Industry-grade non-woven spunbond polypropylene

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Recommended by Health Canada and the WHO.

Our longer article with more photographs is available here.

It may be behind a paywall for you, but if not, this animation explains the physical forces that are responsible for filtration. 

Polypropylene is available as industry-grade materials that are not part of the supply chain for PPE.

They are washable, but should not be tumble dried.

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Pending definitive research, we recommend 70 gsm (2 or 2.25 oz/sq yard), but this is harder to find than lighter-weight material. If you can't find it, we recommend two layers of a lighter material. Two layers will be a bit harder to work. Our plain-language article on what it is, how to identify it, and why we agree with the recommendation to use it, is here.

The best way to use it is likely to build it into your mask as a middle layer. It is fabric-like and can be sewn with a machine or by hand.

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It doesn't fray when cut. If you have pocket masks, you can cut a rectangle of the material to use as a washable filter, though this approach hasn’t been tested. You can sew two layers of lighter polypropylene together if you cannot find 70 gsm polypropylene.

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The material is washable in a hot wash but should not be tumble dried. Wash masks in a mesh bag, remove them after washing, reshape and hang to dry. We do not know how the filtration properties change with repeated washing. We suggest replacing your mask if the outer layers are visibly worn and fraying. We do not know if masks should be replaced after a certain number of washes. Do not iron or put it in the dryer. If polypropylene is ironed or put in a hot dryer, it may no longer be effective, but physical changes may not be noticeable.

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Polyester and nylon interfacing are not the same product.

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We recommend against using fusible products because the glue affects filtration and may be an inhalation hazard.

We are recommending industry-grade non-woven spunbond polypropylene that is intended for the clothing and upholstery industry because it is meant to be washed. Studies of washing are not available. 

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Finally, this material is plastic, and not biodegradeable in any reasonable time frame. Researchers at the de Lannoy Lab and others at the McMaster Centre of Excellence in Protective Equipment and Materials are hoping to identify sustainable, biodegradable materials with excellent filtration efficiency and breathability for use in masks.