A long-standing estimate pins the velocity of a sneeze at roughly 100 meters per second, or 224 miles per hour, but that appears to be a gross exaggeration. The figure originates from a mid-century researcher named William Firth Wells, who analyzed the size of airborne droplets from a sneeze and then inferred the speed at which air must travel across a liquid surface to form them. Wells' figure has been repeated for many years but never directly tested in the lab. "I think people have been waiting for someone to come along and debunk it," says Julian Tang, a medical virologist at the Alberta Provincial Laboratory for Public Health in Edmonton.
For a study published this year, Tang and his colleagues used high-speed cameras to take pictures of pepper-induced sneezes from six volunteers. The team captured each sneeze by positioning the volunteers in front of a concave mirror and then shining an LED beam toward it. The warm air from the sneeze has a different refractive index than the cooler ambient air, so the reflected LED bends differently. The camera records the changes, and scientists can map the sneeze.
The study found that a sneeze's maximum velocity is nowhere near 100 meters per second but instead reaches a high of 4.5 meters per second, or 10 miles per hour. That's comparable to the velocity of air expelled by coughing—and a violent cough can push up a larger volume of air, which requires even more force. "The sneeze is really coming from your upper respiratory tract," Tang explains.
Tang, who did his study in Singapore, acknowledges that his numbers might have come out differently if he'd chosen different subjects. "All my data is from these rather slim Asian students," he says. "If somebody did this in the North American setting, with the bigger body frames that they have here, they might find higher velocities."
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This article originally appeared in the September 2013 issue of Popular Science.