Materials: our suggestions
Plain woven cotton, at least 100 threads per inch
Flannel, either cotton or poly-cotton blend, at least 90 threads per inch
Tea towel material
Heavy, good quality, cotton T shirt material
With fabric that stretches, such as T shirt fabric, it may be important to use a design with edge stitching to prevent the cloth being stretched when worn, which will increase the size of gaps in the material and affect filtration.
Layers: our suggestions
At least 2 layers
3 or 4 layers almost certainly better
Use multiple layers of the same material or combine cotton and flannel
Don’t use a disposable filter, instead add another layer of one of the materials above
There is a trade-off with increased layers: they provide increased filtration efficiency, but also increase the resistance to breathing, which may lead to discomfort and even to not wanting to wear the mask. Increased resistance with increased layers also leads to increased leak around the edges, decreasing the efficiency of the mask.
As public health organisations around the world seek to slow the spread of the SARS-CoV-2 in the COVID 19 pandemic, there is a growing interest in the possibility that cloth masks may reduce community transmission.
Physical distancing and hand hygiene remain the cornerstones of prevention of transmission; we suggest cloth masks be used in addition, and not as an alternative. Correct use and cleaning of the mask is essential to their safe use.
Cloth masks should not be placed on children under 2, or on on anyone unable to remove them without assistance or anyone who has trouble breathing.
There are no high-quality studies that tell us directly that cloth masks will prevent transmission of SARS-CoV-2, or that any mask will prevent transmission outside a health care setting. There is, however, a large body of evidence on the filtration properties of cloth and cloth masks that we have used to suggest a possible materials and design for a cloth mask.
Canadian public health advice is available here.
Kindness & Altruism
Though we are sure that many kinds of cloth block particle transmission, including aerosols, and including viruses, we don’t have direct evidence from clinical trials that wearing a mask reduces actual disease transmission in a community setting. We suggest wearing a mask altruistically, with confidence that this is reducing the contamination of the environment. We suggest remembering people who rely on lip reading to understand what is said, and responding with kindness, considering the level of exposure and risk if asked to remove a mask to facilitate communication. We suggest withholding judgement on those not wearing masks, given that their personal circumstances and physical health are unknown.