Plastic visors are being widely used as a way of protecting people in shops, bars and beauty salons from the coronavirus. But do they really work against a virus thought to be airborne?

With a lungful of air, Kerstin Rosenfeldt steps into position on the raised stage. The lights are down. It is quiet. Then she opens her mouth to blast out a brief segment of an operatic aria.

As she does so, a fine mist of tiny water droplets burst forth from her airways, the bright lights off to one side making their swirling movements visible. With each word she articulates, the cloud of droplets surges forward rapidly before twisting lazily in the air a metre or so in front of her.

This mist is a simulated aerosol, composed of water vapour she breathed in moments before she started to sing. Although exaggerated, it is meant to replicate the fine spray of respiratory fluid released when people speak, breathe and sing.

Every time we utter just a couple of words, we spray thousands of aerosol droplets, which are mostly invisible to the naked eye, into the air in front of us. When someone is infected with a respiratory virus like the one that causes Covid-19, each aerosol droplet can contain thousands of viral particles, each with the potential to infect others nearby who breathe them in.

There is now growing evidence that airborne aerosols carrying the coronavirus may play a major role in spreading Covid-19. The World Health Organization doesn’t consider Covid-19 to be an airborne disease – one that can last for a significant period of time in the air while still remaining infectious – but many scientists now believe it is.

Fortunately, Rosenfeldt isn’t spreading coronavirus as she sings on the stage at the Bavarian Broadcasting’s studio in Unterföhring, Munich. But the plumes of aerosol coming from her mouth and nose are being closely monitored with high-speed cameras set up by Matthais Echternach, head of phoniatrics and paediatric audiology at Ludwig-Maximilians-University Hospital Munich, and Stefan Kniesburges, a fluid mechanics expert at the University Hospital Erlangen. They are hoping to find out how far singers on stage or in choruses may need to keep from each other and their audience to reduce the risk of spreading Covid-19.

It is surprisingly far when a singer is in full flow.

“Some professional singers can create an aerosol cloud up to 1.4m in front of them,” says Echternach. Larger droplets – more like little globules of spit – fly forward and downwards to land on surfaces about 1.5m away. Coughing could send aerosols even further – up to 1.9m, while other studies have shown that a sneeze can project aerosol clouds up to 8m away.

When Rosenfeldt dons a mask – the kind you might see a surgeon wearing in an operating theatre – the situation changes dramatically. Rather than billowing in front of her, the plume is stunted. Jets of aerosol spurt upwards out of gaps where the mask sits over her nose, and out of the sides. Although it is clear the virus might still escape into the air in a mask like this, the amount of aerosol carrying it is greatly reduced.

“There were none of the larger droplets when wearing the mask,” says Echternach. “Aerosols are more problematic, if the mask is not tight fitting, they will find the easiest way out and escape through the masks. The mask does reduce the speed of the aerosols at the front.”

Tests with nine other singers from the Bavarian Radio Chorus revealed similar results. But all struggled to sing with the mask on – as most of us who have worn even home-made masks in recent months will know, breathing is harder and the fabric can muffle the voice.

“Singing with a mask on is almost impossible,” says Echternach, who is also a trained singer himself. “So we did another experiment with two singers wearing face shields.”

These are the plastic shields that doctors treating Covid-19 patients often wore as part of their personal protective equipment, but now these visors are becoming common in other settings such as shops, beauty salons and bars. Some choirs have also started practicing with them too. Many people have chosen them as an alternative to fabric face masks when they venture out of their homes during the pandemic.

There are countless videos online demonstrating how to fashion home-made versions from clear binder covers, empty fizzy drinks bottles or left-over plastic packaging. Major companies including Apple, Nike, Babcock, and Ford, have used their production lines to manufacture face shields, while sports brand Oakley has designed face shields for NFL players to wear on their helmets. 

In some countries, including the UK, governments have issued official advicethat visors be worn by staff who work in close contact with members of the public, such as hairdressers, barbers, beauticians, tattoo artists and studio photographers. People who are testifying in court, giving lectures or performing in public are also recommended to wear face shields by some US states. Similar advice is offered by the government in Singapore while some states in Australia say face shields can be worn instead of face masks in public.

These are certainly great at stopping larger droplets of spit leaving the mouth of the wearer – Echternach and his team’s experiments showed they quickly become speckled with moisture. Similarly they also help to protect the spit of others hitting the wearer’s face.

Where things get difficult is with what happens to the aerosols when wearing a plastic shield.

“Nearly all of the aerosols were coming around the side of the face shield and reached nearly the same distances as without wearing anything,” says Echternach. These results are still to be published, but Echternach says they should serve as a warning for anyone relying on face shields alone to keep them safe as pandemic lockdowns are eased.

“They are certainly not effective when you are in close contact with someone,” he says.

The US Centers for Disease Control and Prevention appears to agree with him – it does not recommend face shields for normal everyday activities or as a substitute for masks. The Swiss health authorities have also warned against using face visors instead of masks after an investigation into an outbreak of Covid-19 at a hotel in the canton of Graubünden revealed all those who were infected had been wearing plastic face shields, while those who avoided infection were in masks.

