emit drops of different sizes that, in addition, they do not necessarily faithfully follow the flow of the air.

that is why it Is recreated mathematically a simple circuit in which a source -that would be the respiratory system of a person – lance as much air as drops; on the other hand, a “sink” or surface point which is reached both the air like the drops -receiver-. “To take into account their differences in size and density, we use the equation of Maxey-Riley , which describes the motion of a rigid sphere of small but finite size through a fluid.” In addition, it took into account the types of breathing, as it does not eject the same amount of fluid when breathing at rest that after sport, for example.

The small droplets, the more dangerous

through a model as “simple”, the researchers suggest that the droplets are at once fluid and air, so that thanks to this formula we could predict when certain drops have ranges of scattering more or less long or short. “Our study shows that there is a linear relationship between the droplet size and the displacement, because the small droplets and large travel further than the mid-size ,” says Felicity Mehendale, co-author and academic, University of Edinburgh.

Left: paths of droplets from the patient to the device of extraction: small droplets, medium, and large. Right: maximum horizontal Distance covered by the fluid exhaled to various diameters of drops: heavy breathing vs. silent. The red dot indicates the global minimum on the distance traveled. In this case, the minimum is reached for droplets of diameters between 50 and 80 microns. – Cathal Cummins

however, according to the researcher, the problem is more unknown is in smaller drops, that could “sneak” in between the fibers of the masks . “We can not underestimate the drops are smaller; the costumes of personal protection are an effective barrier for large particles, but they may be less effective below a certain range”.

A “fan” of drops

As a solution, Mehendale came up with the idea of creating a device extractor that he would keep the medical insurance for many of the medical procedures routine that is in contact with patients. The extraction units, placed near the sources of drops -patients-, can be neutralized if its diameter is less than that of a human hair.

“This has important implications for pandemic Covid-19 -says Cummins-. Larger droplets would be easily captured by the EPP, with masks and protectors. But the smaller droplets can penetrate r some forms of PPE , so that a puller could help to reduce the weakness in our defense today against COVID-19 and future pandemics”.

This mathematical model can also serve as a basis for modeling the impact on the dispersion of droplets of existing ventilation systems within of a range of spaces to clinical and to be applied in other diseases that are spread through the air.

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