That sounds great. It's horrible being in public transport (Europe) with a compromised immune system.

Two weeks after I went on my medicine someone walked past in a close to empty train and sneezed big lumps of snot onto my face.

Every hospital and clinic has them available at the entrance where I am in the US.

I'd be curious how effective any of this is, but that would be hard to test.

Get a sample of ill people, compare the amounts of particles excreted in to the air with and without the mask. You will require a decent sized n, but it doesn’t seem insanely high.