Indeed, the sneezer emerges from a new study less as someone to be handed a Kleenex and comforted than as a smokestack belching infectious clouds into rooms, ventilation systems and beyond. The news is not about how sneezing spreads infectious mucous droplets, which everyone knows. It’s about how they travel and how far the sneeze spreads them: the pathogenic footprint of the sneeze.
The study, published in the “Journal of Fluid Mechanics” and described on MIT’s Web site is entitled “Violent expiratory events: on coughing and sneezing.”
It is co-written by Lydia Bourouiba, an assistant professor in MIT’s civil and environmental engineering department, John Bush, a professor of applied mathematics at MIT, and Eline Dehandschoewercker, a graduate student at ESPCI ParisTech.
Researchers had previously viewed the sneeze as a collection of individual mucous droplets. After an “achoo,” they thought, larger droplets flew farther than smaller ones because of momentum. That’s bad enough. And that would be true, says a summary of the study on MIT’s Web site, if “the trajectory of each droplet were unconnected to those around it.” But close observations show this is not the case.
Droplets are connected. Using high-speed imaging of coughs and sneezes, as well as laboratory simulations and mathematical modeling, the researchers came up with a new measure of the sneeze and its trajectory. The droplets form a gas cloud, they found — a “multiphase turbulent buoyant cloud” which “mixes with surrounding air before its payload of liquid droplets falls out, evaporates into solid residues, or both.”
Because the droplets are in a cloud, they stay suspended and travel further — particularly smaller ones, which travel up to 200 times farther than previously estimated. They can thus penetrate the room and ventilation systems more insidiously. “When you cough or sneeze, you see the droplets, or feel them if someone sneezes on you,” says Bush. “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.” “You can have ventilation contamination in a much more direct way than we would have expected originally,” says Bourouiba. The researchers suggest that architects and engineers may want to reexamine the design of workplaces and hospitals — and air circulation on airplanes — to reduce the chance that airborne pathogens will transmitted among people.