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A team of infectious disease specialists and environmental engineers at Université Claude Bernard Lyon's, École Centrale de Lyon, in France, and the University of Rome La Sapienza, in Italy, has found via experiments that the physical characteristics of exhaled droplets play a role in the transmission of infectious diseases.

In their study in the journal Â鶹ÒùÔºical Review Fluids, the group asked volunteers to breathe normally while silent, to speak normally, and to cough, while their exhausted droplet characteristics were measured.

Prior research has shown that many contagious diseases are spread via tiny droplets expelled through the nose and mouth of infected people as they breathe. These droplets are small enough to hang in the air long enough for others to inhale them, leading to infection. Prior research has also shown that some people can be characterized as superspreaders—for some reason, they infect more people than do others. Some suspect that this might be due to the size of droplets expelled by superspreaders or the distance the droplets travel once expelled.

To learn more about this possibility, the researchers asked 23 adult volunteers to breathe, talk, and cough both with and without a mask. As they did so, a laser sheet was fired across the front of their faces and a high-speed camera captured the size of the droplets expelled and how fast they traveled.

The research team found that while speaking or coughing, droplet size ranged from 2 to 60 micrometers. During normal breathing, they averaged between 2 and 8 micrometers. As expected, coughing made the droplets travel faster and farther, and tended to result in higher concentrations. There was more drifting of droplets during breathing exercises compared to speaking and coughing. And they found that wearing a mask blocked between 74% and 86% of droplets in general.

The researchers also found variations between people and even between different tests of the same person—enough variation to support the idea of droplet characteristics as a basis for superspreaders.

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