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How face masks and ventilation of enclosed spaces can protect against infectious aerosols

KIT researchers are investigating filters, their mode of operation and the dispersion of particles in air

Even though the first German federal states are discussing the abolition of the obligation to wear face masks in shops – at present, mouth and nose protection is part of everyday life in Germany. Researchers now know that aerosols play an important role in the transmission of the COVID-19 pathogen. KIT researchers are investigating the formation, detection, distribution and separation of gas-borne particles and droplets as well as the effectiveness of filters. Using simulations, they analyze aerosols, their distribution and deposition in rooms, filters and the human respiratory tract.

“Masks offer protection,” says Professor Achim Dittler from the KIT Institute of Mechanical Process Engineering and Mechanics (MVM). The expert for gas-particle systems deals with filters and their mode of action. Years ago, a project work carried out at his institute examined various reusable and non-reusable FFP2 class respiratory masks for separation efficiency and breathing resistance. “Such masks protect the wearer and – if they do not have an exhalation valve – also the people around the wearer from particles and droplets,” explains Dittler. The project work showed that the filter performance of the masks tested more than fulfilled the applicable standard. In the corona pandemic, in addition to particle-filtering masks of the FFP2 and FFP3 classes, surgical masks and community or everyday masks made of commercially available materials are used to cover the mouth and nose. “Any community mask is better than no mask at all,” emphasizes Mathias J. Krause, who heads the Lattice Boltzmann Research Group at KIT and uses simulations to investigate, among other things, the spread of aerosols.

Although surgical masks and community masks are mainly intended to protect the people around the wearer, they also offer a certain degree of self-protection, albeit to a much lesser extent than FFP2 and FFP3 masks. The protective effect of all masks is based on deposition: particles hit the fibres of the filter material and adhere to it. The deposition effect is complex and has not yet been researched for the novel coronavirus in all filter materials. It depends, among other things, on the size of the droplets in the aerosol containing the SARS-CoV-2 virus, the structure of the filter material and the flow velocity of the air. Dittler and Krause are sceptical about the plastic face shields also used in the corona pandemic. “Such visors only hold back the large aerosol droplets and reduce the ejection range of aerosols from the exhaled air – they serve primarily as a spit shield, so to speak,” explains Dittler. “However, due to the flow conditions, fine droplets are able to pass through the sometimes quite wide gap between the shield and the face into the ambient air”.

Aerosols from breathing air are a mixture of suspended particles of different sizes – mainly water droplets – and a gas mixture. According to recent findings, these suspended particles play an important role in the transmission of SARS-CoV-2. While larger droplets, such as those ejected by humans when coughing or sneezing, soon sink to the ground, the smaller particles can remain in the air for a long time after exhalation or speaking. “These particles hardly sediment at all, but circulate in space for a very long time,” explains Mathias J. Krause. “This can be compared to cigarette smoke, which can still be smelled even if the smoker is standing a distance away in an unfavorable direction of flow or has even left the space.

The two researchers consider the distance of 1.5 meters, which is generally recommended as protection against infection with SARS-CoV-2, to be too short under certain circumstances, given the accumulation and distribution of aerosols in enclosed spaces. However, how long the viruses remain infectious in aerosols has not yet been conclusively clarified. In any case, concentration and humidity also play a role, as Krause explains: “The higher the concentration, the greater the likelihood of infection with the virus”. Since aerosols move with the air flow, regular and thorough ventilation contributes significantly to infection control in closed rooms. “Ventilation mixes the air and reduces the local virus concentration. Furthermore, ventilation gives the aerosols a chance to escape to the outside. The fresh air in turn has a diluting effect and reduces the concentration of viruses,” Mathias J. Krause continues.

Nevertheless masks remain important. Achim Dittler admits that covering the mouth and nose can also put a strain on the wearer – how much depends on the type of mask: “The wearer has to overcome the flow resistance in front of his face, which means breathing becomes more strenuous. In addition, the flow resistance changes with the length of time the mask is worn – for example, if the mask is saturated with liquid. The masks should therefore be changed regularly. This is very important for correct use. Masks must also fit correctly: They must cover the mouth and nose completely and fit snugly so that the air flows trough the filter medium rather than around it.” Mathias J. Krause adds: “If used improperly, masks also provide a breeding ground for bacteria and fungi. Therefore, the following applies: Do not touch the mask when wearing it, change the mask in time and wash it thoroughly.

Professor Achim Dittler is a member of the Institute Management at the KIT Institute of Mechanical Process Engineering and Mechanics (MVM) where he heads the Gas Particle Systems Group. Further information on his research: https://www.mvm.kit.edu/Mitarbeiter_GPS_4549.php

Dr. Mathias J. Krause heads the Lattice Boltzmann Research Group (LBRG) at KIT. Further information on respiratory simulations: https://www.humanairways.org

Original German publication: https://www.kit.edu/kit/26899.php