Sterile Masks should block miasma.

[Frank_Stein]

I mean, you can block miasma with a gas mask and those are arguably easier to get.

You're right, we should make those harder to get

Spoiler!

Joking aside, a sterile mask shouldn't be as good as a gas mask. My personal opinion is I think sterile masks acting as crappier gas mask, one that doesn't block nearly as many contaminants and offers no eye protection, would be fine. Offer just enough protection to have safe shorter term exposure, so you can move a body someplace else. Breathing in other gases, marginal advantage over someone not wearing a mask, would do nothing for you with mucus membrane irritants like tear gas