The 6-foot rule dates back to the late 1800s, when German scientist Carl Flügge found that pathogens were present in large droplets expelled from the nose and mouth. Most of these droplets fell to the ground within 3 to 6 feet of the person with an infection.
In the 1940s, advances in photography enabled researchers to capture images of these expiratory droplets being sprayed when a person sneezed, coughed, or talked.
Other studies around that time found that large particles quickly fell to the ground near the person expelling them, reinforcing the 6-foot rule — in spite of limitations of the accuracy of these early studies.
These studies tended to group expiratory droplets into two categories: large and small. Scientists thought large droplets would fall quickly to the ground and small droplets would evaporate before they got very far, unless pushed by another airflow.
However, “in the last 90 years, we’ve learned a lot more about what is actually going on … when you speak or cough or sneeze,” said Jesse Capecelatro, PhD, an assistant professor of mechanical engineering at the University of Michigan in Ann Arbor, who wasn’t involved in the new study.
He says many factors can affect how far droplets are spread. If the humidity is low, bigger droplets can shrink and stay in the air longer. Wind outside or ventilation inside can also carry droplets farther away.
“This whole idea that there’s this 6-foot perimeter, and if you’re one inch beyond it then you’re safe, really doesn’t make much sense,” said Capecelatro.
In a recent systematic review, 8 of 10 studies reviewed found that expiratory droplets could travel more than 6 feet away from those with infections, and in some cases up to 26 feet.
Research with the coronavirus that causes COVID-19 supports the idea that 6 feet may not always be enough. In one study, researchers found the transmission distance of the virus may be up to 13 feet. In another, they detected it on multiple air vents in a patient’s room.
There is also the case of the choir practice in Washington state in March, where one person with COVID-19 symptoms transmitted the virus to at least 32 other singers. The forcefulness of the exhalation while singing is thought to have helped the virus spread, but other factors such as sharing snacks may have been involved.