Among all the many different things we didn't expect to happen this year, shopping for a mask in order to enter banks and other shops is way up there as one of the oddest.

The next challenge of knowing which face covering to use is a whole other minefield. As well as some masks being in short supply due to the need to reserve them for healthcare professionals, not all masks are equally effective. Fortunately, scientists are hard at work to determine which masks are best at filtering out ultrafine particles when we cough.

In a new study, researchers from the University of Cambridge and Northwestern University examined the effectiveness of a wide range of masks and fabrics at containing virus particles between 0.02 and 0.1 micrometres at the velocity of an adult coughing. On top of this, they evaluated the breathing resistance of each material to see which material is most suitable for becoming a face mask.

“There was an initial panic around PPE and other types of face masks, and how effective they were,” first author Eugenia O’Kelly, from Cambridge’s Department of Engineering, said in a statement. “As an engineer, I wanted to learn more about them, how well different materials worked under different conditions, and what made for the most effective fit.”

First up, there's good news.

"The current coronavirus pandemic has left many communities without access to N95 face masks," the team conclude in their paper published in the journal BMJ Open. "Our findings suggest that face masks made from layered common fabric can help filter ultrafine particles and provide some protection for the wearer when commercial face masks are unavailable".

Unsurprisingly, the much sought after N95 masks were found to be highly effective at filtering out ultrafine particles expelled at the velocity of a human cough. However, vacuum bags were found to be even more effective, while other available materials were also comparable, with a few caveats.

For the research, the team contacted online sewing groups to see what materials they had been experimenting with to make their own masks. This included everything from vacuum bags to denim jeans and folded socks.

While many homemade mask materials were effective at blocking particles, they were inappropriate to use as masks due to the breathing resistance when tested by team-members.

“A mask which blocks particles really well but restricts your breathing isn’t an effective mask,” O’Kelly explained. “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. N95 masks are much easier to breathe through than any fabric combinations with similar levels of filtration.”

Eugenia O’Kelly et al / BMJ Open (2020) (CC by 4.0)

While N95 masks remain unavailable to most and the best defense remains social distancing or self-isolation, the authors hope their data helps people who don't have access to commercial masks to make themselves "viable protection". As other studies have found, layered fabrics appear to be the most effective in terms of homemade masks.

"Our data suggest that, in times of severe supply shortage, common fabrics can be layered to create face masks which protect wearers and others from a significant percentage of ultrafine particles," the team wrote in the study. "It should not be inferred that these layered fabrics can protect wearers from more viral particles than N95 masks or surgical masks as our study did not discriminate between viral particles and other ultrafine particles. Many viruses are carried on droplets or other particles significantly larger than those tested here."

The team stress that though they studied the effectiveness of masks under damp conditions – shown in the table above – previous studies have found that masks lose their effectiveness after repeated washing, and warn that masks should not be used indefinitely.

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