A short thread on how face masks work, why we wear masks *to protect other people from ourselves* and not the other way around, & why wearing face-shields without a mask, masks with valves, or masks that hang open on the bottom or sides IS ACTUALLY DANGEROUS FOR EVERYONE ELSE...

I am going to start off by citing from a great paper by @trishgreenhalgh about the benefits of mask wearing, which you can read in full here:

Masks can work in 2 ways: protecting the wearer, & protecting other people from exhalations emitted by the wearer. Surgical grade masks are supposed to do both. They block out small particles emitted by the source, AND they filter those particles out from other emissions

For cloth masks, however, they are not so good at filtering out tiny particles which can contain viruses. But what they are good at doing is limiting the large droplets people give off when they breathe or cough – before they become aerosolised.

This means when you wear a cloth mask, the most important role that it plays is not protecting you from other people's exhalations but *protecting them from yours*. @trishgreenhalgh calls this 'source control' i.e. 'protects other people from droplets emitted by the wearer'