Reusable vacuum cleaner bags are the best material for making home-made face masks with, the University of Cambridge has concluded after testing everything from socks to jeans.
The cloth bags, which can be cleaned in the washing machine, were found to be the best at preventing droplets and aerosols from escaping while still enabling wearers to breathe sufficiently well.
Although some materials, such as thick denim and polyester, were better at stopping the spread of ultra-fine airborne particles, they made breathing too difficult to be useful.
"A mask which blocks particles really well but restricts your breathing isn't an effective mask," said the study's first author Eugenia O’Kelly, a doctoral candidate from Cambridge's department of engineering. "Denim, for example, was quite effective at blocking particles – but it's difficult to breathe through, so it's probably not a good idea to make a mask out of an old pair of jeans."
While there are numerous online patterns to guide people through making their own masks, there is little scientific evidence on what the most suitable materials are.
To find out, researchers placed a fabric sample between two sections of tubing and sent through tiny aerolised particles at high speeds, comparable to someone coughing or breathing heavily. Finally, they asked wearers to comment on how well they could breathe through each of the materials to calculate an overall score.
Polyester "windbreaker" material, used for outdoor jackets, was highly effective but was graded in the worst category for breathability.
A folded sock was as easy to breathe through as a vacuum bag but not as effective at catching particles. Likewise, lightweight T-shirt material was extremely easy to breathe through but only stopped around 10 per cent of particles. In contrast, reusable vacuum cloth bags caught more than 40 per cent.
For homemade masks, those with multiple layers of fabric were more effective, and those which also incorporated interfacing – normally used by tailors to stiffen collars – were found to stop up to 60 per cent of particles. However, more layers also made the face coverings more difficult to breathe through.
Ahead of the study, the researchers consulted online sewing communities to find out what types of fabrics they were using to make masks. Due to the severe shortage of proper filters at the time, several people reported that they were experimenting with inserting vacuum bags.
Although researchers found single-use and reusable vacuum bags were effective at blocking particles, they warned that the single-use bags should not be used in face masks as they fall apart when cut and may contain component materials which are unsafe to inhale.
"It's a matter of finding the right balance – we want the materials to be effective at filtering particles, but we also need to know they don’t put users at risk of inhaling fibres or lint, which can be harmful," said Ms O'Kelly.
The researchers also studied the performance of different fabrics when damp and after they had gone through a normal washing and drying cycle.
They found that the fabrics worked well while damp and worked sufficiently after one laundry cycle, but previous studies have shown that repeated washing degrades fabrics and the researchers said masks should not be reused indefinitely.
The researchers said the results may be useful for people when choosing which fabric to use for making masks.
"We've shown that in an emergency situation where N95 masks are not available, such as in the early days of this pandemic, fabric masks are surprisingly effective at filtering particles which may contain viruses, even at high speeds,” said Ms O'Kelly.
The results were reported in the journal BMJ Open.