Hairdressers and barbers could be inadvertently transmitting Covid-19 to their customers as a result of "inadequate" official guidance stipulating that they should wear visors rather than masks, government advisers have warned.
Scientists have expressed fears that plastic face shields being used by workers in hair salons provide insufficient protection for the wearer and client because they leave a significant gap through which small airborne coronavirus droplets could pass.
In the NHS, visors are generally only used in addition to masks, as only the masks are specifically designed to cover the nose and mouth.
The Telegraph understands that scientists raised concerns about the equipment worn by hairdressers and barbers in the latest meeting of The New and Emerging Respiratory Virus Threats Advisory Group (Nervtag) on Friday, as well as in a recent meeting of at least one sub-committee of the Scientific Advisory Group for Emergencies (Sage). Ministers are facing calls from Sage and Nervtag scientists to require barbers and hairdressers to wear masks or face coverings - either alone or with visors as well.
Swiss authorities raised concerns about the use of visors alone last month after an outbreak among staff at a hotel. All of those who became infected wore plastic visors, while those wearing masks came out unscathed.
Prof Mark Wilcox, a professor of medical microbiology who attends Sage and two of its sub committees, said: "I don't think the guidance is correct and I don't understand why visors were chosen in preference to masks.
"We have, in several settings, discussed the issue of masks versus visors and I think it would be fair to say that I am not alone in these views. My colleagues both in those settings and in other settings share my views.
"I've not come across somebody who has said that visors are the best thing."
Dr Ben Killingley, a consultant in infectious diseases who sits on Nervtag, said: "I don't know where it came from, where someone decided that just wearing a visor for hairdressers alone would be enough. That doesn't make sense to me.
"A visor is primarily worn to protect workers from splashes. It does the same job as goggles, protecting the eyes. No healthcare worker would ever wear a visor without a mask.
"You would have thought for their own protection and just in case they have been infected themselves, they should be wearing a mask themselves. In my view, both to protect the hairdresser and the person coming in, just wearing a visor alone is not enough."
Nervtag is understood to have discussed the issue after Prof Peter Openshaw, one of its members, tweeted his concerns, having had a haircut by a barber wearing a face shield.
"His expelled breath and droplets were directed down onto me," Prof Openshaw tweeted. "Zero protection, I'd say." He added that while his own mask provided protection against possible viral droplets, it would have failed to prevent "corneal seeding", or Covid-19 particles entering his eyes.
On Friday, Boris Johnson expanded the number of venues at which members of the public will have to wear face coverings, to include hair salons and barbers, along with museums, galleries, cinemas and places of worship for the first time from August 8.
Dr Killingley cited a study in the US which found that no Covid-19 cases were reported among 139 clients who were exposed to two asymptomatic hair stylists in Missouri. Both of the stylists were wearing surgical masks or cotton face coverings at each appointment, as were their clients.
Prof Wilcox said the study was "some of the best evidence that I've seen to date validating the usefulness of masks to prevent transmission of virus."
Dr Killingley, who sits on Sage's environmental working group, said he understood why visors were "attractive" to those in "customer-facing" roles, because, unlike masks and coverings, they leave workers' entire faces visible. They are also used by many bar and restaurant staff. But visors "are not as good a tool for stopping infection transmission" in a barbers or hair salon, he insisted.
Currently, government advice to those providing "close contact services" such as hairdressers, barbers and beauty salons, states: "Clearly, when providing close contact services, it often may not be possible to maintain social distancing guidelines (2m, or 1m apart with risk mitigation, is acceptable). As a result, personal protective equipment in the form of a visor will be required to mitigate the risk."
The NHBF, the trade body for hairdressers, beauty salons and barbers, states on its website: "Clients may wear face coverings if they wish and staff can wear a face covering in addition to a visor, but not instead of one." The NHBF said it relied on the Government for advice about personal protective equipment.
Last month, Switzerland's Federal Office of Public Health said it had found increased infections among people who only wore face shields.
A Government spokesman said: “Advice for hairdressers remains unchanged and is that they should continue to wear visors.