The more we learn about COVID-19, which has already infected more than 40 million people worldwide , the more evident the importance of transmission through respiratory droplets in the air becomes, particularly in closed spaces and without adequate ventilation.
Until now, studies had focused on how far cough droplets could travel. Now, new research published in the journal Physics of Fluids delves into the size of the “cough cloud” and what the difference is between wearing a mask (and what kind) and not doing it.
Scientists have already studied the properties of the air in the mouth, such as volume, temperature, droplet distribution, and humidity, but how do these properties change as cough clouds move? In this work, the researchers estimated the evolving volume of cough clouds and quantified the volume reduction in the presence of a face mask.
When a person coughs, what is the volume of air that is polluted by the cough that they expel?
“The answer to this question is not easy because the surrounding air is drawn into the cough cloud that comes out of the person’s mouth and eventually becomes part of it; therefore, a much larger volume than initially expelled is affected by coughing, “experts say.
Yes indeed. The conclusion is clear: depending on whether the person who coughs is using a mask or not and the type of mask he uses, we will have the answer : the volume of the cloud without a mask is approximately 7 times greater than that of a surgical mask and 23 times greater than that of an N95 mask; that is, coughing without a mask creates 23 times more volume than an N95 mask! It is confirmed that the masks drastically reduce the size of the cough cloud “and, consequently, significantly reduce the risk of infection for other people in the room.”
Researchers have also studied temperature and humidity changes in cough clouds as determining factors that affect the distribution of water droplets in the clouds. Using jet theory and an analysis based on experimental data, they found that the first 5-8 seconds after coughing the exhaled droplets were still suspended in the air, thus making it a problem with the spread of the disease. After that time, the cough clouds begin to disperse.
“I found that anything that reduces the distance the cloud travels, such as a mask, a handkerchief, and coughing into the elbow, should significantly reduce the area where water droplets are dispersed when coughing, thus reducing the possibility of infection,” he explains. Rajneesh Bhardwaj, co-author of the work.
Interestingly, the researchers found that, along with the significant reduction in the volume of clouds expelled caused by the use of face masks, the force with which a person without a mask coughed did not affect the volume of the cloud they released. However, it was important to wear a face mask. Coughing harder led to the droplets traveling faster and more clouds of coughing.
This research will be useful in determining the maximum number of people that can be accommodated in a hospital ward, for example. It will also help determine the minimum speed at which the air must be circulated to maintain freshness and reduce the possibility of infection, which can be applied to elevators, cinemas, cars, airplanes, restaurants and many other places.
Referencia: “Reducing chances of COVID-19 infection by a cough cloud in a closed space,” is authored by Amit Agrawal and Rajneesh Bhardwaj. Physics of Fluids on Oct. 20, 2020 (DOI: 10.1063/5.0029186). https://aip.scitation.org/doi/10.1063/5.0029186.
