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 gsm, 300 TPI; spunbond polypropylene 70 gsm; outer layers cotton, middle layer polypropylene).
Information on industry-grade spunbond non-woven polypropylene can be downloaded here.
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? The more, the higher the risk.
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 industry grade polypropylene 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.
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 usually 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. Check that masks for children don't have pieces such as toggles and buttons that can become separated and pose a choking hazard.
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.