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Corona pandemic at schools: air filters should reduce risk of infection – not all devices are suitable

Air purifiers against corona: reducing the risk of infection?

Researchers at Goethe-Unversity Frankfurt have investigated various mobile air purifiers. To do this, the team led by Joachim Curtius,

Professor of Experimental Atmospheric Research, set up four air purifiers in a school class of 27 students for a week. The air purifiers had a simple pre-filter for coarse dust and lint, as well as a class H13 HEPA filter and an activated carbon filter.

The result: air purifiers can reduce aerosol concentrations in a classroom by 90 percent in half an hour. “An air purifier reduces the amount of aerosols so much that in a closed room even the risk of infection by a highly infectious person, a superspreader, would be reduced very significantly,” Joachim Curtius sums up after a model calculation based on the measurement data. (Colds and Corona: Regular ventilation in winter helps prevent infections)

Air purifier against corona: sensible technical solution

A study by the German Federal Armed Forces in Munich also came to a similar conclusion. In the Munich study, Christian Kähler of the Institute of Fluid Mechanics and Aerodynamics tested a device with a large volume flow and high-quality H14 filters. According to the study, the room filter had a filter combination that ensures that aerosol particles with a diameter of 0.1 to 0.3 micrometers are separated from the room air by 99.995 percent. (Corona in asthma, COPD and diabetes: How dangerous is the condition for high-risk patients?)

“The results show that aerosol concentrations in a room 80 square meters in size can be reduced to low levels anywhere within a short period of time,” the authors conclude. Accordingly, certain air purifiers are a sensible technical solution for greatly reducing infection risks  from aerosols.

Air purifier against Corona: It depends on the filter

But not all air purifiers are equally suitable. Who plays with the idea of acquiring such a room air filter, should therefore inform themselves sufficiently before the purchase. For example, filters installed in ventilation systems tend to be less suitable. This is because the classic fine dust filters only separate about 50 percent of the aerosols.Better are so-called H13, H14 and ULPA filters. According to aerosol researcher and head of the Hermann Rietschel Institute at the Technical University of Berlin, Martin Kriegel, these filters clean the air almost particle-free.

Air purifiers against corona: also suitable for schools?

The filters could also be a possible alternative for schools. Hesse’s state government, for example, has already announced that it will support school authorities with ten million euros for the purchase of air purification devices. “These are to be purchased in particular for classrooms where it is not possible to ventilate sufficiently because, for example, windows cannot be opened,” explained Education Minister Alexander Lorz (CDU).

Experts from the Federal Environment Agency take a rather critical view. Hence, the Commission for Indoor Air Hygiene continues to advise classic ventilation. In a detailed statement, it says: “The highest possible supply of fresh air is one of the most effective methods of removing potentially virus-containing aerosols  from indoor spaces.” The conclusion: mobile air purifiers in classrooms or at home cannot replace active ventilation, but at best act as a supplement in individual cases. (Ventilation against coronaviruses: Can air purifiers reduce the risk of infection?)

Mit Luftreinigern gegen die Corona-Pandemie: Sinnvolle Ergänzung zu regelmäßigem Lüften in Schulen? | Leben & Alltag (

Experts take a critical view of air purifiers

The cold half of the year is approaching and with it, air purifiers are being massively advertised. They are supposed to rid the indoor air of corona viruses. Experts say: Active ventilation is usually a better alternative.

Expensive model of air purifiers for private use are supposed to remove up to 99.99 percent of possible corona viruses from indoor air. Such devices are just massively advertised. The consumer magazine “Super.Markt” of the Rundfunk Berlin-Brandenburg (rbb) has scrutinized four products and asked experts about them. These doubt the sense of such an investment and point to crucial gaps.

Thus it is possible to achieve a purification from Corona viruses in the air with built-in high quality filters. The crucial factor, however, is the amount of air that a single device can filter. Professor Martin Kriegel of the Technical University of Berlin , who has been studying the spread of airborne particles for years and, since Corona, also studies viruses in the air we breathe, takes a skeptical view: “One hundred cubic meters is nothing. If the device can do 100 cubic meters, then maybe I can get 0.5 percent change in overall risk, but not really make an impact.” Even if 400 cubic meters are cleaned, thorough and regular ventilation has the same effect as expensive devices, whose prices vary from several hundred to over 1,000 euros.

Low ionizer effect

Other devices advertise with a so-called ionizer. This is supposed to ensure that the air particles attract each other, clump together and thus become larger. This is supposed to make them easier to filter or cause them to sink to the ground more quickly. Professor Martin Kriegel is skeptical about this, too: “This effect, according to our studies here in our institute, is so small that it is actually negligible.”

Other manufacturers advertise UV-C rays for disinfection. That UV-C light neutralizes viruses has been known for a long time and is used in the disinfection of surfaces or to sterilize swimming pools. When cleaning air in indoor areas, however, it should be noted that UV-C rays are harmful to health when exposed directly to the skin and eyes. Professor Heinz-Jörn Moriske of the Federal Environment Agency therefore emphasizes “…that it must be prevented that any children play around with the devices and come into contact with the UV lamp.”

Conclusion of the rbb research

If you want to buy an air purifier, you have to pay attention to high-quality filters and the largest possible amount of air to be processed. Under this aspect none of the devices tested by the rbb could convince – despite the high purchase costs. Professor Moriske of the Federal Environment Agency sums up: “These air cleaners make sense as a supplement or in rooms where I simply cannot ventilate via windows… In individual cases, this can make sense. But please do not use it as a substitute for active ventilation.”

Experten beurteilen Luftreiniger kritisch • – Tägliche News aus Medizin, Healthcare & IT