After months dismissing the evidence in favour of face coverings, health secretary Matt Hancock announced on Tuesday that they are to become mandatory in English shops and supermarkets as of 24 July.
Shortly afterwards, the official Downing Street Twitter account posted a new video, which explained the changing “guidance”, concluding: “By wearing a face covering, you are protecting others and slowing the spread of the virus.”
However, it was quickly pointed out that the face masks used in the graphic – which appeared to feature a valve – would be unlikely to offer protection to anyone but the person wearing it.
Masks with valves are typically used in construction work and are unsuitable for medical settings because, while they filter the air consumed by the wearer, they offer no protection to those around them.
The apparent error was spotted by Julia Simons, the Doctors’ Association UK’s medical student representative, who was deployed to the front line of the pandemic during her final year of studies.
“How can the government video not get this right?” she tweeted in response to Downing Street’s video. “Continuous dither, delay and disaster.”
Ms Simons added: “Please do wear a cloth face covering. Don’t wear a facemask with a valve. The valves means your face mask is no use for protecting others.”
The video remained on Twitter nine hours later, having been viewed nearly 140,000 times and also shared by the Department for Health and Social Care's account.
In the US, such masks have already been banned in parts of Florida and California.
“When you wear a mask with a valve, people around you are not protected because the valve lets all of your breath into the air,” Marisa Glucoft, director of infection prevention Children’s Hospital Los Angeles has previously said.
“From that perspective, it’s almost like you’re not wearing a mask at all.”
The guidance on face coverings in England is changing.
From 24 July, it will be mandatory to wear a face covering in shops and supermarkets, as well as on public transport.
Your face covering must cover your nose and mouth at all times. pic.twitter.com/qKKqNDSAu5— UK Prime Minister (@10DowningStreet)July 14, 2020
Dr Marissa Baker, the University of Washington’s industrial hygiene programme director, told AFP: “Masks that have a one way valve, do allow for unfiltered air to be exhaled.
“For this reason, wearing this mask doesn’t protect the people around you. Therefore, this type of mask is not recommended for Covid-19.”
The San Francisco Department of Public Health tweeted in May that masks with valves “are NOT safe and may actually propel your germs further”.
Responding to the government’s video, another Twitter user wrote: “The valve is a one way feature to let the wearer breathe out and not breathe in toxic or irritating fibres, dust etc.
“The purpose of a mask in the pandemic is to stop you breathing out your virus, if you have it, protecting others. So this advice is dangerously wrong.”
Downing Street were approached for comment.