This is a DIY PAPR ( https://en.wikipedia.org/wiki/Powered_air-purifying_respirator) HEPA filtered air is blown in under pressure, exhalation passes through a check valve and is directed out the P100 3M filter. Potentially for high-risk environments- testing it without a CO2 sensor is also a good way to kill yourself.

Part 2- Why 3D printed N95 masks are not a thing.

N95 certification requires an airtight fit. If you are sucking in, or blowing out air from the sides of your 3D printed cosplay prop, viruses carrying droplets are going all over the place- it's not N95.

N95 mask material has a fair amount of air resistance- so the masks have a large surface area, trying to breathe through a little disk of N95 material is like trying to breathe through a straw. But it gets worse- you have to try and exhale through the same little disk of material

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.

That's why industrial respirators have a check valve for exhalation- to avoid that CO2 build up. We can't have a check valve in a healthcare setting since it would blow droplets all over the place. We need to filter inhalation, and exhalation- without CO2 buildup.

So first off- if you are really committed to perfecting your 3D printed N95 masks, you need to have a buddy when you test it, sit cross-legged on carpet (because an uncontrolled fall from standing or even sitting could result in a head impact that will likely kill you).

One arm holds the mask against your face. Raise one arm above your head. If your arm drops- your buddy yanks the mask completely off and put you into recovery position.

So far none of these "3D printed N95 masks" are even remotely airtight so this hasn't been an issue- people are just sucking air in and out the sides, very little is going through the filter medium, but with a heavily customized mask you could get a seal good enough to kill you.

"It's not N95 but it's still better than nothing". Possibly, but not better than cloth. Plastic cannot absorb droplets so any exhalation or cough will tend to blow out the open sides- and eventually drip as condensation forms. Cloth masks are more effective and faster to make.

So if you are a journalist- desperately hunting for clickbait with the usual sociopathic glee of your profession, kindly stop signal-boosting untested, unusable "3D printed face masks". People are wasting time and resources because you are misleading them that these are viable.

Note:
When you learn to use a lathe, you are told leaving the chuck key in will cause it to fly across the room and kill your best friend.
When you learn to use a drill press, you get pics of girls with long hair and scalp degloving injuries.

DIY fucking around with respiration- diving helmets, rebreathers, all that, is right up there with DIY manned flight for huge body count. You plan for, teach and illustrate the absolute worst-case scenario. If you are stupid enough to downplay that in my mentions, expect a block.