Air filtration systems: revolution for classrooms?

Air filtration systems: revolution for classrooms?

Since it became known that the coronavirus can also be passed on via indoor air, mobile air purifiers have become very popular. For a few thousand euros, caterers and event organizers, as well as private individuals or schools, can buy such a box.

The manufacturers promise that the devices almost completely filter out infectious aerosols from the room air. As an alternative to school lessons at the open window in minus temperatures, this sounds like a good idea. But do the goals live up to their promise??

Researchers from the goethe university frankfurt have put the test to the test. Joachim curtius, professor of experimental atmospheric research, and his team set up four air cleaners in a school classroom with teachers and 27 students for a week. The air cleaners had a simple pre-filter for coarse dust and lint as well as a class H13 HEPA filter and an activated carbon filter.

The conclusion: certain air purifiers can reduce the aerosol concentration in a classroom by 90 percent in half an hour. "An air purifier reduces the quantity of aerosols to such an extent that, in a closed room, the risk of infection from a highly infectious person, a superspreader, was also very significantly reduced," curtius summed up after a model calculation based on the measurement data.

The university of the german armed forces in munich came to a similar conclusion. The team around prof. Christian kahler from the institute for fluid mechanics and aerodynamics tested room air cleaners with coarse volume flow and high-quality class H14 filters. The device was equipped with a filter combination that ensures that aerosol particles with a diameter of 0.1 to 0.3 micrometers are separated from the ambient air by 99.995 percent.

"The results show that the aerosol concentration in a room of 80 square meters can be reduced to a low mab everywhere within a short time," write the authors of the analysis. They are "a very sensible technical solution" to "greatly reduce" the risk of infection from aerosols.

However, anyone considering purchasing such a device needs to take a close look. "There are different filter classes," explains prof. Martin kriegel, aersol researcher and head of the hermannrietschel institute at the technical university of berlin. "Classically, fine dust filters are installed in air-conditioning systems, which separate about 50 percent of the aerosols."In his opinion, higher-quality filters – kriegel calls them H13, H14 or ULPA – work so well that "the filtered air is considered particle-free.

Hesse’s state government has already announced that it will support school authorities with ten million euros for the purchase of air purification equipment. "These are to be purchased in particular for classrooms where it is not possible to provide sufficient ventilation because, for example, windows cannot be opened," explained education minister alexander lorz (CDU).

The federal environment agency takes a more critical view of the issue. The office’s indoor air hygiene commission advises on classic airing: "a supply of fresh air as high as possible is one of the most effective methods of removing potentially virus-containing aerosols from indoor spaces," it says in a detailed statement. Mobile air purifiers in classrooms or at home "could not replace active airing, but at best flank it in isolated cases".

Consumer advice centers also warn against blindly trusting manufacturers’ promises. You can make a lot of mistakes when buying. For example, the cleaning capacity had to match the size of the room and the number of people, and the filters had to be changed regularly. In addition to filter performance, some manufacturers advertise additional measures against viruses, such as ozone or UV light. The federal environment agency advises against such advice "for health reasons as well as for safety reasons.

The sars cov 2 virus can be inactivated with ultraviolet light – as shown, among other things, by a study conducted by the university of essen medicine. UV-C lamps can be used to decontaminate medical equipment, for example. However, the federal office for radiation protection warns: "since uv radiation can damage skin and eyes and has been proven to be carcinogenic, uv-c disinfection devices should generally only be used in such a way that no people are exposed to the radiation."

Nevertheless, some cities also use this technology, such as hanau in hesse. In august, the city entered into a "strategic partnership" with a local company that uses uv-C rays for disinfection. The devices suck in air, sterilize it by means of radiation and release it back into the room. The supplier emphasizes that the devices are designed in such a way that neither ozone is released nor radiation escapes.

Meanwhile, the search continues for alternative cleaning techniques. For example, two fraunhofer institutes (IKTS and ITEM) are working on a new system for disinfecting the air in closed rooms. Total electrochemical oxidation completely destroys organic substances such as viruses. "After successful development of a prototype," the institutes say, the aim is to "push ahead with market introduction as soon as possible".

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