Thankfully, the University of Otago’s leading expert on fibre and textile sciences has produced guidance on factors to consider when choosing or creating a non-medical, reusable fabric mask.
Professor Raechel Laing says reusable fabric masks vary, from the extent of coverage over the face, nose, chin; the security of positioning on the face; materials used; number of layers and performance of the layered assembly; and sensitivity to repeated washing cycles.
With mask use mandatory on domestic flights and on public transport in New Zealand, it pays to know what will best protect you.
Reusable fabric masks are typically made from several layers of material, and Laing says the combination of structural and performance properties of these materials determines the overall performance and acceptability by users.
Here are Professor Laing’s suggestions which may assist decisions related to reusable cloth face masks.
Balance between filtration efficacy and porosity required
The principal material properties for a face covering are filtration efficacy (resistance to particle transmission) and porosity (linked to ease of breathing), and a balance between these is needed.
However, these two properties tend to be inversely related, with a material which is an efficient filter typically impairing ease of breathing.
Mask must be able to withstand correct washing
Washing in hot water (~60°C) with soap or laundry detergent is required for a reusable mask, and this needs to be considered in fabric/materials selection.
Use of elastic materials is not advised
Use of ‘elastic’ materials, those containing elastane filaments (e.g. Lycra) or very porous knit structures, is not advised because these stretch during use, which increases interstitial spaces and thus reduces filtration efficacy.
Plus, many ‘elastic’ materials are adversely affected by washing at the high temperature required for cleaning.
Three layers are typically required
Laing says layers of fabric are typically required for a reusable cloth mask depending on both the fabric used, and whether or not the mask design provides for an insert filter.
- The inner layer, in contact with the wearer’s face, should be soft and absorbent. Laing suggests a hydrophilic, closely-woven structure from cotton/cotton blends or silk.
- The middle layer should be either a filter (typically non-woven) in a pocket or a non-woven structure (e.g. melt-blown, needle-punched, other form) for which varied fibres/filaments/films are possible.
- The outer layer is exposed to the environment and functions to limit external contamination and penetration through that layer. It should therefore be a hydrophobic, closely-woven structure made from polyester, polyamide/blends or cotton.