In recent weeks there has been increased discussion around the potential use of face shields as a tool to help exit lockdown and prevent further spikes in infections of Covid-19.
Face shields provide a high level of protection for the wearer. Given they cover the whole face – the mouth, nose and eyes – a high percentage of viral particles are prevented from reaching the wearer. Face shields are arguably best at protecting from coughs and sneezes. (As studies have shown, however, viral droplets, particularly small airborne speech particles, can remain in the air for a period of time and these particles can be sucked in around the shield1.)
Shields can therefore be very useful tools for those facing very regular contact, at close proximity, with others – for instance in medical settings. They also have great value in professions where nonverbal communication through facial expressions is important, such as teaching. They could be valuable in giving protection and confidence to teachers, enabling them to return to a classroom environment.
For this reason we recommend that face shields are procured and supplied to key groups such as teachers, health-care workers, emergency services staff, transport workers and those working in education.
Their use should also be encouraged in private sector settings such as retail, leisure and hospitality.
In our previous paper on masks, The Role of Masks in Exiting Lockdown, we recommended the public use a form of face covering when in large groups and where social distancing is not possible. Unlike face shields, which are designed to protect the wearer, masks inhibit the outward transmission of viral particles and therefore help protect others.
For the general public, a normal medical mask is likely to be more effective, therefore, in helping control transmission of the virus.
Speaking to us about this issue, Jeremy Howard, co-founder of Masks4All2, said that, “The best masks for source control of speech droplets are absorbent and breathable, for instance, using a combination of cotton and paper towel. However, these masks are not as effective for coughs and sneezes, or heavy breathing during exercise. Therefore, they should be combined with face shields, to achieve broader protection. Face shields on their own, however, are not effective at protecting the wearer from airborne small speech droplets.”
Combining both a mask and face shield would, of course, provide significant protection.
What Is a Face Shield?
Face shields come in many forms, but all form a clear plastic face covering. They are mainly deployed in health-care settings.
To provide optimal protection, they are designed to cover the full face (from the top of the head to the chin and covering the ears horizontally) and protect the wearer from viral spray particles.
Face shields are generally cheap to produce, easier and more comfortable to wear than masks and can, potentially, be reused indefinitely (as long as they are correctly cleaned).
The Purpose of a Face Shield
Face shields are valuable in protecting the wearer from inward inhalation of aerosol droplets that may be carrying the virus. While they offer some level of source control, since particles emitted through coughing and sneezing would be somewhat contained, they are less effective than masks when it comes to preventing the wearer from transmitting viral droplets outwards.
With this mind, the main purpose of face shields is to protect the wearer. Their effectiveness in this regard is enhanced because they are more likely than masks to be worn properly, and to be kept on for long periods of time. They are particularly useful, therefore, for those who are likely to have a lot of contact with the public. This is especially the case for those in jobs where nonverbal communication using facial expressions is important, such as teaching young children.
Shield-wearing is not an isolated measure. If the population adopts mask wearing in public places, as recommended in our previous paper “The Importance of Masks in Exiting Lockdown”, then transmission will be reduced. Combined with face shields, we the wearers would have enhanced protection.
The data on the efficacy of face shields is more limited than on masks. We include below the key study on shields and some of the recent commentary about their efficacy in responding to Covid-19.
It appears clear that face shields offer a high degree of protection to the wearer from close range exposure to viral particles emitted through coughing and sneezing.
They do, however, allow particles – particularly small airborne speech droplets that hang in the air – to be sucked in around the shield. And, while they offer some level of source control, face shields are not as effective as masks in preventing the outward transmission of particles.
National Institute for Occupational Safety and Health
A 2014 study on face shields titled “Efficacy of Face Shields Against Cough Aerosol Droplets From a Cough Simulator” has been cited in many places as the core evidence available.
The study used a coughing-patient simulator and a breathing-worker simulator to investigate the exposure of health-care workers to cough droplets and examine the efficacy of face shields in reducing the level of exposure.
The results of the study found the following:
- 9 per cent of the initial burst of aerosol from a cough can be inhaled by a worker 46 cm (18 inches) from the patient.
- During testing of an influenza-laden cough aerosol with a volume median diameter (VMD) of 8.5 μm, wearing a face shield reduced the inhalational exposure of the worker by 96 per cent in the period immediately after a cough.
- The face shield also reduced the surface contamination of a respirator by 97 per cent.
- When a smaller cough aerosol was used (VMD = 3.4 μm), the face shield was less effective, blocking only 68 per cent of the cough and 76 per cent of the surface contamination. In the period from one to 30 minutes after a cough, during which the aerosol had dispersed throughout the room and larger particles had settled, the face shield reduced aerosol inhalation by only 23 per cent.
- Increasing the distance between the patient and worker to 183 cm (72 inches) reduced the exposure to influenza that occurred immediately after a cough by 92 per cent.
The authors conclude that their “…results show that health care workers can inhale infectious airborne particles while treating a coughing patient. Face shields can substantially reduce the short-term exposure of health care workers to large infectious aerosol particles, but smaller particles can remain airborne longer and flow around the face shield more easily to be inhaled. Thus, face shields provide a useful adjunct to respiratory protection for workers caring for patients with respiratory infections. However, they cannot be used as a substitute for respiratory protection when it is needed.”
The Tony Blair Institute for Global Change
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