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Material test for mouthguards

Cotton, paper towels, vacuum cleaner fleece?

14. April 2020, 15:14 Uhr   |  Nicole Wörner

Cotton, paper towels, vacuum cleaner fleece?
© Dom Jack

Which face mask material is best suited?

Scientists at the Max Planck Institute for Chemistry have tested how well different substances filter particles from the air - and are thus suitable as a possible corona mouthguard. Almost all the materials tested performed quite well.

The National Academy of Sciences Leopoldina recommends masks for mouth and nose as protection against the coronavirus - also self-sewn. But which material is suitable and which is not?

In addition to cotton and paper towels, researchers at the Max Planck Institute for Chemistry have tested coffee filters and fleeces from vacuum cleaner bags to see how well they separate particles and thus potentially infectious droplets.

"We found that all of the filter materials we examined were very efficient at separating large particles of five micrometers and larger," says Frank Drewnick, head of the research group in the Particle Chemistry Department. "The efficiency is usually 90 percent and above." This would enable the materials to trap a large proportion of the droplets that play the major role in corona virus infection. Because, according to everything currently known, these droplets are significantly larger than a few microns.

However, the material used for a face mask not only has to efficiently trap particles and droplets, it is also important how easily you can breathe through it. To test this, atmospheric researchers measured the pressure drop of the air as it flows through the filter fabric. It is best to keep it low - while at the same time achieving a high filter effect. The tests showed that the pressure drop values for double-layer firm cotton fabric and a combination of vacuum cleaner bag material and cotton fabric were slightly worse than those for professional surgical masks or a combination of jersey and beaver fabric. Although larger particles are well separated on a coffee filter, they are hardly permeable to air and therefore not very practical.

It all depends on use

"Our data makes no statement about how well a face mask actually protects. However, they may help in the selection of suitable filter materials for self-sewn masks," says Frank Drewnick, who plans to test other materials and examine larger samples. He stresses that the effectiveness of a mask depends on many factors. Not only correct handling is crucial, but also what proportion of the air actually flows through the mask or through an air gap between the mask and the face when breathing, coughing or sneezing, or how often and how the mask is cleaned.

By the way, how efficiently a filter separates particles depends not only on the particle size, but also on the electrical charge of both the particles and the fibres of the filter material and the speed of the air flow through the material.

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