But the scientific evidence around the effectiveness of face shields is mixed. While Echternach’s research looked at what happens when someone who might be carrying the virus is wearing a shield, other researchers have tried to assess how they might protect the wearer from other people around them.

One recent, but as yet unpublished, study by Israel Institute for Biological Research used water-sensitive paper stuck around the face of a mannequin wearing a plastic face shield. For larger droplets sprayed directly into the wearers face from 60cm (2ft) away by a cough or sneeze, it was as effective as a face mask. The shield also appeared to be up to 10 times more effective against fine aerosols than the mask.

Visors bring a number of advantages over masks, such protecting the eyes – which can be an entry point into the body for some viruses. They can also help to reduce the risk of people touching their face and inoculating themselves with any virus they have picked up on their hands. They are also generally considered to be more comfortable and fog glasses less when the wearer breathes.

“Face shields offer a number of advantages,” wrote Eli Perencevich, an epidemiologist at the University of Iowa in a recent article in the Journal of the American Medical Association calling for their widespread use. “While medical masks have limited durability and little potential for reprocessing, face shields can be reused indefinitely and are easily cleaned with soap and water, or common household disinfectants. People wearing medical masks often have to remove them to communicate with others around them; this is not necessary with face shields.” (Learn more about how face masks can affect our communication.)

But face shields appear to be only truly effective in ideal conditions: when someone is coughing directly onto the plastic surface. In most everyday situations, such as in a hair salon, the wearer might be moving around in close proximity to their customers as they cut hair.

When the Israeli researchers moved the “cough source” just 30cm (1ft) above or below the mask – but still kept it 60cm away horizontally – the mannequin's face became spattered with droplets that spilled in around the sides of the visor. In these situations they estimated that the visor was only 45% effective at blocking droplets.

Tests with actual aerosols and droplets that contain virus are few, but analysis carried out by the US National Institute for Occupational Safety and Health in Morgantown, West Virginia, found that a face shield could block 96% of larger cough droplets when they were laden with influenza virus. But with smaller cough aerosols less than 3.4 micrometres in size, the face shield blocked just 68%.

But there is another complication. As Echternach’s tests with the singers showed, the fine mist that is released as we talk, sing or cough doesn’t just disappear once it reaches a metre or so away. While the larger droplets will fall quickly to the ground or other surfaces, the microdroplets we produce can remain suspended in the air for several minutes, and in some cases hours in very still conditions. In well-ventilated rooms, or those with disturbed air, the time they remain airborne is thought to be much lower.

There are even some reports that droplets contaminated with coronavirus can spread through the ventilation systems of buildings – swabs of air exhaust outlets at a hospital in Singapore treating Covid-19 patients tested positive for the virus.

With virus-laden aerosol drifting around a room, these can then easily creep in the large openings at the side of plastic visors. The tests by the National Institute for Occupational Safety and Health showed the aerosols dispersed throughout the room in the 30 minutes after a cough. In this situation the face shield reduced inhalation of the virus-filled aerosol by just 23%.

Of course, it is still unclear just how many copies of the Covid-19 virus can be carried in aerosol droplets. Flu research suggests finer microdroplets of the type capable of lingering in the air can contain many tens of thousands of copies of the influenza virus. More recent research has suggested that the virus could survive in these droplets for up to three hours while suspended in the air.

Most researchers, Echternach among them, agree that perhaps the most effective way to use a face shield is to wear a mask underneath it.

“Just wearing a face shield isn’t going to be very protective,” says Echternach. The tighter the fit of the mask underneath, the better, he adds. Gaps can cause aerosols to escape and get in – even with tight fitting masks like the N95 respirator, wisps of facial hair around the seal can reduce their effectiveness.

For those hoping to re-join choirs or sing in religious services, Echternach and his colleagues have some further advice – keep a distance of at least 2.5m (8ft) apart at the front and 1.5m (5ft) on either side when singing. Sticking to well-ventilated rooms is also important, Echternach adds.

While the findings could equally be applied to other settings, his concern about singing is well placed.

One infamous case of Covid-19 superspreading occurred during a two-and-a-half-hour-long choir practice of the Skagit Valley Chorale in Mount Vernon, Washington. Fifty-three of the 61 choir members who attended the rehearsal later fell ill and were confirmed or suspected to have Covid-19. Two of the group later died. It is thought a single member of the choir who had developed “cold-like symptoms” a couple of days before the rehearsal may have been responsible for the spread.

“It is very bad as it has brought some very bad connotations to singing,” says Echternach. “It is a shame because singing is very good for our health and mental wellbeing.”

For Kersten Rosenfeldt and her fellow singers at the Bavarian Radio Chorus, any information about how to stay safe from the virus is welcome. “There is a great thirst for knowledge on this issue,” says Susanne Vongries, manager of the choir.

Like the rest of the world, they are now waiting to see just how the pandemic plays out.

BBC

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