Publication from WHO containing revised homemade mask recommendations

[KY_EnviroGuy]

This document relates to a message issued by Dr. Tedros Adhanom Ghebreyesus, Director-General of the World Health Organization on Friday where he stated: "The updated guidance also contains new information on the composition of fabric masks. @WHO advises that fabric masks should consist of at least three layers of different material. We also provide guidance on how to wash and maintain a fabric mask & how to use a mask safely."

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Advice on the use of masks in the context of COVID-19 (PDF file)
Interim guidance
5 June 2020

This document is an update of the guidance published on 6 April 2020 and includes updated scientific evidence relevant to the use of masks for preventing transmission of Coronavirus disease 2019 (COVID-19) as well as practical considerations.

Go to: https://www.who.int/publications/i/item/advice-on-the-use-of-masks-in-the-community-during-home-care-and-in-healthcare-settings-in-the-context-of-the-novel-coronavirus-(2019-ncov)-outbreak

Click on "Download" and either open the PDF or save it and then open it.

Go to Pages 9 and 10.

There, it says:

b) Number of layers

A minimum of three layers is required for non-medical masks, depending on the fabric used. The innermost layer of the mask is in contact with the wearer’s face. The outermost layer is exposed to the environment.
Fabric cloths (e.g., nylon blends and 100% polyester) when folded into two layers, provides 2-5 times increased filtration efficiency compared to a single layer of the same cloth, and filtration efficiency increases 2-7 times if it is folded into 4 layers. Masks made of cotton handkerchiefs alone should consist of at least 4 layers, but have achieved only 13% filtration efficiency. Very porous materials, such as gauze, even with multiple layers will not provide sufficient filtration; only 3% filtration efficiency.

It is important to note that with more tightly woven materials, as the number of layers increases, the breathability may be reduced. A quick check for breathability may be performed by attempting to breathe, through the mouth, and through the multiple layers.

c) Combination of material used

The ideal combination of material for non-medical masks should include three layers as follows: 1) an innermost layer of a hydrophilic material (e.g. cotton or cotton blends); 2), an outermost layer made of hydrophobic material (e.g., polypropylene, polyester, or their blends) which may limit external contamination from penetration through to the wearer’s nose and mouth; 3) a middle hydrophobic layer of synthetic non-woven material such as polyproplylene or a cotton layer which may enhance filtration or retain droplets.

Also note Table 3. Non-medical mask filtration efficiency, pressure drop and filter quality factor on Page 9 which shows the effectiveness of various household-type fabrics regarding filtration effectiveness.

There are other commonly available materials available which are far more effective than those listed which I'll try to address in a separate reply or OP later if anyone requests it.

KY........

[Warpy]

1. Link is borked

but I'm sending on your excerpts to people I know who are making them.

[KY_EnviroGuy]

2. Thanks for letting me know.

The link is actually OK but the DU system does not recognize the full address for some reason. It won't allow loading because the link has parenthesis in it and I don't know how to fix that.

If you copy and paste the entire link into your browser address window, it will work OK and I conformed it.

Sorry, I should have tested the link via DU's page before posting.

KY