Researchers at the Institute of Physics have conducted experiments with medical face masks made by five manufacturers, trying to destroy the virus using three methods: machine washing at 60 degrees, keeping the mask in boiling water for five minutes and keeping the mask in hot water for five minutes.

Each method was applied 10 times, and the masks were allowed to dry for 24 hours between each heat treatment. After 10 times, the masks' filtration efficiency and breathability were measured.

Professor Heikki Junninen, head of the Laboratory of Environmental Physics of the University of Tartu Institute of Physics, said the results of the study show there is no reason to fear that the quality of masks would be significantly decreased by heat treatment.

There are roughly three types of face masks: personal protective equipment which are subject to the strictest requirements, medical "blue" masks and community masks, which count as all other types, including homemade masks.

After applying a virus-destroying re-use method, the size of particles that passed through the mask was examined. This was done by experiments with particles of three sizes: 100 nm particles that are the smallest, about the size of viruses; 300 nm particles that pass through the mask best, and 3000 nm particles that represent very small droplets, the filtration size required for certified community masks. The purpose of the experiment was to determine whether the filtration efficiency of medical masks had changed after heat treatment.

The results showed that the quality of disposable masks changes very little after heat treatment. The medical mask may be worn again after keeping it in hot water for five minutes and the hot water procedure may be repeated at least ten times. The breathability of the masks did not change significantly.

Even after a tenfold treatment, the quality of the masks remained better than that of several reusable cloth masks that do not have a filter layer. The properties of reusable masks containing a separate filtering layer are similar to those of the disposable medical masks now tested.

Junninen said that based on the results of this study, it is possible to reduce the number of masks which are thrown away which do not biodegrade.

"Other than that, the quality-price ratio of medical masks is by far the best: they are very efficient in filtering particles of all sizes, they have very good breathability and they are cheap," he said.

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