Face Shields Are No Match For The Peculiar Physics Of A Sneeze

Face shields are a popular alternative for people who, for various reasons, can’t wear a mask. By design, they are not as effective as a mask as they don’t filter the air breathed in and out. New research shows that even a sneeze directly aimed at a face shield would not be halted due to the peculiarity of fluid dynamics… or in this case sneeze physics.

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When a person sneezes, air and droplets are released at high speed in a straight line. But that’s not all. The sneeze also produces a vortex ring – just picture your sneeze being surrounded by a donut of turbulent air.

Published in Physics of Fluids, researchers have simulated what happens if a sneeze is directed at a person wearing a shield from 1 meter (3.3 feet) away. The data showed that between 0.5 and 1 second after the start of the sneeze, droplets reach the shield and get near the face thanks to the vortex ring.

"A vortex ring is a donut-shaped vortex that is generated by an instantaneous ejection of fluid from a circular orifice," lead author Fujio Akagi, from Fukuoka University, said in a statement. "The vortex rings generated by the sneeze capture the microscopic droplets within the sneeze and transport them to the top and bottom edges of the face shield. If this arrival time is synchronized with inhalation, the shield wearer will inhale the droplets."

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While the setup is an idealized case, the finding shows the importance of considering the complexity of sneeze turbulence when it comes to the spread of viruses. Droplets are spread by both the high-velocity sneeze and the vortex ring, so they can easily get inside the shield.

Face shields alone are not highly effective at stopping the spread of Covid-19 and other airborne pathogens. When possible, they should be worn alongside a mask. However, the team hopes that understanding the physics of sneezes might help them design better face shields.

"We are currently developing and demonstrating several improved shields," added Akagi. "We want to contribute to keeping people safe from infection, and believe that one day in the near future, medical workers will be able to prevent infection using only a face shield and a regular mask or, ideally, with only a face shield."

As we wait for a vaccine, remember to wear a mask, keep a safe distance from others, and regularly wash your hands. These small gestures are saving lives.

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