How to improve the effectiveness of a DIY face mask

Do-it-yourself protection

Although face masks were more commonly worn on the streets in Asia before the coronavirus pandemic, they are now becoming part of everyday life around the world. Medical face protection and respiratory masks are now scarce and not easy to obtain. Dwindling stocks of medical masks should be left for the protection of healthcare professionals. Many people are making their own masks to help protect themselves and others. However, the effectiveness of these DIY masks is rather low and they convey a false sense of security, as Prof. Dr. Dirk Schubert, Chair of Materials Science (Polymer Materials) at Friedrich-Alexander-Universität Erlangen Nürnberg (FAU) emphasises. He explains in this interview how to improve DIY masks.

Prof. Schubert, you criticise the many different instructions for building DIY masks on the internet. Why?

It seems to me that many people do not know how a filtering facepiece (FFP) mask works. Statements such as using suitable filter material makes breathing too difficult make me doubt that people really know how FFP masks work. There are different types of masks: For use in hospitals and in dealing with infected persons, masks with the protection level FFP-2 or FFP-3 are required. In these situations, the filter effectiveness is sufficient enough to protect against infection with the virus. Simple surgical masks, however, do not offer sufficient protection. They are intended to protect patients from the saliva of the medical staff. To a certain extent, they prevent others from becoming infected.

Now, I also like to design and make things outside the University. That’s why I like to see people taking the initiative and sewing masks themselves. But I am also a scientist who deals with fibrous materials and I have worked for years in the industrial development of medical hygiene products at leading national and international companies. That is why I have to say quite clearly: these DIY masks do not work well or barely work at all. They convey a false sense of security. But even a poorly functioning mask is better than no mask.

What is the technical problem with DIY masks?

The problems are primarily caused by the material used. SARS-CoV-2 is mainly spread by droplet infection. The larger the diameter of the material fibres in a protective mask, the poorer it protects. This is because the diameter of the fibres determines the pore size, and the larger it is, the easier it is for droplets to reach the outside. Conversely, this means that material with finer fibres will offer better protection It is also important how hydrophobic – i.e. water-repellent – the material used is. Materials that are more hydrophobic allow fewer water droplets to pass through, for example, when speaking or coughing. Cotton is therefore less suitable.

DIY masks which are constructed with pockets into which the filter material is inserted are also problematic. If these are not attached accurately enough to the edge and only cover the mouth and nose, the wearer virtually breathes past the filter.

How can the design of DIY masks be improved?

First of all, it must be clear that any useful level of protection will come with some restriction, meaning that masks that filter more effectively make breathing more difficult. Adequate DIY masks can be made from materials around the home depending on their availability. For example, although microfibre cloths have finer fibres than conventional clothing textiles, they are hydrophilic. However, they should still be used rather than conventional textiles.

Melt-blown materials are more suitable: they are hydrophobic and have a very small fibre diameter. These are also found in commercially available nappies or vacuum cleaner bags as a barrier layer. In the case of the vacuum cleaner bag, the fibres often even carry an electric charge (electret), since they are intended as particle filters similar to an FFP mask. This significantly increases the filter performance without increasing the respiratory resistance. Some vacuum cleaner bag manufacturers clearly label their products as a ‘three layer electret microfilter bag’.

It is of course also important that masks are worn close to the face and that the effective filtering area is as large as possible in order to keep the respiratory resistance as low as possible. Unfortunately, many of the designs found on the internet have filtering areas which are far to small to be effective – this even applies to some designs produced by companies in the injection moulding sector who want to switch their production to masks. A rough guide for manufacturers switching production: In the case of filter material suitable for FFP2, the filtering area should be greater than 150 cm².

Please note that these instructions do not constitute medical or safety advice and any implementation is done at the users’ own risk.

Further information

Prof. Dr. Dirk W. Schubert
Chair of Materials Science (Polymer Materials)
Phone +49 9131 8527752
dirk.schubert@fau.de