Some reusable face masks found on the high street have "alarming flaws" in their effectiveness, with one popular mask only blocking a third of potentially harmful particles, consumer group Which? has said.
Masks from a range of popular brands were tested by the consumer watchdog to see how well they filtered particles, how breathable they were, and whether they became less effective after washing.
The worst product, a pack of three Adidas masks being sold for £17, only filtered a third of particles in the tests.
Meanwhile, the Vita Shield face covering (£9.99 for three) filtered 60% of particles on the first use but this dropped to 38% after five washes.
Adidas said its mask had been designed early on in the pandemic, before there were defined performance criteria for face coverings, and that it had tested its product for comfort and breathability.
The Vita Group said external accredited testing facilities had found filtration levels of more than 90% for its mask.
The best performing face covering – the Airpop Pocket – managed to block more than 99% of potentially harmful particles, making it just as effective as a disposable mask.
The covering, sold at £24.99 for a pack of four, is made from similar materials to disposable masks but is "semi-reusable" – meaning it can only be washed and reused a set number of times before it should be thrown away.
Other masks that fared well were from Superdrug (£4.99 for three), The Big Silk (£16.90 for one) and Marks and Spencer (£9.50 for five), scoring highly for filtration, breathability and comfort.
Some face masks did not have as high filtration levels as they stated on the packet when tested by Which?.
The Wise Protec (£10 for one), sold at John Lewis, Matalan and Home Bargains, and Alvita (£7 for two), sold at Boots, both claim to have more than 90% filtration efficiency.
However, Which? tests found their filtration levels were at 80% and 82% respectively. This dropped to the high 60s for both after washing.
Both companies said their filtration levels had been verified in independent, accredited labs.
Boots’ Adult Reusable Face Mask (£10 for two) did initially have the more than 95% filtration it claimed on its packet, but this dropped to 75% after five washes.
The high street chemist said its mask had been "rigorously tested".
Which? magazine editor Harry Rose said too many face coverings still "fall short of the mark", adding: "We would like to see more manufacturers committing to minimum performance standards for filtration and ensuring basic safety instructions are present, but for now it is worth taking time to research the best option for yourself and your loved ones before buying."