Coughs and sneezes float much farther than you think

Smaller droplets that emerge in a cough or sneeze may travel five to 200 times further than previously believed, MIT scientists say.
Researchers from Massachusetts Institute of Technology (MIT) found that coughs and sneezes have associated gas clouds that keep their potentially infectious droplets aloft over much greater distances than previously realised.
"When you cough or sneeze, you see the droplets, or feel them if someone sneezes on you," said John Bush, a professor of applied mathematics at MIT, and co-author of the new study.
"But you don't see the cloud, the invisible gas phase. The influence of this gas cloud is to extend the range of the individual droplets, particularly the small ones," Bush said.

The study found that the smaller droplets that emerge in a cough or sneeze may travel five to 200 times further than they would if those droplets simply moved as groups of unconnected particles - which is what previous estimates had assumed.
The tendency of these droplets to stay airborne, resuspended by gas clouds, means that ventilation systems may be more prone to transmitting potentially infectious particles than had been suspected.

"You can have ventilation contamination in a much more direct way than we would have expected originally," said Lydia Bourouiba, an assistant professor in MIT's Department of Civil and Environmental Engineering, and another co-author of the study.

The researchers used high-speed imaging of coughs and sneezes, as well as laboratory simulations and mathematical modelling, to produce a new analysis of coughs and sneezes from a fluid-mechanics perspective.
Their conclusions upend some prior thinking on the subject. For instance: Researchers had previously assumed that larger mucus droplets fly farther than smaller ones, because they have more momentum, classically defined as mass times velocity.
That would be true if the trajectory of each droplet were unconnected to those around it. But close observations show this is not the case; the interactions of the droplets with the gas cloud make all the difference in their trajectories.

Specifically, the study found that droplets 100 micrometres in diameter travel five times farther than previously estimated, while droplets 10 micrometres in diameter travel 200 times farther.
Droplets less than 50 micrometres in size can frequently remain airborne long enough to reach ceiling ventilation units, researchers said.
The study was published in the Journal of Fluid Mechanics.