Check out the research from Texaas A&M University Research Department - it should answer all of your questions, they tell you which MERV filter to use and how many layers you need depending on the rating.
I've looked at a small amount of data on this question. I think it's a really important one (see a related question of mine), but am extremely not an expert. The most actionable item is this study that essentially "salting" a surgical mask might make it significantly more protective against flu viruses. The study's in vivo section with mice strikes me as a bit sketchy (small n, and unclear how representative of mask filtration their mouse procdeure actually is), but their in vitro section seems legit, and the study is in Scientific Reports (part of the Nature publishing group). If you're making a DIY mask/filter and it's not too thick already, it can't hurt to include a salted layer. Their proposed mechanism of action is by the salt killing the virus particles, not filtering them, so it should stack well with particulate filters. The recipe in the paper is to coat a hydrophobic filter in solution of salt and surfactant (they used polysorbate 20, which is approved to use as a food additive), then let it dry.
The 100% efficacy for a middle filter layer that's had a saltwater + surfactant sprayed onto it sounds really good; but I wonder how tight the filter material has to be, for that level of efficacy. I also wonder how much air resistance the salt coat adds.
A HEPA filter + carbon would be less restrictive if the carbon part were salted than if the HEPA filter itself were salted, but that might not deactivate all of the virus.
I've read that Hepa vaccume filters can be dangerous as they can contain fiberglass particles. Is this true? Are any of them actually safe to breathe through?
I don't have an answer, but maybe you can reach out to the people at MUSC who designed this: https://web.musc.edu/innovation/covid-19-innovation
I notice that that mask has an exhaust valve, which means it filters air only on the way in, not on the way out. But I understand that a large part of the reason for wearing a mask is to protect other people from you, and an exhaust valve defeats that.
Agreed.
I'm just hoping that that they can give the OP some information about using HEPA filters.
I've noticed that many N95 masks also have an exhaust valve.
A local organization is looking to produce masks for the hospital, and they reached out to me for guidance. Since this is outside of my expertise and I haven't been following all of the recent research, I'm passing the questions along here.