Back in the late 30s, researchers at the University of Chicago stumbled onto its effectiveness as a germ-killer, as related in this Time magazine story from Nov. 16, 1942:
A powerful preventive against pneumonia, influenza and other respiratory diseases may be promised by a brilliant series of experiments conducted during the last three years at the University of Chicagos Billings Hospital. Dr. Oswald Hope Robertson last week was making final tests with a new germicidal vapor propylene glycol to sterilize air. If the results so far obtained are confirmed, one of the age-old searches of man will finally achieve its goal
the researchers 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.
How did it work? Respiratory disease bacteria float about in tiny droplets of water breathed, sneezed and coughed from human beings. The germicidal glycol also floats in infinitesimally small particles. Calculations showed that if droplet had to hit droplet, it would take two to 200 hours for sterilization of sprayed air to take place. Since sterilization took place in seconds, Dr. Robertson concluded that the glycol droplets must give off gas molecules which dissolve in the water droplets and kill the germs within them.
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 complete Time story can be read here: Air Germicide TIME
In a scientific summary of the discovery, it was noted that Tests on possible deleterious effects of breathing propylene glycol containing atmospheres over long periods of time are being carried out.
Those tests were done and a second summary report on propylene glycol vapor was released:
Propylene glycol is harmless to man when swallowed or injected into the veins. It is also harmless to mice who have breathed it for long periods. But medical science is cautious there was still a remote chance that glycol might accumulate harmfully in the erect human lungs which, unlike those of mice, do not drain themselves. So last June Dr. Robertson began studying the effect of glycol vapor on monkeys imported from the University of Puerto Ricos School of Tropical Medicine. So far, after many months exposure to the vapor, the monkeys are happy and fatter than ever. Dr. Robertson does not expect mankind to live, like his monkeys, continuously in an atmosphere of glycol vapor; but it should be most valuable in such crowded places as schools and theaters, where most respiratory diseases are picked up.
The monkeys lived in enclosures filled with propylene glycol vapor. No deleterious effect was ever reported. And the concentrations of PG we inhale on a regular basis surely must equal the amount inhaled by the monkeys for this test. Obviously, no scientist saw a time when a device would atomize a PG mist that would then be inhaled for fun. But time and technology has given us the electronic cigarette. With each inhalation, we are washing our lungs with a germicidal agent used today in some air sanitizers.
Glycerine, by the way, has some germicidal impact, but not, apparently, to the degree provided by inhaling propylene glycol vapor. Glycerine is now used by dairy farmers to help prevent bacteria entering a cows teats after milking. Glycerine both softens the teats and kills bacteria.
One more quote on PG: The vapour from as little as 0.5 mg of propylene glycol can kill nearly all the microorganisms in a liter of heavily contaminated air within 15 seconds.
The initial experiments with PG vapor were part of a search to find ways to create clean rooms, so the 1918 Spanish Flu pandemic that killed so many millions would never be repeated. Today, researchers have wondered online if propylene glycol vapor might not offer protection against a widely feared coming pandemic of bird flu, tagged H5N1.
Imagine e-smokers being healthier than non-smokers in such a scenario.