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We all remember the advice frequently repeated during the COVID pandemic: maintain six feet of distance from every other human when waiting in a line to avoid transmitting the virus. While reasonable, the advice did not take into account the complicated fluid dynamics governing how the airborne particles actually travel through the air if people are also walking and stopping. Now, a team of researchers led by two undergraduate physics majors at the University of Massachusetts Amherst has modeled how aerosol plumes spread when people are waiting and walking in a line.

The results, published recently in , grew out of a question that many of us may have asked ourselves when standing in marked locations six-feet apart while waiting for a vaccine, to pay for groceries or to get a cup of coffee: what's the science behind six-feet of separation? If you are a physicist, you might even have asked yourself, "What is happening physically to the aerosol plumes we're all breathing out while waiting in a line, and is the six-foot guideline the best way to design a queue?"

To find answers to these questions, two UMass Amherst undergrads, Ruixi Lou and Milo Van Mooy, took the lead.

"We wanted to know how the aerosols we breathe out are transported, but it turns out this is very difficult to do in a real waiting line," says Lou, who is now a graduate student at the University of Chicago.

The ideal situation would be to have real humans standing in a real, moving line to test how their exhalations travel—a far-too risky proposition. Instead, Lou and Van Mooy decided to 3D-print a set of cylinders and human-shaped models and put them on a conveyor belt to see how the plumes moved. Their models "exhaled" colored dyes mimicking sneezes, coughs and regular breathing. They also ran computer simulations in collaboration with the group of Rodolfo Ostilla at the University of Cadiz, in Spain.

"What we found was really surprising," says Van Mooy.

Since warm air rises, there is a slight updraft surrounding our bodies—and so the team expected to see the aerosol plumes rising. But instead, they observed a "downwash" effect, where the simple act of walking and waiting in a line caused the plumes to sink.

Even more surprising was that, if the ambient temperature is close to our body temperature, as would be the case in a non-air-conditioned room in summer, those aerosols could be pushed toward the floor due to air currents. However, in a climate-controlled room, the difference in temperature between what we exhale and the ambient conditions are enough to drive those plumes aloft. If the temperature is in an intermediate range, it is quite possible that the aerosols can hover at just the right height for the next person in the line to inhale them as the line moves forward.

"Ultimately, there are no hard-and-fast rules about social distancing that will keep us safe or unsafe," says senior author Varghese Mathai, assistant professor of physics at UMass Amherst. "The fluid dynamics of air are marvelously complex and general intuition often misleads, even for something as simple as standing in a line. We need to take space and time into account as we come up with our public health guidelines."