First things first: Masks are NOT respirators . In order for a respirator to function as intended, there are three major things that have to happen:
1. The material must filter particles out.
Respirators like N95s are tested and certified (by NIOSH in the US) to see if they can capture particles that are generally 0.3 microns in diameter (approximately 200 times smaller than the width of a human hair)—why that size?
Well, it turns out that size is particularly difficult to capture . N95 respirators generally get a significant percentage of their performance from an electrical charge that is put into the material—which is one reason you should NOT wash an N95 respirator with soap and water: it can remove the charge and make the respirator less effective. It’s also one of the reasons that even the best sewn masks/respirators fail a quantitative N95 fit test: a needle will poke a hole that is hundreds or thousands of times larger than 0.3 microns.
3D printing a respirator? Well, if you’re using a filament/FDM printer, there will be gaps large enough for those pesky particles to sneak through in between the layers of plastic.
2. It must fit correctly and tightly to the face.
In order for a respirator to achieve and pass an N95 fit test, it MUST fit correctly. In practice, everyone who works in an environment that requires them to wear an N95 should regularly undergo a fit test to ensure they are wearing the correct size mask.
In our experience, the most common places for a mask or respirator to leak are around the nose (particularly where the nose meets the cheeks), cheeks, and under the chin. This is the biggest reason that ear-loop and surgical tie masks are so inefficient—large gaps.
Want to put a flap over a hole in your mask for your straw so you can drink? Think again—that is analogous to drilling a basketball-sized hole in your tub and trying to fill it up with nothing more than a towel covering the hole. It turns out getting a proper fit is probably the most challenging part of getting a respirator to perform to the best of its capability…the difference between 95% and 70% could be a leak the size of a pinhole.
3. There must be enough material.
This one may be less intuitive. When you breathe in, your lungs generate a negative pressure to draw air into your body. If you cover your nose and mouth with material, it increases breathing resistance because it effectively reduces the total area through which air passes on its way in (which is why it’s harder to breathe with a respirator or mask on).
The effect is similar to having to breathe through a straw. The difference is that instead of a single passage for air to go through, a mask/respirator has millions of tiny holes between the material that allow air to pass through.
Why does this matter? Well, if you decrease the total area of the material (ex: a 3D printed respirator with a 2”x3” filter), the pressure drop across the mask will increase (smaller straw: have to breathe harder to fill lungs). When the pressure-drop increases, the particles can sneak their way through and around the mask into your lungs. This turns out to be one of the reasons that 3D-printed respirators with small filters are less effective than they could be.