Summary of Advice
What the evidence tells us about masks
Wear the best mask available
We describe the efficiency of different masks, including medical masks and respirators here. The rest of the page below is about reusable cloth masks, worn either alone or over a certified or uncertified disposable mask. It includes what to look for in materials and design for cloth masks, whether you are thinking of making masks or assessing a reusable mask made by someone else.
Materials for reusable masks
Quilting cotton, either mass-market or high-quality
Cotton fashion fabric
Cotton T-shirt material (needs special handling: see Knits)
At least one layer of non-woven industry-grade spunbond polypropylene, recommended by Health Canada and the World Health Organization. Polypropylene requires special handling: see details here.
Other materials (poly-cottons, silk, and synthetics) sometimes perform in a similar range to cotton. We do not recommend them because there is less direct published evidence of good filtration, breathability and consistency.
Other materials that have been tested, which we do not recommend, include tea towel fabrics (good filtration but some fabrics in this category are thick, leading to problems with mask construction), batiks and bedsheets (variable breathability, with some fabrics not meeting breathability standards), and bandana fabric (less good filtration).
Layers for reusable masks
2 to 3 layers
Cotton against the face with polypropylene for middle or middle and outer layers.
If resources are an issue, 2-layer cotton masks with head ties are a good option
When made from the materials above they provide filtration of 50-60%, comparable with a level 1 ASTM-certified medical mask (Drouillard, Davies)
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. Increased resistance with increased layers also leads to increased leak around the edges, decreasing the efficiency of the mask.
Fit
Well-made cloth masks can have equivalent filtration to medical masks because they fit better and there is less leak of completely unfiltered air around the edges.
Here are four questions to answer when assessing your mask. Fit and breathability can't be assessed until you have purchased the mask. We acknowledge the difficulty this causes, and that these are subjective and challenging to assess.
What makes a good mask?
How many layers does it have and what are the materials? A sandwich of two outer layers of cloth (evidence is strongest for cotton) and a middle layer of ~70 gsm (2 - 2.25 oz/square meter) polypropylene, or two middle layers of lighter-weight polypropylene. In many countries, manufacturers are not permitted to make filtration claims for non-medical masks, but they are allowed and encouraged to specify what materials are used.
Is the material breathable? Less breathable material will pull in when breathing in and puff out when breathing out. There may be a sensation that it is hard to breathe. The concern is that if the fabric is less breathable, air will take the path of least resistance around the edges of the mask. This completely unfiltered air is called edge leak. Some, but not all, high-threadcount fabrics, such as batik and bedsheets, have low breathability.
Is there a good subjective fit? Do you see gaps under the eyes or at the sides? Is there good contact with the face over the cheeks and along the jawline or under the chin? When you breathe in and out, is there obvious leak - can you feel airflow on your face, or with your cupped hands. Leaks are usually under the eyes, at the sides, and under the chin. A nose wire helps reduce leak under the eyes, but completely eliminating leak in this area is challenging.
Are the head fastenings secure? This goes back to fit. Does the edge of the mask make good contact with the face and stay in position, or does it feel loose and move or fall down as you move or talk? Earloops are popular and convenient - fixed-size earloops work well if they are the right size for your face; over time they can stretch. Adjustable earloops may be preferable for these reasons. There is also some evidence that ties over the head may improve the overall filtration of a mask.
See also double masking and mask hacks for ways of improving the overall filtration of masks.
A note on edge leak: Air that goes around the edge of the mask, whether inward or outward, is completely unfiltered. This means that respiratory particles in that air can travel from a person to the environment (air and surfaces), and from the air to a person. This edge leak reduces how good the mask is at filtering particles, the overall efficiency of the mask. We try to minimize edge leak by addressing breathability and fit.
Our academic evidence summary is available here, with a plain language version here.
Other pages have more information on cotton, knits, polypropylene, and unsafe materials. We have a page on research around fit which translates into advice on getting the best fit, as well as summaries around specific design features (head attachments, nosewires) and masks for children.
Patterns and instructions for reusable cloth masks are here.
Mask Standards
The ASTM International standard for cloth masks is F3502. Masks made to this standard are available for purchase, though they can be difficult to find. The standard requires a certain level of filtration, assessed by testing the whole mask sealed to a plate. Design features that lead to good fit should also be addressed by the manufacturer but there is no single specified way of doing this.
The F3502 standard operates within a paradigm that cloth masks do not provide protection of the wearer. The standard is designed around the idea of providing source control, ie, preventing contamination of the environment. No standard method of assessing how well a mask provides source control exists.
We agree with the writers of F3502 that cloth masks do not provide the same protection as a respirator and should not replace respirators. However, there is ample peer-reviewed information showing that unselected cloth masks reduce aerosols reaching the wearer: filtration efficiency of 23 - 52% has been reported in studies of haphazardly-selected masks (O’Kelly, Clapp, Blachere), 50% was reported for a 2-layer T-shirt mask on overhead ties (Davies), and in our research (Drouillard) cotton masks filtered 40 - 66% of particles, compared with a control medical mask which filtered 55% of particles.
Our evidence-based summary of the relative efficiency of masks and respirators in protecting the wearer is here.
Public health advice on cloth masks
WHO advice is here.
CDC advice on cloth face coverings is here.
Government of Canada advice on cloth face coverings is here.
Caution
Cloth masks should not be placed on:
children under 2
anyone unable to remove them without assistance
anyone who has trouble breathing
Remember not to use detachable parts such as toggles for young children because they are a choking hazard.