Coughs and Sneezes Float Farther Than You Think

MIT researchers say that most ventilation systems may not be up to par.

Dr. Oz once filmed an episode where he had a machine that mimicked the effect of a sneeze with dyed powder. He placed a dummy several feet from the sneeze machine in order to show just how far that sneeze really traveled. It traveled really, really far. The mannequin was covered in powder. The audience gasped. It was gross. Now, it turns out, that Dr. Oz was correct. And it’s backed up by MIT science.

MIT researchers suggest that the next time you feel a sneeze coming on, raise your elbow to cover up that “multiphase turbulent buoyant cloud” you’re about to expel. 

In a new study, published in the Journal of Fluid Mechanics, researchers report that coughs and sneezes have “associated gas clouds that keep their potentially infectious droplets aloft over much greater distances than previously realized.” The study found that the smaller droplets that materialize in a cough or sneeze may travel five to 200 times farther than what was previously assumed. Basically, the study suggests that ventilation systems, (such as the one in your office building) may be more prone to transmitting potentially infectious particles than had been previously thought, due to the tendency of these droplets to stay airborne, resuspended by gas clouds.

“When you cough or sneeze, you see the droplets, or feel them if someone sneezes on you,” says John Bush, a professor of applied mathematics at MIT, and co-author of the new paper. “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.”

According to the study:

The researchers used high-speed imaging of coughs and sneezes, as well as laboratory simulations and mathematical modeling, 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. Indeed, the cough or sneeze resembles, say, a puff emerging from a smokestack.

“If you ignored the presence of the gas cloud, your first guess would be that larger drops go farther than the smaller ones, and travel at most a couple of meters,” Bush says. “But by elucidating the dynamics of the gas cloud, we have shown that there’s a circulation within the cloud — the smaller drops can be swept around and resuspended by the eddies within a cloud, and so settle more slowly. Basically, small drops can be carried a great distance by this gas cloud while the larger drops fall out. So you have a reversal in the dependence of range on size.”

Specifically, the study found:

Droplets 100 micrometers — or millionths of a meter — in diameter travel five times farther than previously estimated, while droplets 10 micrometers in diameter travel 200 times farther. Droplets less than 50 micrometers in size can frequently remain airborne long enough to reach ceiling ventilation units.

The study’s authors suggest that with this in mind, architects and engineers may want to re-examine the design of workplaces and hospitals, or air circulation on airplanes, in order to reduce the chances of airborne pathogens being transmitted among people.

In the future, MIT researchers hope to better estimate the reach of a given expelled pathogen.

“An important feature to characterize is the pathogen footprint,” Bush says. “Where does the pathogen actually go? The answer has changed dramatically as a result of our revised physical picture.”

Melissa Malamut
Melissa Malamut Melissa Malamut, Senior Editor, Health, at Boston Magazine

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