Why Face Shields And Plexiglass Barriers Don’t Block Coronavirus

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Face masks probably help slow the spread of Covid-19, but wearing a mask isn’t practical in certain situations. Customers in restaurants and bars can’t cover their faces while eating and drinking, for example.

When masks aren’t worn, they’re sometimes replaced with face shields or plexiglass barriers. Staff wear plastic visors, for instance, while venues might install transparent screens between two people — as seen on TV during the US vice-presidential debate.

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Shields and barriers are not a safe alternative to covering your nose and mouth, however. Not wearing a mask is dangerous, especially inside buildings, and reflects misconceptions about how Coronavirus is transmitted, and what masks are for.

Coronavirus is airborne

The first misconception stems from thinking that the SARS-CoV-2 coronavirus only spreads through fast, sudden actions like a cough or sneeze, which shoots liquid into the air to create ‘ballistic droplets’.

But droplets are also generated from fluid in the respiratory tract through less violent activity, such as simply talking or breathing. Scientists say those respiratory droplets from an infected person can hang around in the air and be inhaled by other people. The microscopic droplets create a fine mist that’s invisible to the naked eye — an airborne solution, or aerosol.

The word ‘aerosol’ probably contributes to the public misunderstanding of virus transmission because when people imagine aerosols, they might think of an aerosol spray — something released in a particular direction, not the mist that’s left behind.

Droplets aren’t always bullets

Once you understand that an aerosol doesn’t necessarily travel in one direction, it’s obvious why a barrier won’t block its movement. When a customer and cashier are facing each other at the supermarket checkout, a plexiglass screen may well block droplets if one person coughs or sneezes, but it can’t stop mist.

A group of scientists organized by aerosol researcher Jose-Luis Jimenez of the University of Colorado, Boulder, recently produced a document entitled ‘FAQs on Protecting Yourself from COVID-19 Aerosol Transmission‘. As the FAQs say, “face shields do not offer much protection against aerosols […] while masks do.”

The document explains that a shield is good for blocking ballistic droplets and can supplement eye protection (safety goggles) but can’t substitute for a mask. Likewise, “Plexiglass barriers are generally useful to avoid […] direct aerosol transmission whenever people are in close proximity and distance cannot be kept.”

Aerosols can spread like smoke

Understanding how an aerosol can transmit the virus becomes even easier once you compare aerosols to cigarette smoke instead of a spray. Air currents can drive virus ‘smoke’ around a barrier to reach you indirectly, but shouldn’t pass through a mask.

Even if you can’t see smoke, you still know it’s there because you can smell it. And though you can’t sniff-out viruses, you can spot when someone isn’t wearing a mask, allowing you to treat that ‘smoker’ as a potentially infected individual — someone who might not show symptoms because they’re an asymptomatic carrier of the virus.

Smoke also offers a way to understand asymptomatic transmission. In the smoking analogy, a pregnant woman inhales passive smoke from a smoker that might affect her unborn baby’s health, just as someone infected by an asymptomatic carrier could pass Coronavirus to an unseen third party, such as an elderly person who’s vulnerable to suffering from symptoms and potential life-threatening effects.

Masks help protect other people

The second misconception which leads to thinking that shields and plexiglass protect people stems from misunderstanding what masks are for. A face shield is a form of ‘Personal Protective Equipment‘. Full PPE is often worn by hospital staff and other healthcare workers if they’re at a high risk of being sprayed directly by droplets that might contain germs.

PPE should be appropriate for the situation in which it’s used. You wouldn’t wear a hard hat to protect yourself from noise, for instance, nor would you use a pair of ear defenders to prevent objects hitting your head. Likewise, a face shield won’t create an effective barrier against a virus carried in an aerosol.

As the ‘Personal’ in PPE implies, it’s designed for the person wearing it. But while you might assume the main purpose of a mask is to protect the wearer, it’s actually to stop them spreading the virus (possibly via asymptomatic transmission). As the Centers of Disease Control says, “Wear a mask. Protect others.”

You’re morally responsible for others

Plastic face shields and plexiglass barriers have many advantages over masks. Shields are more comfortable, for instance, while barriers are more convenient. Both allow for clear communication because you can see a person’s mouth moving.

But those benefits don’t outweigh the obvious costs. Businesses let employees wear shields because they believe that visors block Coronavirus. They don’t. The virus is spread by aerosols, which won’t necessarily travel in one direction and will spread according to air currents. Without proper indoor ventilation, people will get sick.

If you work in a bar or restaurant, point your employer to this article as they need to understand that wearing masks will reduce the risk of customers catching Covid. The guidelines provided by governments and public health authorities like CDC don’t follow the science strictly enough.

On the flip side, people who need to be mask-less should avoid visiting places where the staff aren’t masked. It’s not worth the risk of infection to you or those you come into contact with. Helping stop Covid-19 from spreading is a moral responsibility.

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