@ Bunk.“ Second, if you are wearing a shield and not a mask and you cough, sneeze or talk, how much is the risk that those droplets will spread out of the side of the shield. With a mask, isn't it more likely that the droplets will be confined?“
That is not what I understand.
A proper shield goes all the way around your face, almost to your ears, so droplets do not spread out the side of the shield.
The problem with some face masks that people wear is they are not tight enough, do not go around your face and gape,
so would let droplets out the side.
Not true.
A face shield has open space even if it goes "around your face", because there is no closed SEAL against the face.
That's why the CDC and every other "scientific" authority makes such a big deal over the seal of masks which is critical.
The seal therefore must also apply to the face shield to do what you said, ie, disallow droplets to spread.
The CDC and even OSHA both have extensive protocols for testing one's mask to assure it's being applied and worn properly.
A face shield requires no video explanation because by definition it does not, and is incapable of, creating a true seal.
A "proper" face shield therefore only provides a modicum of protection, and so it remains an "imperfect" shield.
Valuable of course, but
not impervious.
For many instances then, a face shield could be a nice adjunct to a mask, but never a substitute.
The thing is the
open space, even if less by virtue of extending farther around, because it is still what allows aerosol droplets
to either enter or go out,
depending on whether you're causing a spread or accepting one.
So, bottom line, going farther around your face is better than not, but the true vulnerability has not been diminished much,
given that the ubiquitous aerosol droplets travel freely in and around the face.
And your other statement (re masks) is in fact true, and actually speaks to the problem with your first statement (seal or no seal being the key).
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