MIT: Coughs and sneezes can traverse ventilation systems

The State Column, Ian Lang | April 09, 2014

Germ-riddled particles can travel up to 200 times further than previously thought

You would think that, when coughing or sneezing, even unobstructed ones will only make it so far. Maybe just across the room, or to your cubicle mate if you aim it juuussst so. Think again. According to researchers at MIT, the tiny droplets released during a cough or sneeze may travel anywhere from five to 200 times further than previously believed. That means that among other things, rather than supplying you with fresh air, your office ventilation may actually help spread germs.

Prior to the study, prevailing wisdom held that the goop released from a cough or sneeze came out in droplets and moved as unconnected groups of particles. They do, in fact, but that’s not the whole story. Using high-speed imaging of coughs and sneezes, they discovered that there’s more going on beyond what you can see or feel when someone sneezes or coughs near (or on) you.

“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,” says John Bush, a professor of applied mathematics at MIT, and co-author of a new paper on the subject. “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.”

When the microparticles remain suspended in cloud form, that means ventilation systems spread them rather than contain them. With this new knowledge, engineers might want to consider re-designing everything, from HVAC systems in hospitals and offices to the way air circulates in airplanes. The goal, obviously, is to reduce the instance of human-transmitted illnesses.

“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.”

The research might have a much bigger impact beyond the number of sick days taken in your office – by continuing to apply principles of fluid dynamics, the researchers hope to to better understand the mechanisms underlying the epidemic patterns that occur in populations.


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