How effective are room air filters?
It all depends on the darned aerosols. They do not move in space as predictably as we would like them to - namely sinking to the ground quickly. "In principle, they are nothing more than small mini-droplets, perhaps ten or twenty times smaller than a droplet we produce when we cough or sneeze," explains physician and Professor Ulrike Protzer, Director of the Institute of Virology at Helmholtz Zentrum München and Technische Universität München. These mini-droplets remain in the air for a long time - up to several hours - and it is mainly through them that SARS-CoV-2 is transmitted from person to person. This makes lessons at school or lectures at university, visits to nursing homes, restaurants and theaters a risk.
Ulrike Protzer and her team have been working flat out over the past few months on a model that can help to greatly reduce the concentration of aerosols in enclosed spaces. The scientists have achieved promising results with the "Next Generation Classroom": In collaboration with specialists from OHB Systems AG and HT Group, they have developed a design for a ventilation system which could soon make school lessons, events in seminar rooms or concert halls possible again with a substantially lower risk of infection.
Ventilation concept from aerospace and medical technology
The principle of this "classroom": a vertical ventilation system installed under the ceiling draws the rising exhaled air in the room upwards, filters it using so-called hepa filters and feeds it back in as fresh air at the bottom, on the floor. The strong vertical flow prevents the aerosols contained in the exhaled air from spreading uncontrollably. Low-pulse extraction is used to ensure that the people sitting in the room can conference, work and listen without being disturbed by excessive air flow or noise.
Hepa filters consist of many layers of very fine layers of synthetic fibers, glass fibers or cellulose and can filter tiny particles of up to 0.0003 millimeters from the air. Space technology (OHB Systems AG) and medical technology (HT Group) applications such as the filters are used wherever clean, pollution-free air is absolutely essential. At the German Aerospace Center (DLR) in Göttingen, fluid physicist Andreas Westhoff and his colleagues are testing the "Next Generation Classroom" of so-called thermal human models.
"The more we move around in a room, the more the aerosols shoot across, in the truest sense of the word," explains Andreas Westhoff. This is exactly what makes school lessons or lively daycare groups, but also waiting rooms in doctors' offices or parties in closed rooms a challenge in Corona times.
Room fans can reduce the spread of aerosols
The test dummies in the "classroom of the future" are human models wrapped with a heating wire that simulates the heat emission of a seated person. Some of the dummies were equipped with a breathing air simulator to recreate the real situation as closely as possible. "Each person's heat emission creates a lift current that has a significant effect on the flow in a room," says Andreas Westhoff. DLR also uses the dummies to simulate the spread of human spit particles. Initial findings on this are expected to be available soon.
"Investigation of the space ventilation concept has shown that with the help of the tested prototype, a stable flow from a seated person directly to the extraction system can be realized," explains the scientist. "The warm upflow generated by the human body supports this effect. An uncontrolled spread of virus and bacteria contaminated aerosols from the breathing air can thus be reduced under the conditions investigated".
For their measurements, the researchers not only simulate the situation in the classroom, but also typical scenarios in the doctor's waiting room, restaurant and cinema. Practical tests in schools and restaurants are also planned. To equip rooms accordingly, all that is needed is a retrofit kit for existing ventilation systems that is quite easy to install, says Westhoff.
Filter systems do not replace distance rules
Conclusion: This vertical displacement ventilation seems to be more effective and in winter at cold temperatures more practicable than frequent ventilation or mobile room air cleaners, which - depending on the size of the room - would have to be placed in larger numbers.
But mobile devices can also provide initial relief. Achim Dittler investigated this. The Professor of Mechanical Processing Technology at the Karlsruhe Institute of Technology (KIT) specializes in research on gas-particle systems. Together with colleagues, he has investigated the effectiveness of indoor air cleaners and says: "Our investigations have shown that indoor air cleaners greatly reduce the global particle concentration in closed rooms, in times when ventilation is not possible".
The fresh air requirement of a 22-member school class, for example, is 1,100 cubic meters per hour. High-performance units that can circulate several thousand cubic meters of air per hour cost around 1,000 euros or more.
However: "Good room air filters can only be a further component, but by no means a substitute for the AHA-L rules," says Dittler. Without distance, hygiene, an everyday mask and regular airing, it would not work. This is because wherever people without a protective mask and far away from a room air filter are standing or sitting particularly close together, the mobile room air filters could do much less to protect against possible exposure to viruses in the room. "Mobile room air filters also do not remove CO2 from the air we breathe. Therefore, a sufficiently high fresh air supply must still be ensured".
The practical test of the "Next Generation Classroom" is currently taking place in a seminar room at the TU Munich. "We have chosen a room measuring around 50 square meters, which is currently used by about a dozen employees during breaks," explains virologist Ulrike Protzer. "They also take meals there and do not wear a mask. An endurance test, then. And if it works with a dozen people, it may soon be possible to transfer the concept to large school classes.
KIT Researchers Develop Room Air Filter Aerobuster
Researchers of the Karlsruhe Institute of Technology (KIT) have developed an apparatus that can take pathogens from the ambient air and inactivate them. According to the scientists, the so-called aerobuster is simple, compact and effective. Simulations of aerosol movements in an average classroom with 20 pupils show that the aerobuster can significantly reduce the concentration of active viruses in the indoor air and thus permanently reduce the risk of infection. This makes the device suitable for use in schools, kindergartens, university lecture halls, hospitals, medical practices, offices or restaurants.