N95 respirators and ASTM Level 1-3 Surgical masks have passed specific tests to ensure their safety and usability. These tests include bacterial filtration efficiency at 3 microns (BFE), particle filtration efficiency at 0.1 micron (PFE), differential pressure (Delta P), fluid resistance (mmHg) and flammability.
A N95 respirator is an item of PPE designed to reduce exposure to airborne contaminants. Known airborne pathogens include TB, SARS, Anthrax, and Influenza. Respirators must be individually selected to fit the wearers face and shown to provide a good seal (via Fit Testing). They also must be certified by NIOSH or equivalent standards.
Homemade masks are less efficient at filtering particles compared to N95 respirators and surgical masks because of the materials used, but they are still effective at blocking the transmission of microbes by droplets.
For the analytical mind, aerosol researcher Timothy Sipkens has created an interactive tool to help visualize the filtration efficiency and breathability of different DIY materials in comparison to commercial medical and non-medical masks.
DIY homemade masks can achieve varying degrees of filtering efficiency based on the fit and the materials used. However, there will always be a compromise between high filtration efficiency and breathability. A mask may be so efficient at filtering particles that it becomes difficult for the wearer to tolerate because the material does not allow adequate gas exchange for ventilation - a large surface area is needed for gas exchange which is why many N95 masks have an air pocket in front of the nose and mouth to allow for better gas exchange. A N95 mask has a resistance (pressure drop) target around 8 mmH20 — if the pressure exceeds 15 mm H20, a vacuum effect can be generated that sucks in air from the sides of the mask; of course, it will also be laboured to breathe.
Studies show that the fit of a mask is just as important as the filtration efficiency of the mask material. Leakages around the mask area can degrade efficiencies by ∼50% or more, pointing out the importance of fit. Aside from the design of the mask, the tighter your mask, the tighter your fit, the better you will benefit from the filtration efficiency of the mask material. A loose style surgical mask will help block large particles but will not stop small particles from entering your mask.
The best homemade cloth masks can achieve high filtration efficiency with the insertion of filters (eg. dried baby wipes, Halyard H100-400) - and studies show the filter need to cover the entire area of the mask to be the most effective. Many filters can be effective (multiple layers of paper towel, coffee filter, AC filter, floor wipes) but most are not breathable and some not biocompatible. The Halyard sterilization wrapping can be used as a standalone material for masks. It's advantage is that it is by far the most effective reusable filtration material and it is biocompatible and waterproof. Compared to AC filters (some are not biocompatible) and nanofibers - its cost is significantly less - it is free if recycled from medical facilities after sterilization!
Source:
https://www.maskfaq.com/test-results
https://www.maskfaq.com/other-studies
https://www.medrxiv.org/content/10.1101/2020.04.10.20060632v1
https://www.masktestingatNU.com