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Virus protection from face shields

Face shields are widely used in healthcare to protect workers from virus contamination in droplets (i.e. large visible drops of mucous) and aerosols (i.e. very small particles that can be inhaled) from patient coughs, but how effective are they?

Research by the US National Institute for Occupational Safety and Health (NIOSH) helps us understand what protection they can provide. The researchers simulated a coughing patient using machinery that generates a droplet spray and placed a breathing mannequin in front of it. They varied the distance between the coughing and the breathing headforms and had either large (8.5 μm) or small diameter (3.4 μm) aerosol produced with each cough. The experiment did not measure the protection from droplets splattering into the face, which we can assume was 100% In the experiments the visor was very good at protecting against the initial larger aerosol exposure, where more than 95% was diverted away by the visor. The face shield also reduced the surface contamination of a respirator worn behind the visor by a similar amount. However, it was less effective for the smaller diameter aerosol where it blocked around 70% of the initial cough aerosol and 76% of the respirator surface contamination.

The visor was much less effective at protecting the worker in the period after the cough, and in the 30 minutes following a cough the aerosol dispersed through the room and the face shield only reduced aerosol inhalation by 23%. This underlines the need to wear the visor in combination with an effective respirator to achieve effective protection. The authors concluded that the:

‘…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.

Reference

Lindsley WG, Noti JD, Blachere FM, Szalajda JV, Beezhold DH. (2014) Efficacy of face shields against cough aerosol droplets from a cough simulator. Journal of Occupational and Environmental Hygiene; 11: 509-18.