I’ve been making masks using HEPA material. Everything I have read leads me to believe that HEPA is a higher standard than N95. It clogs up from moisture but if you can figure out how to inhale through the filter and exhale through a valve, it works pretty well.
I am using a cheap snorkel purge valve to try and achieve this and it seems to work pretty well. The HEPA filter material doesn’t come close to clogging up during the course of a day and I can replace the HEPA insert every morning.
There is a New York Times article:
What’s the Best Material for a Mask?
If a mask is more to protect others from yourself than vice versa, then it seems to defeat the purpose to exhale through a valve…
Since there’s no way that a face mask is making a gas-tight seal at the edges, it’s probably self-defeating to use a high-pressure-drop material, which will face un-filtered air around the mask.
HEPA filters are designed to work in HVAC systems that generate a measurable constant pressure differential. Air is drawn through because of that pressure differential. When you breath, you have a very low and unsteady pressure differential so a mask with a good seal is going to require a lot of effort to draw air through the material. HEPA filters can also produce fibers that are generally no worse than what you get in the ambient environment but if you are breathing them in constantly may irritate the respiratory passages, actually make you more vulnerable.
I don’t know what the actual effectiveness of HEPA materials are in filtering viruses but clean rooms and biocontainment facilities actually use ultra-low particulate air (ULPA) rather than HEPA standard filters, and then decontaminate the exhaust with high heat or UV-C irradiation. It would seem that standard HEPA filters are just not adequate for that purpose, although they are obviously better than plain woven textiles like cotton or polyester blends.
The filter material in N95 and normal surgical masks is a non-woven textile produced by special melt-blown process. This material actually has particularly electrostatic properties that attract small particulates like bacteria and virions out of the air. Because there only a few companies that make machines that produce this material, and getting them to work reliably is a very finicky process engineering problem, the ability to rapidly increase material production is limited.
Stranger
There’s some DIY info here. Scroll down to “Step 8: Filters”.
The valve is inside the fabric layer.