There continues to be a dispute about whether a face shield (sometimes called a “visor”) is as effective or more effective than a face mask. Frankly, I do not understand why. Face shields will not protect someone from airborne Covid-19 – period. A face mask must always be worn with a face shield.
This is not an opinion – it is physics. As long as there are gaps on the bottom and sides of the face shield, any airflow (even created by walking) will create negative pressure inside the shield that will draw particles into the breathing zone of the wearer.
Several weeks ago I wrote this article in a semi-humorous tone thinking that as soon as someone read it they would say – “Of course, that makes sense.” https://www.texairfilters.com/the-adventures-of-hairy-manne-or-why-you-should-always-wear-a-face-mask-with-a-face-shield/
Since I wrote the original article, the dispute between face shields and face masks has continued. One individual touted a particular type of face shield. I purchased it and did what I am now calling “the tissue test.” It is pretty simple. Position the face shield in an airstream. Hold the tissue (either facial or toilet) in the airstream outside of the mask and observe it going away from the source of the airflow. Then hold the tissue inside the face shield. Voila! The tissue will move toward the source ie. into the breathing zone of the wearer.
This “test” is not hard. It takes about 15 seconds and would be a great demonstration in a 7th grade science class to help explain the concept of differential pressure.
Consequently, there is a good argument to be made that a face shield without a face mask is actually MORE dangerous than nothing at all. So, what can be done? Face shields have some advantages – particularly from droplets that travel more in straight lines.
To answer this question I contacted my lifelong friend Jim Hodgson. Jim has always loved aviation. He is a Vietnam vet who flew single wing reconnaissance airplanes, was a commercial pilot and after retirement became the Executive Director of the Fort Worth Aviation Museum.
After he read my original article, his comment was: “I agree with your conclusion. There is definitely negative pressure inside the face shield. In aviation we call this ‘turbulence.'” So, in effect, the face shield on a walking person is a very slow moving aircraft.
I asked him what is done in aviation to avoid turbulence. He had three suggestions – wind vortex generators, stall fences and spoilers. It is beyond the scope of this article to explain these, but there are potential alternative designs that will make face shields more effective. Developments in the redesign of the face shield could have significant benefits in our fight against Covid-19.
Researchers who have tested face shields have arrived at the same conclusion. For example, the noted indoor air quality expert, Professor Linsey Marr of Virginia Tech University, has done extensive testing of various different types of face masks. As a part of this research her team also tested face shields and found that they offered almost no protection from aerosols, at best a 10% reduction on a manikin, sampling through the mouth at a realistic velocity.
The bottom line is this. People think that face shields will protect them from Covid-19. On their own – they will not. I have teacher friends who are intending to teach classes this fall with a face shield only. I see workers at retail stores wearing face shields with no mask. Texas now has a stockpile of 500,000 face shields.
People need to be made aware of the risks of wearing a face shield only. Thanks for whatever you can do to share this fact.