Thread by @DrEricDing

📍FACE SHIELDS NOT GREAT for #COVID19: Costco says wear “mask **or** face shield”—worrisome. Face shields don’t stop airborne aerosols.

“Nearly all of aerosols were coming around the side of shield & reached nearly the same distances as without wearing anything.” says study. 🧵

2) “They are certainly not effective when you are in close contact with someone”

That’s why also CDC doesn’t recommend face shields.
bbc.com/future/article…

3) The Swiss health authorities have also warned against using face visors instead of masks after an investigation into an outbreak of Covid-19 at a hotel revealed all those infected wore plastic face shields, while those who avoided infection wore masks. bag.admin.ch/bag/en/home/kr…

4) “But the scientific evidence around the effectiveness of face shields is mixed. While Echternach looked at what happens when someone who might be carrying the virus is wearing a shield, other researchers assessed how they might protect the wearer from other people around them.

5) “One recent study used water-sensitive paper stuck around the face of a mannequin wearing a plastic face shield. For larger droplets sprayed directly into the wearers face from 60cm (2ft) away by a cough or sneeze, it was as effective as a face mask...”

6) “The shield also appeared to be up to 10 times more effective against fine aerosols than the mask.” But this for self protection from others, and it’s for water test. Not for one who can transmit to others. medrxiv.org/content/10.110…

7) “But face shields appear to be only truly effective in ideal conditions: when someone is coughing directly onto the plastic surface. In most everyday situations, such as in a hair salon, the wearer might be moving around in close proximity to their customers as they cut hair.

8) “When researchers moved the “cough source” just 30cm (1ft) above or below the mask – but still kept it 60cm away horizontally – the mannequin's face became spattered with droplets that spilled in around the sides of the visor...”

9) “In these situations they estimated that the visor was only 45% effective at blocking droplets.” medrxiv.org/content/10.110…

10) US National Institute for Occupational Safety and Health found that a face shield could block 96% of larger cough droplets when they were laden with influenza virus. But with smaller cough aerosols less than 3.4 micrometres in size, the face shield blocked just 68%.

11) Another complication—Echternach’s tests with the singers showed, the fine mist that is released as we talk, sing or cough doesn’t just disappear once it reaches a metre or so away...

12) “While the larger droplets will fall quickly to the ground or other surfaces, the microdroplets we produce can remain suspended in the air for several minutes, and in some cases hours in very still conditions.” jamanetwork.com/journals/jama/…

13) “With virus-laden aerosol drifting around room, these can then easily creep in large openings at the side of plastic visors. The tests showed the aerosols dispersed throughout the room 30 min after a cough—face shield reduced inhalation of virus-filled aerosol by just 23%.”

14) Hence the authors concluded, “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.” ncbi.nlm.nih.gov/pmc/articles/P…

15) BOTTOMLINE: Don’t use face shields by itself in isolation and hope your get much protection from airborne aerosol virus particles. It must be used together with other protection and measures. Hence, for COSTCO shopper, use mask *and* face shield. Not “or”. #COVID19