Is there any point in wearing a transparent face shield?
Text updated on 2020-06-05
The face shield is much more convenient to wear than a mask, in many ways. It protects well against direct projections, and is therefore equivalent to standing 2m away from the person you are talking to. However, it does not replace the mask, especially indoors, because the aerosol of the micro-particles can pass through the sides to reach the mucous membranes of the face. We, therefore, recommend wearing it with a mask indoors!
The face shield has many advantages over fibre or fabric masks: the face is fully visible, allowing non-verbal communication, especially smiling and lip-reading for the hearing impaired. Unlike masks, the face shield does not get too hot, does not scratch, and does not interfere with breathing. It protects the entire face from splashes and prevents touching the face, thus preventing contamination from hands. The face shield is more durable than a mask, it is easy to wash (water and soap) and disinfect (70% alcohol). The protection is best when the face shield covers the cheeks up to the ears, and goes down below the chin.
But does the face shield provide adequate protection against coronavirus infection, since it is not closed at the sides? There are no published studies that formally demonstrate this. However, one study suggests that the face shield strongly reduces the number of viruses inhaled by the wearer: simulations indicate that the face shield would reduce direct viral exposure by 96% when the exposed person is less than 50 cm away from the coughing person: the large droplets that contain most of the viral load are stopped. The face shield is less effective at trapping small particles, those that stay in the air for a long time and spread further. The face shield stops only a third of the aerosol in suspension within half an hour of emission, but this aerosol contains a very small proportion of the total viruses emitted.
If I already have a mask, why put on a face shield too?
- The face shield added above a mask protects it from splashes. This increases its effectiveness and duration of use, especially in hospitals, and the possibility of reusing it after decontamination.
- For other types of viruses, the face shield has been used in addition to the mask to prevent eye infections (herpes, flu). It advantageously replaces the waterproof glasses that fog up too easily, and simple glasses that let viral particles pass through the sides.
- The eyes are a gateway for the SARS-CoV-2 coronavirus as well. Cells in the conjunctiva and cornea express the ACE2 receptor, so the virus can attach to them and infect them. This quite often results in conjunctivitis associated with COVID-19.
- Tears flow from the eyes to the nasal passages through the tear duct. This can accelerate the spread of infection to the lungs. It is therefore useful to protect the eyes.
Overall, the face shield is therefore quite effective, but much less than a non-woven polypropylene mask (surgery and FFP2 types). As a result, the face shield has almost the same effect on particle and virus transmission as maintaining a distance of 2 m from a person. The face shield does not replace a mask and is not a respiratory protection device since aerosols pass through the sides. The face shield should, therefore, be used in combination with a mask as additional protection for the eyes, facial skin, and mask, especially for caregivers who are exposed to splashes and/or have too few masks to change them often.
The face shield has its place in the fight against COVID-19. One point of view explains the advantages of the face shield: the authors argue that it is useless to stop 100% of the emitted viruses, and to protect 100% of the people, to stop the epidemic.Perencevich, E. N., Diekema, D. J., & Edmond, M. B. (2020). Moving Personal Protective Equipment Into the Community: Face Shields and Containment of COVID-19. JAMA.
An experimental study models the passage of influenza viruses in particles of different sizes, with and without face shield, at various distances. Direct comparison with a mask is not performed.Lindsley, W. G., Noti, J. D., Blachere, F. M., Szalajda, J. V., & Beezhold, D. H. (2014). Efficacy of face shields against cough aerosol droplets from a cough simulator. Journal of occupational and environmental hygiene, 11(8), 509-518.
An interesting review on the different types of face shield, which cites some work on their effectiveness, and the regulations on their wearing in the United States.Roberge, R. J. (2016). Face shields for infection control: A review. Journal of occupational and environmental hygiene, 13(4), 235-242.
Coronavirus receptors are present on cells in the ocular conjunctiva and cornea.Zhou, L., Xu, Z., Castiglione, G. M., Soiberman, U. S., Eberhart, C. G., & Duh, E. J. (2020). ACE2 and TMPRSS2 are expressed on the human ocular surface, suggesting susceptibility to SARS-CoV-2 infection.
SARS-CoV-2 infects cells in the ocular conjunctiva, bronchial epithelium, and pneumocytes in the lungs.Hui, K. P., Cheung, M. C., Perera, R. A., Ng, K. C., Bui, C. H., Ho, J. C., ... & Poon, L. L. (2020). Tropism, replication competence, and innate immune responses of the SARS-CoV-2 coronavirus in human respiratory tract and conjunctiva: an analysis in ex-vivo and in-vitro cultures. The Lancet Respiratory Medicine.