Which? has carried out extensive testing on face masks that are on the market and raised concerns about the effectiveness of some. The consumer organisation has outlined what consumers need to look out for.

Reusable face masks and coronavirus

It should be noted that coronavirus particles can be much smaller (as little as 0.1 micrometre in diameter), so what we are measuring is not the face covering’s ability to protect against coronavirus, per se.

READ MORE: 'Alarming' - New tests raise concerns over effectiveness of some Covid-19 face masks

Face coverings are not medical devices and aren't designed to block all particles down to these ultra-fine particles, like a higher-grade medical respirator mask would.

Like basic disposable surgical masks, they are intended to help block larger droplets and aerosols emitting from the wearer, who may be asymptomatic, helping to create community protection by minimising exhalation of virus particles.

Basic surgical disposable face masks

While disposable surgical masks must adhere to strict standards on filtration in order to be sold, there is no such binding standard for reusable fabric masks. However, surgical masks are single-use, non-recyclable products, so not really a practical solution for sustained daily use. They could also have a grave impact on the environment if millions end up in landfill.

At around 45p per mask, the cost of using just one disposable per day over the course of a year could be £164 - significantly more than Which?’s Best Buy reusable face coverings which come in packs of 3 for £15.

How to choose a reusable face covering

Try to go for three layers, but at least two – our tests showed a clear difference between single-layer face coverings and those with a double or triple layer.

Choose your material wisely – tightly woven cotton is a good option – the homemade mask we tested did well on filtration – and worst performing masks were a single layer of mostly polyester. Ideally, a mix of different fabrics like cotton, polypropylene, and different types of polyester.

Make sure it’s adjustable, or comes in different sizes – our testers rated face coverings higher for fit and comfort when they were adjustable, either by the ear loops and mouldable nose wire, or because the mask came in different sizes. Find out more in our face mask buying guide.

How Which? Tests face masks

To get at bacterial filtration, Which? used an aerosol generator to shoot bacterial particles that were 3 micrometers in diameter at sections of mask fabric and see what percentage of bacteria made it through.

To test breathability Which? measured the pressure required to draw air through each of the covering at a rate of eight litres a minute, which is two litres above the average breathing volume.

To find out how well the masks would last with repeated use,testers put on and took off the masks 80 times, and to understand how comfortable the face covering is, testers with different face shapes were asked to assess the fit – looking for tightness, gaps and ease of adjustment.

Which? also look for comfort while wearing and talking, and for glasses wearers – whether it made them fog up.