In June, I authored a blog post regarding the use of masks to decrease transmission and susceptibility to COVID-19 infection. As I noted in that post, recommendations change as new information becomes available particularly in a pandemic where little is known at the onset. It is important to note that this disease has been known to humankind for less than one year.  However, our knowledge is improving on a daily basis. In October, the Centers for Disease Control and Prevention (CDC) updated its COVID-19 guidance to include that COVID-19 can be spread through airborne aerosols. What does this mean in terms of COVID-19 transmission and how can you best protect yourself?

The evidence is now clear that COVID-19 can be spread both by droplet and airborne aerosol transmission.

Droplets are larger particles that are produced when people cough, sneeze or simply talk. Because they are heavier, they tend to drop to the ground quicker and are unlikely to spread more than six feet. They are the most likely source of COVID-19 spread, but aerosol transmission also occurs.

Aerosols are smaller, lighter gasses which can linger in the air for minutes to hours and travel larger distances. Cigarette smoke is an aerosol and provides a good analogy of how far and long COVID-19 could linger in an enclosed, poorly-ventilated area.

So how do we protect others and ourselves from COVID-19? Until–and even after a vaccine is available–the steps we take to minimize the risk of infection tend to be additive. The virus spreads from person to person, so the less you interact in person with those you don’t live in the same household, the lower your chance of infection. Distancing and facemasks remain the primary way to limit the chance of becoming infected or infecting others should you be unaware that you are carrying the virus.

More recently, face shields have been getting a lot of attention as being potentially useful to decrease the spread of COVID-19, but they are not as beneficial as facemasks.

Face shields are curved plastic or plexiglass panels attached to a headband and worn to cover the face. Their primary use is as eye protection for the person wearing them. Face shields protect the eyes from droplet exposure from persons who are near to the person wearing the mask.

Evidence for benefit in COVID-19 is limited and the CDC does not recommend substituting face shields for facemasks.

One potential benefit of wearing a face shield is that they naturally decrease the amount of times a person will touch their own eyes, mouth or nose.  It is also hard to wear a face shield incorrectly whereas many people seem to struggle to keep a facemask over their nose. Face shields might be useful for those who care for someone who is hearing impaired and dependent upon lip reading, but that is a very limited use and they will not offer adequate protection for either the wearer or those around them.

The problem with face shields as a primary COVID-19 tool is that face shields do not fit snugly. While they may prevent droplets from an infected person from spreading beyond the mask, aerosols can spread around the mask. This was demonstrated in a model published in the journal Physics of Fluids on September 1, 2020. While face shields stopped an initial jet of expelled droplets, aerosols readily spread around the shield. Swiss health authorities also have described a hotel where employees wore either facemasks or face shields. There was an outbreak of COVID-19 that infected only the employees who wore face shields and a guest was infected by one of those employees.

If there is a role for face shields, it is to use them in addition to facemasks and distancing to further decrease the risk of COVID-19 spread. Frontline healthcare workers and other first responders who are working with known or suspected COVID-19 patients regularly utilize face shields in addition to masks. The general public could consider using them in addition to masks particularly if they are employed in settings or are in other situations that bring them into close proximity with potential COVID-19 infected people. If face shields are used, they should be cleaned after use with warm soapy water or alcohol wipes containing at least 70% alcohol.

Distancing, wearing masks and staying home, particularly if you are not feeling well, remain the primary ways to avoid COVID-19. Infections increase when guidelines are not adhered to and people gather in close proximity, in enclosed spaces and without masks. We have the knowledge to help minimize spread of this virus, but we need the will to use that knowledge appropriately and effectively. Not adhering to guidelines puts you and your family, friends and neighbors at risk. If you or someone you are regularly in close contact with have higher risks for severe disease, you should take the maximum possible steps to avoid infection. Please listen to your local public health guidance, wear a mask, stay healthy and save lives.

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