Both fabric choice and mask design can impact the effectiveness and wearability of masks.

Mask effectiveness is often measured as filtration efficiency which represents the percent of particles (usually within a defined size range) that are expected to be blocked by the mask. Masks made of fabrics with higher filtration efficiencies mean more particles are blocked and therefore may offer better protection to you and others.

Wearability relates to the ease of breathing in the mask and is usually reported as the difference in pressures measured on each side of the mask (often reported as ‘pressure drop’ in millibars or Pascals across the whole mask or per area, e.g. Pa/mask or mbar/centimeter2). Masks with large pressure differences would make breathing more difficult. The WHO recommends the pressure difference across the whole mask should be < 100 Pascals. For hygiene promoters, masks can be tested before field use by wearing them around the house during daily activities to make sure they offer adequate breathability.

When selecting or making a fabric mask, it is important to consider the following characteristics which will impact filtration efficiency and breathability:

Number of layers: Masks should be made of multiple layers of material. Masks made out of multiple layers of fabric or material often have improved filtration efficiency compared with single-layer masks. The WHO recommends a minimum of three layers for most fabric masks - described in this video. Ideally, the mask should consist of an inner layer made of an absorbent material (e.g. cotton or cotton blend), an outer layer made of water-resistant material (polyester blends, polypropylene), and a middle layer either made of natural or synthetics materials. For commonly used fabrics (e.g. polyester, nylon, cotton), filtration efficiency may increase by up to 2-5 times when using two layers of instead of one, or 2-7 times when using four layers (WHO, Study 1, Study 2). The following graph illustrates how filtration efficiency increases with the number of layers.

Data source: ACS Nano, 2020 ; Data visualization: Mona Chalabi

The video below, based on a single case study, illustrates how 1- and 2-layer fabric masks block the release of respiratory droplets and aerosols during speaking, coughing, and sneezing compared with wearing no mask or a surgical mask.

The gains in filtration efficiency achieved by using masks with multiple layers will depend on the type of fabric used as well as the number of layers used. For example, the filtration efficiency of a mask made of loose weave cotton (80 TPI) increased by four times when using 2 layers instead of one. In contrast, when using a tightly woven cotton fabric (600 TPI), the gain in filtration efficiency was negligible when using two versus one layer of fabric (Study 1).

Choice of fabric: Different materials and fabric weaves have different filtration efficiencies and therefore some materials are better suited for constructing fabric masks (Study 1, Study 2). Fabrics with tighter weaves may offer better filtration, particularly for small particles, but may impact breathability. For example, the filtration efficiency of cotton fabrics with tighter weaves (600 threads per inch [TPI]) was almost 9 times higher than cotton with looser weaves (80 threads per inch). Cotton, polyester and blends can all be used, as well as materials usually found in surgical masks (like polypropylene). Avoid using materials with very loose weaves (e.g. gauze) or stretchy materials which may distort the weave and impair filtration. Materials with visible pores when held up to a light source should be avoided or used only as part of a multilayer mask.

Combining materials: Hybrid masks that combine different materials such as cotton with silk, or polyester blends, have been shown to increase filtration efficiency. The different fabrics of the hybrid masks may work together to allow effective filtration of a range of particle sizes.

Fit: Masks come in a variety of acceptable shapes (flat-fold, duckbilled, molded, etc). Fit is more important than shape. The mask should fit closely around the nose, cheeks, and chin. Poor fit can reduce the performance characteristics of the mask. Unfiltered air may enter or exit the mask at gaps created by poorly fit masks. If you feel lots of air exiting around the edges of the mask then this is an indication that the fit is poor.

Exhalation valves: Some commercially available fabric masks include exhalation or one-way valves that are meant to make it easier to breathe. Masks with such valves are not recommended as they allow the user to expel respiratory secretions and limit the function of the mask for source control.

Several online resources provide simple step-by-step instructions for making your own fabric face mask. The CDC provides instructions for making three simple masks at home. A video demonstrating the third option is available here in English and here in Spanish. The WHO has also made a video on sewing tips for making fabric face masks. Fabric face masks are also widely available for purchase from online retailers but do consider the mask characteristics described above before purchasing them.

In some countries community groups are also producing masks which are more inclusive. These masks have a clear plastic window at the front of the mask allowing people with hearing impairments and who rely on lip reading, to more easily communicate at this time. As with many homemade fabric masks, the safety of most of these masks has not been tested but they are being encouraged by support groups for people with hearing impairments. Surgical masks with similar adaptations are approved for use in healthcare settings in the USA. Ensure any mask modifications such as these do not increase the likelihood for exhaled air to escape the sides of the masks.

Want to learn more about mask use for interrupting the spread of COVID-19?

Editor's note

Author: Jackie Knee
Review: Peter Winch, Julie Watson
Last update: 13.08.2020

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