Characterization of Face Masks
Face masks that cover the wearer’s mouth and nose, such as surgical masks, are intended to prevent large particles from being expelled by the wearer into the environment and most have a fluid resistant outward-facing layer to protect the wearer against splashes and droplets. The pore size and pore volume of these protective masks can be evaluated by capillary porometry and mercury intrusion porosimetry, while moisture interaction with a mask can be evaluated by manometric water sorption isotherms.
Surgical or procedural masks intended to prevent large particles expelled by the wearer into the environment are thin, multi-layer fabric masks. The outer layer is generally fluid resistant to mitigate the penetration of bodily fluids (by splash or droplet). The middle layer is a filtration pad that serves to prevent the passage of droplets and particulates, and a third layer lies against the face, mouth and nose. The three layers are sealed around the edges to maintain structure.
The fluid-resistant outer layer is usually large-pore, nonwoven, thermo- or spun-bonded hydrophobic polypropylene. The filter pad–the small pore layer that is responsible for filtering efficiency–is normally made of nonwoven, melt-blown polypropylene. The inner layer, another large pore non-woven fabric, provides comfort and prevents saliva and perspiration from saturating the filter pad.
Particulate filtration[1-3] is achieved by:
- Sieving: physically preventing passage of particles through the filter based on the size of the particle having a dimension larger than any given passage (pore) in the filter.
- Inertial impact: retaining fast moving and larger particles as they move through a tortuous pore network and collide with the fibers of the pad.
- Diffusional impact: holding impacted particles in place by van der Waals forces when they contact the fibers of the filter at low velocity.
- Electrostatic interaction: capturing particles by taking advantage of the attraction between a charged particle and a surface charge on the filter.
The pore size distribution of all three layers, but primarily the middle layer, determines the overall filtration efficiency. Smaller pores in the layers enhance the sieving effect and provide more surface area for particle capture. The non-woven nature of the fabric creates a tortuous pore network and increases impact (inertial and diffusional) efficiency.
Surgical masks afford the wearer, e.g., healthcare workers, some protection against air-borne pathogens, but because they can be loose-fitting they are not intended to function with the efficiency of a respirator type mask. However, surgical face masks have been found to significantly reduce detection of influenza virus RNA in respiratory droplets and coronavirus RNA in aerosols, resulting in the conclusion that surgical face masks can prevent transmission of human coronaviruses and influenza viruses.
- A. Balazy et al, American Journal of Infection Control, (2006) 34, 51-57
- E. Sanchez (2010) “Filtration Efficiency of Surgical Masks”; thesis, University of South Florida.
- A.S. Zhou et al (2018) J. Thorac. Dis. 10(3): 2059–2069.
- T. Greenhalgh et al, (March, 2020) Oxford COVID-19 Evidence Service Team
- N.H.L. Leung, et al. Nat Med (2020). doi.org/10.1038/s41591-020-0843-2
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