We suggest materials, and a possible design for a cloth mask. The mask is based on a pleated medical mask. It can be hand or machine sewn. The suggestions for materials are based on our evidence review. Our plain language summary is here.
Design: our suggestions
A pleated mask design, based on the common pleated design for medical masks, results in a mask with subjectively good fit that is relatively simple to make. Paper fasteners, florists’ or electricians’ wire, or pipecleaner can be inserted across the top to improve fit at the nose.
Other mask shapes may perform as well. There are no head-to-head comparisons. A historic duckbill-shaped mask performed very well.
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.
Though data are not available that conform with any modern standard method, from the studies available, muslin (a type of unfinished cotton), cotton, and flannel are the best supported and are our suggestions for evidence-informed cloth masks.
Successful masks have used muslin (an unfinished type of plain weave cotton) at about 100 threads per inch (TPI) in 3-4 layers (4-layer muslin or a muslin-flannel-muslin sandwich), cotton tea towels (also known as dish towels in the US, i.e., flat cloths used for drying dishes), studied as one-layer, and two-layer expected to be better, and good-quality cotton T shirts in 2 layers.
In flat-cloth experiments, cotton 600 TPI in two layers, or cotton 600 TPI with flannel 90 TPI, performed well. We have no information on breathability for cotton 600 TPI and no information on this material made into an actual mask.
Two-layer cotton 80 TPI did not perform well in flat-cloth experiments.
Multiple layers should be used, at least two, and preferably three or four.
Specifications for masks for sale
People making masks for sale should specify the materials (composition, weave, weight, thread count) for each layer and the number of layers (e.g., cotton 100%, plain weave, 150 g/m2, 300 TPI; 3 layers).
Useful information for mask manufacturers can be found here.
Choosing materials, and choosing the right mask for the task
We think that most cloth masks in current use are likely reducing contamination of the environment and reducing particles reaching the wearer. However, if people are making their own masks or choosing a mask they can consider choosing materials from the selections above. People may also consider their risk exposure, and their planned activities while wearing the mask, in selecting the ideal mask.
Level of exposure
How many people?
Are the other people likely to be low risk themselves: usually able to comply with physical distancing and hand hygiene and other public health recommendations, or is that not likely, or unknown?
Will the other people be wearing masks?
How close will other people be, and is there a risk that physical distance might be unexpectedly compromised?
What is the duration of the activity? The longer, the higher the risk.
Will people be talking loudly, shouting, singing or eating? These activities create many more aerosol particles than quiet talking.
Is the activity indoors or outdoors? Outdoor activities appear to be lower risk.
Level of activity
People will likely tolerate more layers when sitting comfortably than when moving around or when exercising.
Level of personal risk
People who are older or from a vulnerable group, and people who are risk-averse for any reason (e.g., living with or caring for someone from a vulnerable group), may be prepared to wear more layers, even though it is less comfortable.
Bandanas and scarfs
These same materials would be recommended if using a bandana or scarf-type design, though we would anticipate that this would be less efficient. Optimally, this will include a prefolded shape, and a clear differentiation of outside and inside, such as this video showing a multi-layered suggestion.
Evidence on household filters is limited. We found one study of tissue paper and paper towel, which did not report high efficiencies: we think a third or fourth layer of cloth is preferable to a disposable filter. Our suggested design incorporates an optional internal unhemmed extra piece of cloth to provide the third and fourth layers, if desired.
No evidence is available on fastenings. To simplify the design and construction, our suggestion has buttons sewn on the corners and ordinary rubber bands, looped together, to provide over-the-head elastic. We borrowed this idea from N95s, which offer a high level of protection to health-care workers; these invariably have over-the-head elastic straps. Elastic perishes with multiple washes, and the button design facilitates easy replacement. Sewn elastic, ear loops and cloth ties are alternatives.
Don’t touch your face when wearing your mask. Learn how to take off a mask safely here, and wash your hands afterwards.
Children under 2 years, people having breathing difficulties, and people who cannot remove their own masks should not wear masks.
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.