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How do I choose a cloth face mask?

MIT Medical answers your COVID-19 questions. Got a question about COVID-19? Send it to us at CovidQ@mit.edu, and we’ll do our best to provide an answer.

Which cloth masks does MIT Medical recommend? My family is trying to find some to purchase, but it’s confusing, since there are so many options available. In addition, would you recommend using masks with filters?

 Illustration of an assortment of cloth PPE masks

MIT Medical can’t recommend specific products, but your question offers a good opportunity to discuss the characteristics that make a cloth mask most effective. Whether you’re purchasing a cloth mask or making one yourself, there are three main considerations: fabric, fit, and thickness.

Fabric: When it comes to fabric, the tightness of the weave is crucial. At a bare minimum, you want the weave to be tight enough that you don’t see the outline of the individual fibers when you hold the material up to light. But tighter is better. A study comparing the aerosol filtration efficiencies of a number of different fabrics found that a high-thread-count (600 TPI) cotton fabric far outperformed a moderate-thread-count (80 TPI) quilter’s cotton for particles of all sizes.

As far as fabric type, filtering experiments show tightly woven 100% cotton outperforming most synthetics. This may be because synthetic fibers are relatively smooth at the microscopic level, while cotton fibers have a somewhat three-dimensional structure that likely creates additional barriers to both outgoing and incoming particles.

Another study suggests that you can increase the effectiveness of a multi-layer mask by combining one layer of cotton with a different material. Researchers evaluated the filtering efficiency of masks made from one layer of 600-thread-count cotton and either two layers of natural silk or chiffon (in this case, a 90% polyester–10% Spandex weave) or a single layer of flannel (a 65% cotton–35% polyester blend). Materials chosen for the non-cotton layers were thought likely to provide good electrostatic filtering, a process that traps particles through the same kind of “cling” effect created by static electricity. Not only did the hybrid masks outperform all other two- or three-layer masks made of a single material, they were superior to N95 masks for particles smaller than 300 nanometers and only slightly inferior for larger particles.

Fit: Of course, no fabric or fabric combination will work as intended if your mask doesn’t fit properly. Research indicates that leakages around the sides of a mask can degrade filtering efficiencies by 50 percent or more. Lack of such leakage is one reason why properly worn N95 masks work so well.

A well-fitted mask will hug your face, covering both nose and chin with no obvious gaps. Everyone is shaped differently, so you might end up trying a few different designs before you find one that fits well and feels comfortable — part of the reason we can’t recommend any one product over another. It’s also important that your mask stays put, even when you talk, so you’re not constantly touching it to readjust. Masks with a bendable metal nose strip may help to create a tight seal and hold the mask in place; this can also help prevent glasses from fogging up.

Thickness: Multiple layers are recommended. A well fitting cloth mask should have at least two layers of tightly woven fabric. A third layer provides additional protection, as does the addition of a filter. At least one study suggests that filters made from polypropylene material, which is derived from plastic, are particularly effective. While some people are recommending coffee filters for this purpose, this is not something we would advise. As it turns out, it’s very difficult to breathe through a coffee filter.

Finally, speaking of breathing, avoid those masks that come with valves at the front. While the valve makes it easier to breathe out, it also releases unfiltered air, so it doesn’t protect others if you’re contagious.

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