Most of us have got used to the idea of wearing COVID-19 face masks in shops and on transport - but new research has shown that not all masks are equal.
Researchers from the Georgia Institute of Technology measured the ability of common fabrics to filter out ‘submicron particles’ (a human hair is about 50 microns in diameter while one millimeter is 1,000 microns in size).
Filtering such particles is crucial for stopping coronavirus transmission, the researchers say - and so offer useful guidelines on what materials to use (and not to use) in home-made masks.
The scientists found that masks vary wildly in their ability to filter such tiny particles - with the best-performing materials for homemade masks being blackout drapes and sterilisation wrap for packing surgical instruments.
Multi-layered masks also performed far better than single-layer masks in tests on 33 different materials, including single-layer woven fabrics such as cotton and woven polyester and various filter materials, they added.
The researchers found that two-layered and three-layered samples tested show an overall filtration efficiency of about 50% for submicron particles.
Mask fit is also important since particles can easily escape through gaps at the nose or through the sides of the mask.
The analysis showed that properly fitted and multilayer masks reject 84% of particles expelled by a person when one person wears it.
Two people donning these types of masks reduces particle transmission by 96%.
The researchers said that people should avoid using loose-knitted material, batting fabric, felt, fleece, or shiny, reusable shopping bags.
They also advised against using vacuum bags unless they are certified to be fibreglass-free since often such filters on their own may release glass fibres that can be inhaled.
Nga Lee Ng, associate professor and Tanner Faculty Fellow in the School of Chemical and Biomolecular Engineering and the School of Earth and Atmospheric Sciences, said: "A submicron particle can stay in the air for hours and days, depending on the ventilation, so if you have a room that is not ventilated or poorly ventilated then these small particles can stay there for a very long period of time."
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"The best way to protect ourselves and others is to reduce exhaled particles at the source, and the source is our face. That really gets amplified when everyone starts wearing masks," Dr Ng added.
"Not everyone understands the importance of airborne virus transmission, and the importance of wearing a mask.
"I hope that the practice will continue to help reduce the release of these viral particles into the environment and help protect others.'
Ryan Lively, an associate professor and John H Woody Faculty Fellow in the School of Chemical and Biomolecular Engineering, said: "We found commercially available materials that provide acceptable levels of submicron particle rejection while still maintaining air flow resistance similar to a surgical mask.
"These materials combine fabric fiber density, a maze-like structure, and fiber surface chemistry to effectively reject submicron particles."
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