Interesting that a fairly thorough search has found the claim that PG is such an ingredient
numerous times in different vape related forums, but never a mention of which.
I haven't scrolled up to read the premise of the OP
but I personally believe from personal experience that indeed
both PG and VG act as expectorants when inhaled and thus help asthmatics.
The mucus in the bronchials gets softened with every inhale.
While looking around for the answer to your question, which inhalers.
I came across some interesting information that supports my premise or at least is relative.
http://jpet.aspetjournals.org/content/91/1/52.abstract
This from 1942,
http://content.time.com/time/magazine/article/0,9171,932876,00.html
and this from where the above info came, I'll post link at the end:
Propylene glycol, the primary ingredient in the electronic cigarette cartridge, may be a powerful deterrent against pneumonia, influenza, and other respiratory diseases when vaporized and inhaled according to a study by Dr. Oswald Hope Robertson. Decades before the e cigarette was invented, a study was conducted by Dr. Robertson of the University of Chicago's Billings Hospital in 1942 on inhalation of vaporized propylene glycol in laboratory mice. A more in-depth article was printed in the 1942 issue of TIME Magazine
http://www.time.com/time/magazine/article/0,9171,932876,00.html for November 16th. "Dr. Robertson placed groups of mice in a chamber and sprayed its air first with propylene glycol, then with influenza virus. All the mice lived. Then he sprayed the chamber with virus alone. All the mice died."
The researchers also found that "the propylene glycol itself was a potent germicide. One part of glycol in 2,000,000 parts of air would--within a few seconds--kill concentrations of air-suspended pneumococci, streptococci and other bacteria numbering millions to the cubic foot."
http://www.news-medical.net/news/20...s-might-keep-us-healthy-says-researchers.aspx