You said , " What I haven't figured out is why I am able to nebulize 3 ml. Levalbuterol in a much shorter time period than 4 ml. saline." I don't have empirical proof for this, but my guess is that the suspended particles in the levalbuterol solution are much smaller than those in the saline. This would be the same reason saline tends to "clog up" mesh nebulizers unless it is soaked out.

Here is another question to ponder. If the purpose of the saline solution is to thin the mucus and help us cough it out, why have we all concluded that 4 ml is the "magic dose"? I have purchased 7% saline in 3 ml, 4 ml and 10 ml ampules.

When I was struggling with making it through all 4 ml, the RT in my pulmonologist's office said, "Use it until you can huff cough. Huff cough/Aerobika for up to 15 minutes, nebbing in between if you need to. Then stop."

This process has kept me healthy for 3 1/2 years now. I don't fret about the last drops of saline, or force myself to try to clear until my throat and ribs hurt.
sue