[This essay is adapted and expanded from a comment I made in another essay.]
One of my businesses is an online vacuum cleaner supplies store. I just tested HEPA vacuum cleaner bag material for breathability when used as material in a do-it-yourself face mask. It turns out that HEPA is extremely difficult to breathe through when attached to the face with a good seal. It will not make a good DIY mask.
The typical vacuum cleaner motor produces about 3 pounds per square inch (psi) of suction. Maximum human suction via lung power is 1.38 psi for men and 0.94 psi for women. That's maximum. Normal breathing psi is considerably less.
The current gold standard of non-powered respirators, colloquially known as N95 masks, filter out 95% of very small (0.3 micron) particles and trap almost all bacteria and most viruses in their filter matrix. N95 respirators must be fitted to the individual, won't work for some people, a seal against the face must be established and maintained to achieve high efficiency, and they're virtually impossible for non-medical personnel to obtain.
In comparison, HEPA vacuum bags filter 99.7% of 0.3 micron particles and WOULD make efficient face masks IF they could be breathed through when sealed against the face. But that's not practical when air movement is powered by human lungs.
Here's an excerpt of an article I wrote for my website that explains a bit about HEPA:
What is HEPA Filtration?
High-Efficiency Particulate Air (HEPA) filtratiion is an extended-surface dry-type filtration system with a minimum particle removal efficiency of 99.97% for particles down to 0.3 micron diameter (300 nanometers) as well as providing high efficiency filtration of both larger and smaller particles. A HEPA rating is established using a smoke challenge test consisting of particles of 0.3 micron average diameter.
An alternative to HEPA vacuum bags are what are known as micron or allergen filtration bags. Micron Filtration retains 100% of dust mites, grass pollen and ragweed plus particles 15 times smaller than the diameter of a human hair (5 microns). Vacuum bags with Micron Filtration will trap a large portion of lung irritants as well as most bacteria and viruses. They're somewhat easier to breath through but more susceptible to damage from moisture.
Testing of Various Mask Materials
Cambridge University researchers tested a wide range of common household materials for homemade masks. The coronavirus is just 0.1 microns. Could homemade masks capture virus particles that small? The scientists tested 0.02 micron Bacteriophage MS2 particles (5 times smaller than the coronavirus) against various materials.
The Scientific Conclusion: Based on particle capture and breathability, the researchers concluded that cotton t-shirts and pillow cases are the best choices for DIY masks. Paper towels were tested separately and found to be far less effective than the other materials tested.
Although the tea towel and the vacuum bag captured the most particles, they were also the hardest to breath through. With two layers, the tea towel was over twice as hard to breathe through as the surgical mask. In contrast, the pillow case, t-shirt, scarf, and linen were all easier to breathe through than the surgical mask.