An interesting article published in the Journal of M1 by Guha and others January 13, 2021.  

Face coverings constitute an important strategy for containing pandemics. Infection  from airborne respiratory viruses such as SARS-CoV-2 can occur in at least three  modes; 

Tiny and dash or dried aerosols, which are typically less than 1 μm generated  through multiple mechanisms which include talking, breathing, singing. 

Large droplets greater than 0.5 μm generated during coughing and sneezing, Macro droplets transmitted via formites.  

Whilst there is a growing number of studies looking at the performance of household  materials against some of these situations, there has not been any systematic  characterisation of household materials against all three modes.  

This study developed and used a methodology to characterise the performance of 21  different household materials with various material compositions such as cotton, polyester, polypropylene, cellulose and blends. 

They used sub-micron sodium chloride aerosols, water droplets, and mucus  mimicking macro droplets over an aerosol droplet size range of 20 nm to 0.6 cm. 

Except for 1000 thread count cotton, most single layer materials have filtration  efficiencies less than 20% for sub-micron solid aerosols.  

However, several of these materials stopped greater than 80% of larger droplets, even at sneeze velocities of up to 1700 cm/s.  

Three or four layers of the same material, or a combination of materials, would be  required to stop macro droplets from permeating out or into the face covering. Such materials are able to be boiled for reuse. 

Four layers of loose knit or woven fabrics independent of the composition e.g. cotton, polyester, nylon or blends are likely to be efficient source controls.  

One layer of a tightly woven fabric combined with multiple layers of loosely knit or  woven fabrics in addition to being source controls can be found to have sub-micron filtration efficiencies greater than 40% and may offer some protection to the wearer.  

However, the pressure drop across such fabrics can be high. 

The study authors concluded that layering materials therefore with the same or  different material compositions demonstrate higher filtration efficiencies for larger  droplets ranging from 0.5 to 0.6 cm and are likely to be sufficient for control of  transmission for a wearer who is infected with SARS-CoV-2. 

The London General Practice has always recommended the wearing of face masks  and continues to do so. 

Dr Paul Ettlinger 

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