When someone coughs or sneezes, just how far do bystanders have to be to avoid the germy spray? Lydia Bourouiba, an associate professor directing the Fluid Dynamics of Disease Transmission Laboratory at MIT, says it’s farther than once thought. 

Through experiments in the lab and clinical environment, she and her team found that what we’re dealing with isn’t a spray of individual droplets that quickly fall to the ground and evaporate; instead, it’s a cloud of hot, moist air that traps droplets of different sizes together, propelling them much farther than any one would be able to travel on its own. A cough can transmit droplets up to 13 to 16 feet, while a sneeze can eject them up to 26 feet. Surrounding air conditions can further disperse the residual droplets in upper levels of rooms.

“A surgical mask is not protective against inhalation of a pathogen from the cloud,” says Bourouiba, who has published a paper on the implications of her work in the Journal of the American Medical Association. “For an infected patient wearing it, it can contain some of the forward ejecta from coughs or sneezes, but these are very violent ejections and masks are completely open on all sides.” She recommends that health-care workers wear a respirator whenever possible.