Crowded, enclosed spaces are the most dangerous when it comes to picking up the coronavirus. This has prompted a surge in demand for mobile air purifiers in the hospitality industry, schools and private homes.

The manufacturers promise that the devices filter infectious aerosols from indoor air almost completely. This sounds like a good idea as an alternative to school lessons conducted with wide-open windows in sub-zero temperatures – but does the technology actually work?

Researchers in Germany have put the devices to the test. Joachim Curtius, professor of Experimental Atmospheric Research from the Goethe University in Frankfurt, and his team set up four air purifiers for a week in a classroom with teachers and 27 pupils. They concluded that some air purifiers can reduce the aerosol concentration in a classroom by 90% in half an hour.

"An air purifier reduces the number of aerosols so much that in an enclosed room the risk of infection by a highly infectious person, a super spreader, would also be very significantly reduced," Curtius said.

A team from the Army University in Munich came to a similar conclusion. They tested room air cleaners with a large volume flow and high-quality class H14 filters. Their study suggests that 99.995% of aerosol particles with a diameter of 0.1 to 0.3 micrometers are taken out of circulation.

"The results show that the aerosol concentration in a room with a size of 80 square meters (861 square feet) can be reduced to a low level everywhere within a short period of time," the team said. It found that purifiers are "a very sensible technical solution" to "greatly reduce" the risk of infection by aerosols.

However, researchers say anyone considering purchasing such a device needs to take a close look.

"There are different filter classes," explained professor Martin Kriegel from Berlin's Technical University. He added that high-quality filters work so well "that the filtered air is considered particle-free."

Some countries have already allocated funds for the purchase of air purification equipment for classrooms and other places where windows can't always be kept open.

However, health experts still warn that air purifiers in classrooms or at home are not a replacement for active ventilation from open windows.

Consumer groups are also worried that many will blindly trust the promises of manufacturers and say that users will need to make sure the performance matches the size of the room and the number of people and that filters need to be changed regularly.

Some manufacturers advertise additional measures against viruses, such as ozone or UV light, in addition to filter performance, but the German government, for example, advises against such devices "for both health and for safety reasons."