("Supplementary Data" to you science types.)
Ensuring a good fit:
In occupational settings, the all-important fit of the mask to the face must be professionally tested before use and every year thereafter. Here is another link re fit testing (both last accessed 02-06-2021). I got my present mask at a hardware store outside of an occupational context but did a DIY fit test with powdered stevia sweetener puffed out of a rubber bulb just after giving the bulb a good shake. The stevia seems to lose its sweetness slowly after removal from the packet, so I grind it fresh for each test. Will try saccharin next. (02-20-2021: saccharin test passed, but self-administered.) NB: I have two years of experience in wearing a respirator occupationally. If you are new to this, you should practice a lot in a safe setting until you know what a good and bad face seal feel like and the moves you need to get a good one. My big discovery was that moving the mask up or down on my face often did the trick.
I learned how to wear a respirator in a paint shop where there are smelly fumes that you can smell if they leak into your mask. This facilitated the learning process and I am not surprised that OSHA provides a smell option for fit testing involving water solutions of isoamyl acetate; using smell is so much more convenient than taste (no need for a "nebulizer" that often blocks up) and makes a more stringent and foolproof test because to pass, the mask has to keep out individual molecules, which are vastly smaller than the tiniest particles of concern in infection control. You just swap organic vapor cartridges for filter cartridges for the duration of the test, then swap 'em back afterward.
The above smell-based procedure seems designed for testing only the mask-to-face seal. What if you are concerned about the seal between the filter cartridges and the mask, or whether there is a hole in a filter? Let's say you have visions of an enemy, who is a dead ringer for the Green Goblin, sneaking into your apartment while you are on the shitter and sticking a nail through the intake grille (or maybe you have kids). Then, I guess you need to use small, taste-able particles for testing, but having previously tested the face seal (e.g., by smell) should simplify the process of testing the filter cartridges, because the cartridges are smaller, more rigid, and geometrically simpler than the face seal. For testing the cartridges and their seals, perhaps all the bowing and talking to be performed during the fit test, as prescribed by OSHA, would be unnecessary.
In cold weather, a mask leak can fog up my glasses and thus be detected easily if it occurs alongside my nose, which is a problematic area anyway. Fogged glasses can be good for detecting leaks.
Making a flared exhalation tube:
I first prepared a 6-cm dia. plastic funnel by sanding the places where future hot glue would go. I then cleaned off the sandings. I cut off the hanger tab and then encircled the funnel with a bead of glue to help hold the filter. I then cut off the stem and drilled small holes near the bottom to pass the hold-down wire. I then assembled the flared tube on the mask and used hot glue to seal the seam between the funnel and the straight part of the tube (cut from a 1.5" dia. aspirin bottle along the top and bottom of the label). I then secured the ends of the hold-down wire with more glue. (See below). The straight part of the tube fits snugly into the exhalation valve without leaks, but getting it to go in required a combination of force, accurate alignment, and stuffing in the parts that boinked out. A lubricant might have helped; haven't tried it. Something else to try would be filing the end of the tube on a bevel.