Air filters as a weapon against corona
Since it has been known that the coronavirus can also be passed on via the indoor air, mobile air purifiers have been very popular. For a few thousand euros, restaurateurs and event organizers, but also private individuals or schools, can buy such a box. The manufacturers promise that the devices will almost completely filter out infectious aerosols from the room air. As an alternative to school lessons at the open window in freezing temperatures, that sounds like a good idea. But do the devices keep what they promise?
Researchers at Goethe University Frankfurt have put the test to the test. Joachim Curtius, Professor for Experimental Atmospheric Research, and his team set up four air purifiers for a week in a school class with teachers and 27 students. The air purifiers had a simple pre-filter for coarse dust and lint as well as a HEPA filter of class H13 and an activated carbon filter.
The bottom line: certain air purifiers can reduce the aerosol concentration 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 the risk of infection from a highly infectious person, a super spreader, would also be reduced very significantly,” said Curtius after a model calculation based on the measurement data.
The University of the Federal Armed Forces in Munich came to a similar conclusion. The team led by Prof. Christian Kähler from the Institute for Fluid Mechanics and Aerodynamics has tested room air purifiers with a large volume flow and high-quality filters of class H14. The device therefore had a filter combination that ensures that 99.995 percent of aerosol particles with a diameter of 0.1 to 0.3 micrometers are separated from the room air.
“The results show that the aerosol concentration in a room with a size of 80 square meters can be reduced to a low level within a short period of time,” the authors of the analysis write. They are “a very useful technical solution” to “greatly reduce” the risk of infection from aerosols.
However, if you are thinking of purchasing such a device, you have to look carefully. “There are different filter classes,” explains Prof. Martin Kriegel, aerosol researcher and head of the Hermann Rietschel Institute at the TU Berlin. “Fine dust filters, which separate around 50 percent of the aerosols, are traditionally built into ventilation systems.” In his opinion, higher-quality filters – Kriegel calls 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 the school authorities with ten million euros in the purchase of air cleaning devices. “These are to be purchased especially for classrooms where it is not possible to ventilate adequately because, for example, windows cannot be opened,” said Minister of Education Alexander Lorz (CDU).
The Federal Environment Agency sees the issue more critically. The Office’s Indoor Air Hygiene Commission advises classic ventilation: “The highest possible supply of fresh air is one of the most effective methods of removing aerosols that may contain viruses from indoor spaces,” says a detailed statement. Mobile air purifiers in classrooms or at home could “not replace active ventilation, but at best support it in individual cases”. (dpa)
