A study just published on Physics of Fluids (a scientific journal dealing with fluid dynamics, founded by the American Institute of Physics and created by AIP Publishing) analyzes in detail the behavior of droplets emitted from a subject who coughs in the absence of wind, with light and moderate wind. The authors, from the University of Nicosia in Cyprus, set out to understand in this way whether the rule of 6 foot distance (used in Anglo-Saxon countries and corresponding to just under 2 meters by us) is worth protecting people from the SARS-Cov-2 virus. The simulation experiments were carried out assuming an ambient temperature of 20 C and 50% relative humidity.
Safe at 1 meter without wind
By studying the kinetics of the cloud of saliva droplets resulting from a human cough, they found that 2 meters is a safe distance in case of total absence of wind. As seen above, in the Figure 1, even 1 meter away (the distancing measure in force in Italy) a safe threshold, since after 49 seconds from the cough all the droplets did not exceed the horizontal distance of 1 meter from the mouth. Equally essential that the droplets took about 15 seconds to fall below the level of a person’s life, which considered a safe measure (vertically) such as to prevent bystanders from inhaling infected particles. In case of absence of wind – note the study authors -, young children will be the most vulnerable, when they are in the immediate vicinity of the cloud of droplets falling. Without wind, the droplets will fall to the ground a short distance from the person who exhales or coughs no more than 1 meter away. A small number of particles can move slightly forward. However, their trajectory at that point will already be significantly less than half a meter above the ground.
In light winds it reaches 6 meters
The experiment then measured the dynamics of the droplet cloud in the presence of light wind at 4 km / h: in this case the distance of 2 meters is insufficient because the droplets can move up to 6 meters away in just 5 seconds, as seen in the Figure 2a under. After 5 seconds from the cough, the cloud of droplets loses mass and the minimum dimensions gradually decrease until they disappear completely. Another phenomenon that we notice the vertical elongation of the cloud of droplets as it moves away from the mouth. At this low wind speed, the cloud always remains below the horizontal line that starts from the mouth of the subject who emits the cough.
With moderate wind more dispersion
With the wind speed increasing from 4 to 15 km / h, we observe a different kinetics. Saliva droplets move away faster and reach 6 meters in 1.6 seconds with greater dispersion (see Figure 2b above). Similarly, evaporation accompanied by a mass reduction of saliva droplets. The cloud lengthens and remains higher, with droplets of saliva above the height of the mouth, a dispersion that can therefore affect adults and children of different heights.
Air flows are also important inside
For all environmental conditions, including different wind speeds, the diameter of saliva droplets decreases over time. As the wind speed increases, the diameter reduction is faster because the droplets evaporate more quickly. At 4 km / h instead, the total mass reduction occurs more slowly than in the case of 15 km / h. This finding indicates that at light wind speeds, exposure to the cloud of droplets may be longer, potentially increasing the risk of virus transmission. These aspects may also be important in the case of current flows in indoor places – the authors write – who remember how the quantity of inhaled virus and the time of exposure to the respiratory droplets determine the probability of infection (HERE the article on the most risky places).
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