Face Masks 101
World Health Organization Guidelines
This is the advice that the World Health Organization published, as of June 2020.
“Non-medical masks may be made of different combinations of fabrics, layering sequences and available in diverse shapes. Few of these combinations have been systematically evaluated and there is no single design, choice of material, layering or shape among the nonmedical masks that are available. The unlimited combination of fabrics and materials results in variable filtration and breathability. “
“It is important to ensure that the mask can be held in place comfortably with little adjustment using elastic bands or ties.”
Recommended layers and materials
“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.”
This is recommended because just double-layering some materials can increase the filtration efficacy many times over, without significantly compromising on breathability.
“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.”
This means that the layer closest to your face should be good at absorbing water, allowing any droplets from your breath to be caught and trapped in the material. The layer farthest from your face should be water resistant, preventing water droplets from going through the mask. The middle layer should also be water-resistant, and non-woven in order to catch droplets and act as a filter.
Materials to avoid
“It is preferable not to select elastic material for making masks; during wear, the mask material may be stretched over the face, resulting in increased pore size and lower filtration efficiency throughout use. Also, elastic materials may degrade over time and are sensitive to washing at high temperatures.”
“Coating the fabric with compounds like wax may increase the barrier and render the mask fluid resistant; however, such coatings may inadvertently completely block the pores and make the mask difficult to breathe through. In addition to decreased breathability unfiltered air may more likely escape the sides of the mask upon exhalation. Coating is therefore not recommended.”
This advice is taken directly from the Advice on the use of masks in the context of COVID-19
Interim guidance, published by the World Health Organization, 5 June 2020.
WHO Reference Number: WHO/2019-nCov/IPC_Masks/2020.4