Is PLA safe for masks?
PLA itself should be safe, at least chemically, but there's no guarantee that additives in a particular filament manufacturer's material are safe. From this standpoint it would be best to use an uncolored "natural" (comes out translucent but cloudy when printed) PLA from a manufacturer who documents that it has no added ingredients. It's also plausible (not saying this is necessarily the case) that there's fine particulate matter produced by heating or in the extruder gear that ends up on the surfaces.

However, in the bigger picture/XY-question, it's unlikely that printed masks provide safety against viral transmission. Especially with a rigid material like PLA, they're not going to make an air-tight fit with your face, and they're likely not air-tight themselves even if they do (due to imperfect extrusion, slight gaps between layers at least intermittently). This could perhaps be mitigated by using a separate edge material to make a tight fit with face, and sealing the print like you suggested. However, once you make it air-tight, it's unsafe for another reason: it's a suffocation hazard! Just because you insert a filter to breathe through doesn't mean that you're actually going to be able to breathe through it properly.

Naomi Wu a.k.a. Sexy Cyborg (well known for working with Creality, open source/OSHW compliance in China, and popularizing the Ender 3) has done a number of twitter threads on what the dangers are and why it's unsafe and irresponsible to be creating air-tight 3D-printed masks if you're not qualified for designing this type of device. Here is one. A highlight:


  This results in CO2 buildup. After about 30 minutes your plastic mask, if actually airtight and strapped securely to your face will, before you can take it off, kill you- very peacefully. You'll just slump over and go to sleep, but you'll still be dead.