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So in light of the in going pandemic, I have begun printing masks for my family and a few friends. As I was printing them the thought occurred to me, "Is PLA safe to be breathing off of?" So that is my question. Is PLA safe to breathe off of or does it need to be coated with, for example, a food grade epoxy?

I am using Matterhackers standard black PLA and a stock ender-3 printer.

Thank you for your time and stay safe!

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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.

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    $\begingroup$ On the other hand, ear savers, bracing/frame for face shields, and no-touch openers are all more likely to be good uses for PLA. $\endgroup$
    – Joel Coehoorn
    Commented Apr 10, 2020 at 15:38
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    $\begingroup$ @JoelCoehoorn, PLA's too rigid to make a good face shield frame. I'd use something like PETG or ABS. $\endgroup$
    – Mark
    Commented Apr 10, 2020 at 23:40
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    $\begingroup$ This is correct - the frames for face shields are the products bring community sourced around the world right now, not masks. I gather they’re printed thin enough and need little enough flexibility that PLA is fine for those. $\endgroup$
    – Tim
    Commented Apr 11, 2020 at 1:33
  • $\begingroup$ PLA does not provide the needed flex to make a faceshield holder that is sturdy enough to withstand more than a few bends before snapping. $\endgroup$
    – Trish
    Commented Apr 11, 2020 at 7:34

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