OK, not very steampunk, but with the current conditions, this may at least be informative for some people.
Recently, the city council where I live has made the wearing of face masks in retail establishments mandatory. No definition of what amounts to a 'face mask' was stipulated, so I've been wearing a black bandana so I look like an old west outlaw. Heh. But the whole mask business seems to be a big deal, and I've done a fair amount of research into masks as protection dating back at least 15 years (with some recent updates), and I feel like passing this information along.
It's important to understand that, with respect to biological contagions, a face mask can serve two purposes: One, to protect the wearer, and Two, to protect others FROM the wearer. An ordinary 'surgical' mask is intended to fulfill the second purpose. It offers little protection to the wearer. But if you're like me, and I know I am, you're more concerned about protecting yourself. So here's some information that could help in selecting a mask.
'Filter masks' have an alpha-numeric rating system. This takes the form of designations such as N95. There are three letters that I'm aware of. N means no protection from oil. R means oil resistant. And P means oil proof. This may be significant, since I've read that coronavirus particles have a 'slippery' coating, so a P rating might help. (Note that this is speculation on my part.) As for the numbers, the higher the number, the smaller the particle the mask can filter out. The highest number seems to be 100, which is HEPA filter quality. Technically, this will NOT filter out flu, or coronavirus particles, BUT, when airborne, these particles are nearly always surrounded by water vapor, which does make them large enough to stop.
Additionally, it is important that a mask have a good seal around its borders. This means NO facial hair in these regions for the wearer!
I'll also point out that these types of masks offer no protection from gasses. That would require an additional activated charcoal element, or an entirely isolating system, like a Scott air pack, or SCUBA gear.
This is about as short, and simple as I can make it. If it helps even one person, I'll be very satisfied.