New research suggests coughs and sneezes produce gas clouds that allow germs to travel much further than researchers previously believed was possible.
"When you cough or sneeze, you see the droplets, or feel them if someone sneezes on you," John Bush, a professor of applied mathematics at Massachusetts Institute of Technology (MIT) , and co-author of a new paper on the subject, said in a news release. "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 believe the droplets travel between five and 200 times further than they would if traveling in disconnected particles. This finding could mean ventilation systems are more prone to spreading germs than was previously thought,
"You can have ventilation contamination in a much more direct way than we would have expected originally," Lydia Bourouiba, an assistant professor in MIT's Department of Civil and Environmental Engineering, and another co-author of the study said in the news release. The researchers suggest that architects rethink modern ventilation systems to reduce the spread of these airborne pathogens.
The researchers made their findings using high-speed images of coughs and sneezes couples with mathematical modeling. The researchers determined a cough or sneeze resembles "a puff emerging from a haystack."
"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 noticed that droplets measuring 100 micrometers traveled five times as fast as was previously expected while those measuring 10 micrometers could travel up to 200 times further.
"The cloud entrains ambient air into it and continues to grow and mix," Bourouiba said. "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 researchers hope to gain more insight into this subject in the future.
"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."
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