Face shields were touted at the onset of the coronavirus pandemic as the 'friendlier' alternative to face masks. And while it is true wearing a face shield may ensure your smile isn't obscured, this form of personal protective equipment is no match for Covid-19.
To understand why, you need to watch this video.
Made by a team of engineers from Japan, the video shows face shields are ineffective at protecting a wearer from the minuscule aerosol droplets sprayed from a person's mouth and nose when they sneeze. The video, and the study around it, were published Tuesday in the journal Physics of Fluids.
What's new — When it comes to modes of infection, a sneeze or cough is a pretty powerful vector. If not caught, a sneeze can expel thousands of microdroplets from your mouth and nose at up to 100 miles per hour. And these droplets don't all fly straight or drop off to the ground, the authors explain.
Rather, the velocity of a sneeze, as well as temperature differences in the air, can cause the droplets to form what scientists call a "sneeze vortex," or vortex rings.
"A vortex ring is a donut-shaped vortex that is generated by an instantaneous ejection of fluid from a circular orifice," Fujio Akagi, the study's first author and an engineer at Fukuoka University, explains in a statement. "This resembles bubble rings made by dolphins."
"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," Akagi says. "If this arrival time is synchronized with inhalation, the shield wearer will inhale the droplets."
In other words, if you are wearing a face shield and someone sneezes on you, then you best hope they don't have coronavirus or any other respiratory infection, because it is coming for you.
How they did it — There's a common misunderstanding that fluid dynamics describe only materials that flow — like water. But for scientists, fluid dynamics apply to air as well.
To test how sneeze vortexes may be affecting face-shield wearers, the scientists designed a simulation experiment.
The real environment of a hospital is full of thousands of unknown or changing variables that could affect how disease and infection are actually spread, but for the sake of their simulation, the team narrowed it down to three critical factors — distance, breath, and fit.
The researchers assumed a frontline worker wearing only a face shield would stand approximately 1 meter away from an infectious, mask and shield-less sneezer. The frontline worker was also assumed to be primarily breathing through their nose, and the distance between their face and their shield was 25 millimeters.
The team then simulated how sneezes might traverse the meter distance, and sneak underneath a face shield.
What they found — In the simulations, a sneeze heading toward the face shield actually split into a number of different vortex rings, with some moving upwards toward the top of the mask, and others heading towards the wearer's chin and chest.
The differing directions may be explained by a buoyant force created by the difference in temperature of the room and the sneeze, the researchers explain.
Why it matters — After analyzing the potential infection risk of the vortex rings, the researchers estimate 4.4 percent of expelled droplets from the sneeze made their way under the face shield and to the wearer's nose — posing the risk of infection if inhaled.
The findings jibe with previous research into the relative benefits and drawbacks of certain kinds of face coverings. A study published earlier this year also demonstrated similar weaknesses with face shields when the roles are reversed, too — in other words, when the face shield wearer is infectious.
The Inverse Analysis — To lower the risk of infection when wearing a face shield, the solution is simple: wear a face mask underneath it. But the team behind this paper also plan to use these weaknesses to design a new, more effective face shield which can protect against infection from diseases like coronavirus.
"We are currently developing and demonstrating several improved shields," Akagi said. "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."
Abstract: A flow analysis around face shield was performed to examine the risk of virus infection when a medical worker wearing a face shield is exposed to a patient's sneeze from the front. We ensured a space between the shield surface and the face of the human model to imitate the most popularly used face shields. In the present simulation, a large eddy simulation was conducted to simulate the vortex structure generated by the sneezing flow near the face shield. It was confirmed that the airflow in the space between the face shield and the face was observed to vary with human respiration. The high-velocity flow created by sneezing or coughing generates vortex ring structures, which gradually become unstable and deform in three dimensions. Vortex rings reach the top and bottom edges of the shield and form a high-velocity entrainment flow. It is suggested that vortex rings capture small-sized particles, i.e., sneezing droplets and aerosols, and transport them to the top and bottom edges of the face shield because vortex rings have the ability to transport microparticles. It was also confirmed that some particles (in this simulation, 4.4% of the released droplets) entered the inside of the face shield and reached in the vicinity of the nose. This indicates that a medical worker wearing a face shield may inhale the transported droplets or aerosol if the time when the vortex rings reach the face shield is synchronized with the inhalation period of breathing.