WEDNESDAY, 17 JUNE 2020As the country begins to open after lockdown, it is vital that people are fully aware of the best ways to protect themselves in order to prevent a second wave of COVID-19. Social distancing is a good start to ensuring safety but, as has become clear over the last few months, it is in many cases impractical and often ignored or accidentally broken. Wearing face masks while in contact with other people is likely to significantly reduce the spread of SARS-CoV-2 and other respiratory viruses by reducing the spread of droplets carrying the virus. Therefore it is a good step towards protecting both the wearer and the people around them.
In light of the new government advice to wear face coverings while on public transport, supported by WHO recommendation, it is important to consider how much protection they provide. While it is helpful to some degree for people to wear any kind of face covering, using suitable materials and having an effective design is essential for the mask to be successful. Many people are under the impression that anything in front of their face is good enough, be it a bandana, a clear plastic visor or a respirator. This may not be the case. Currently, the European standards for respirators require 99% filtration efficiency of particles between 0.02 and 2μm as well as be of a low enough resistance for the user to breathe comfortably (3mbar) . It seems reasonable that we should be creating masks that approach this effectiveness if we wish to use face masks as a successful strategy to combat the spread of the virus.
After reviewing the evidence surrounding the materials and designs for masks which people could make at home, it is clear that it can be possible for every person to equip themselves properly using materials that are easily available. Here, I provide a summary of research conducted on designing the best mask to ensure that we can protect ourselves in the best possible way. From material to design to maintenance, there are simple and effective measures that we can all take to maximise the worth of our homemade masks.
A consensus in relevant literature that investigates the filtration efficiency of various household materials is emerging, indicating that high thread-count cotton, such as that used in bedding, is a good start for homemade masks. The literature suggests that cotton of this type can provide at least 70% filtration efficiency with a sufficiently low resistance     with some finding even higher efficiencies, especially with multiple layers of material. It is important to ensure that the cotton is high quality as not all cotton fabric will provide the same levels of filtration . Therefore this seems like a good place to start for creating homemade masks. Of course, the breathability of the mask is another essential factor in deciding what materials to use. This is why porous, cloth materials have been explored here.
It is thought that filtering usually occurs by two groups of methods; mechanical and electrostatic. It is thought that larger particles (1 to 10μm) are filtered by their inertial settling, medium sized particles (100nm to 1μm) are filtered by their impact into the fibres of the material and smaller particles are captured by the electrostatic attraction between the particles and the fibres of the materials. There is some evidence that materials such as silk, chiffon (polyester/spandex mix) or flannel (cotton/polyester mix) which are ineffective on their own can significantly improve the filtration efficiency to ≥90% when combined with a cotton layer . It is thought that the improvement in efficiency could be due to the combination of electrostatic and mechanical filtering.
There are many suggestions that other materials may also be effective filters. There is some evidence that vacuum cleaner bags could be effective filters   and several brands, particularly those claiming to relieve allergies, make similar claims  although these are unregulated and unverifiable. It could also be reasonable to incorporate air conditioning filters into masks as these are regulated by European Standards and so some (those with a H13 or higher rating) provide >99.95% filtration efficiency . However, the breathability of these materials is limited, questioning their suitability in this context.
Equipped with this information, it seems that hybrid masks are the most efficient. Using a combination of cotton and different material or household filters is the first step for creating your own protective device.
The particular design is not a primary important factor in their efficacy alone, but the fit of the mask to the users’ face certainly is. This is because an improper fit could reduce filtration efficiency by more than 60% . Proper respirators have to be fit tested for each user. Testing is conducted either by the user judging if they can taste a flavoured aerosol when they are wearing the mask or by comparing the particulate count inside and outside the mask as the user breathes . Since it is impossible for everyone to test homemade masks, it seems reasonable to look at designs which have passed fit tests by large numbers of other people. It is important to note that users are recommended to do seal checks after putting on any mask where they deeply exhale while bracing the mask against their face with their hands, and feeling if the air goes through or around the mask . This provides no guarantee but is a helpful step.
There are a huge range of fabric masks but fit tests bear out a few common themes in those that have high filtration efficacy . Effective designs commonly have structures that maintain its shape, such as a metal strip which can be fitted around the nose. These are similar to the “duckbill” designs of commercially available masks.
Some examples can be found at the following links:
- UFHealth Mask alternative - https://anest.ufl.edu/clinical- divisions/ mask- alternative/
- HKMask - https://diymask.site
- Weibel Surgical Mask - https://3dprint.nih.gov/discover/3dpx-014003
- Suay Community Mask Coalition - https://suayla.com/pages/suay-community-mask-coalition
Furthermore, a large variety of designs, usually 3D printed, have a ridged frame to which a piece of filtering material is attached. This design allows the use of stiffer materials like vacuum cleaner bags and air conditioning filters. A lot of these designs are used in hospitals in America at the moment  . Those that will prove the most useful are the designs that are quick to make and require little skill to put together. As many people will know, earlier on in the pandemic the 3D printing community quickly came together to produce hundreds of thousands of visors for hospitals . It will be interesting to see how people with these skills can help as many people as possible to return safely to work in jobs that require close contact such as retail and public transport.
Some examples of printed masks can be found at the following links:
- The Montana Mask – https://makethemask.com
- Wiles Covid Pandemic Mask - https://3dprint.nih.gov/discover/3DPX-013805
And some much quicker printing designs:
- Sani-Mask - https://nemotech3d.com/sani-mask
- “CEG Extreme” Mask Frame - https://3dprint.nih.gov/discover/3dpx-013690
- GE Additive Community Face Mask Frame - https://3dprint.nih.gov/discover/3dpx-013512
There are many more designs that people across the world have been making which may be suitable. It is just important to think about the fit of a mask when making them.
Cleaning and Hygiene
Proper cleaning and practicing good hygiene is also essential for the use and maintenance of an effective mask. There is evidence that a successful way to clean items such as masks and clothes could be to wash them at more than 70°C for 25 minutes or to wash them at a cooler temperature with a strong disinfectant such as bleach . Some materials cannot tolerate this type of cleaning regime though, so it is important to keep in mind how the device will be cleaned when building a mask. A better method may be to heat the mask to more than 70°C with high humidity for an hour, such as by placing them in the oven at a low temperature . When cleaning items at home, it is also important to remember that surfaces such as the outside of washing machines, work tops or ovens can get contaminated and spread contamination. A last resort method of decontamination could be to leave the mask sealed in a bag at room temperature for a minimum of 5 to 7 days, although the effectiveness of this method is expected to vary greatly on how clean the mask already is .
A final factor to consider is taking the mask on and off properly to ensure that the user does not get contaminated. The guidelines from Public Health England and the CDC are clear on the importance for the user to wash their hands before and after touching the mask, to dispose of it/store it hygienically and to remove it after other protective items (e.g. gloves)  .
Everyone should have access to a mask that is effective and properly protects them and everyone else around them, especially those who are more vulnerable. As we potentially move out of lockdown and into the next stages of returning to normality, it is essential to keep in mind social distancing practices, personal hygiene and the use of effective personal protection equipment. Designing a face mask that optimises filtration and is easy to clean will be a great aid in ensuring as many people as possible are protected.
Jago Strong-Wright is a second year undergraduate at St John’s College studying Physics. For a more detailed look at the factors to consider when making an effective mask he has written a more full review which is available at: https://www.preprints.org/manuscript/202006.0132/v1
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