Gas Clouds Help Infectious Droplets From Sneezes Travel Farther
April 8, 2014

Gas Clouds Help Infectious Droplets From Sneezes Travel Farther

Lawrence LeBlond for - Your Universe Online

It is common knowledge that when you sneeze, you are spreading potentially infectious droplets of germs and bacteria into the air around you. While most people have the common courtesy to cover their face when they cough and sneeze, those who don’t could be spreading disease farther than they think.

New research from MIT has found that the contents of your sneeze can travel much farther than previously estimated, especially when the droplets remain “in the cloud.”

This novel study, funded by the National Science Foundation and published in the Journal of Fluid Mechanics, shows that when we cough or sneeze, we tend to release a cloud of gas that can keep potentially infectious droplets aloft far longer than previously realized.

“When you cough or sneeze, you see the droplets, or feel them if someone sneezes on you,” study coauthor John Bush, a professor of applied mathematics at MIT, said in a statement. “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.”

The researchers, whom included MIT’s Lydia Bourouiba and Eline Dehandschoewercker, a graduate student at ESPCI ParisTech, found that smaller droplets that emerge in a cough or sneeze gas cloud can travel five to 200 times farther than they would if they simply moved as groups of unconnected particles – as was assumed in previous studies.

The fact that these droplets can stay airborne for so long, means that ventilation systems may be more prone to transmitting infectious particles than had been suspected. This may lead architects and engineers to re-examine the design of workplaces and hospitals to reduce the risks associated with a viral cougher or sneezer.

“You can have ventilation contamination in a much more direct way than we would have expected originally,” says Bourouiba, an assistant professor in MIT’s Department of Civil and Environmental Engineering.

For the study, the team used high-speed cameras to capture images of coughs and sneezes. They also used lab simulations and mathematical models to produce a new analysis of coughs and sneezes from a fluid-mechanics perspective.

Before this study, it was generally assumed that larger mucus droplets fly farther than the smaller ones, because they have more momentum. This would be true if the trajectory of each droplet were unconnected to those around it. But in the MIT observations this was not seen. Instead, the interactions of the droplets with the emitted gas cloud make all the difference in the trajectories of the droplets.

The team liken the gas cloud from a cough or sneeze to that of 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 said. “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.”

The team’s high-speed images confirmed that smaller droplets carry much farther than their larger counterparts. The team found that droplets about 100 micrometers – millionths of a meter – in diameter travel five times farther than previously estimated; droplets 10 micrometers in diameter can travel 200 times farther. Surprisingly, droplets that are under 50 micrometers in size can remain airborne long enough to reach ceiling ventilation units.

According to the team, a cough or sneeze is a “multiphase turbulent buoyant cloud.” They come to this conclusion due to the fact that the cloud mixes with the surrounding air before the payload of liquid droplets falls out, evaporates into solids, or both.

“The cloud entrains ambient air into it and continues to grow and mix,” Bourouiba says. “But as the cloud grows, it slows down, and so is less able to suspend the droplets within it. You thus cannot model this as isolated droplets moving ballistically.”

The MIT researchers said they are now developing additional tools and studies to extend their knowledge of sneeze science. For example, they want to look into how air conditioners either help or inhibit expelled pathogens.

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

“Bourouiba’s continuing research focuses on the fluid dynamics of fragmentation, or fluid breakup, which governs the formation of the pathogen-bearing droplets responsible for indoor transmission of respiratory and other infectious diseases. Her aim is to better understand the mechanisms underlying the epidemic patterns that occur in populations,” wrote Peter Dizikes for MIT News.

“We’re trying to rationalize the droplet size distribution resulting from the fluid breakup in the respiratory tract and exit of the mouth,” said Bourouiba. “That requires zooming in close to see precisely how these droplets are formed and ejected.”