After more than nine months of the corona pandemic, SARS-CoV-2, COVID-19, and mask-wearing in our lives, you can now see them everywhere: the see-through shields. They are enjoying increasing popularity as an alternative to conventional mouth-nose coverings. Even if they do not really cover the mouth and nose … because there’s a gap of one to several centimeters between the face and the lower side of these shields. Nevertheless, according to the current infection control regulations in the German state of Bavaria, they are officially considered to be mouth and nose coverings.
Practical … yes, but…
The advantages of these types of masks are also obvious. You can see someone’s facial expressions – in contrast to the traditional mouth-nose protection, which covers the face from the chin to almost the eyes – and they are more convenient to wear and more comfortable. But do they also serve their purpose? Do they catch exhaled aerosols? Many experts are critical of this design. These transparent masks have now been tested for their effectiveness at the Munich University of Applied Sciences (HM). “We can measure exactly how the spread of aerosols is influenced by masks of this type with our test set-up,” says Prof. Dr. Christian Schwarzbauer. He is a Professor of Medical Technology and Medical Informatics at the Faculty of Applied Natural Sciences and Mechatronics at the University of Applied Sciences in Munich and the science director of this pilot study.
The researchers tested a transparent shield made by a Southern German manufacturer, which is very common and is increasingly being used in schools and daycare centers. The mask was tested under realistic and real-life conditions, says the scientist. Typical everyday situations were taken into account, such as those often found in schools, daycare centers, offices, and on public transport.
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Example 1: The test person sits on a chair and exhales through their nose
Figure 1 of the title picture shows the typical dispersion of aerosols. At first, they flow down along the body. Just like it is advertised by the manufacturer. However, shortly afterward the aerosol is deflected forward and then expands far into space in front of the test person. In other words, if you were standing or sitting directly opposite someone wearing one of these mouth and nose shields, you would get the full charge of aerosols in your face. Furthermore, it is clearly visible that the aerosol rises more and more upwards as it spreads and spreads further into the room in the process. This is facilitated by the typical temperature differences between exhaled air and room air.
Example 2: The test person walks through a room and coughs
In another scenario, the subject coughed several times while walking through a room. In this case, the aerosol cloud exhibited a particularly pronounced spread, which expanded further into the room immediately after coughing. The spread of the aerosols happened relatively quickly. People sitting on chairs in the background would have been exposed to a high aerosol concentration, especially around the facial and upper body areas.
The findings of this pilot study illustrate the problems of this shield design, according to the researchers. “There is no doubt that these shields are comfortable to wear, but they do not provide effective protection against infection,” says Schwarzbauer. Especially in closed rooms, such as schools, daycare centers, offices, or on public transport, the use of such masks is strongly discouraged, stresses Prof. Dr. med. Christian Hanshans, Professor of Medical Principles and Medical Technology at HM. He supervised this study as a physician and project engineer.
Title picture: Aerosol dispersion during nasal exhalation. The test subject is exhaling evenly through their nose without moving or speaking (Photo: Christian Schwarzbauer)
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