Generally Recognized as Safe to Use
The oral toxicity of propylene glycol is extremely low, and large amounts are required to instigate the dangerous effects. The potential for long-term toxicity is also very low. In one study, rats were given feed with as much as 5% PG over a period of 104 weeks and showed no apparent ill effects. Due to propylene glycols low chronic oral toxicity, propylene glycol is generally recognized as safe (GRAS) for use as a direct food additive. Cases of propylene glycol poisoning are related to either inappropriate levels of intravenous or subcutaneous fluid injection use or the accidental consumption of large quantities by children.
So, if you've been using these products in your day-to-day life, then it's safe to say that you've been absorbing a fair amount of propylene glycol - whether in liquid, solid or vaporous form - into your system for many years.
Furthermore, "[Propylene Glycol] is licensed for long-term high-volume inhalation under all employee health regulations," which is why companies have been using it as a carrier solution for the medicine in asthma inhalants and as the main ingredient in fog machines. And, this chemical is used as the main ingredient in today's antifreeze because it's much less toxic than ethylene glycol.
These may all seem contrived to make PG look harmless. If you're looking for real-world experimentation, then the results of a controlled lab experiment done in 1947 should convince you of the safety of using vaporous propylene glycol in everyday products, including e-cigarettes.
Exposure to highly concentrated amounts of the foggy substance from 12 to 18 months did not result to extensive internal organ damage to the rats and monkeys. However, the rats exposed to the thick chemical fog had consistently higher weight gains while the monkeys endured some dryness of the skin around their faces compared to the control groups of each animal.
After the amount of vaporized propylene glycol was dialed down to just below saturation point, the lab animals didn't show any of these physical symptoms at all. Furthermore, autopsy investigations of all lab animals showed there were no signs of internal damage to their lungs as well as their bone marrow, kidneys, liver and spleen.
And so, the researchers concluded that:
The results of these experiments in conjunction with the absence of any observed ill effects in patients exposed to both triethylene glycol and propylene glycol vapors for months at a time, provide assurance that air containing these vapors in amounts up to the saturation point is completely harmless.
Though the propylene glycol (PG) used to vaporize the nicotine in e-liquids is generally safe to use, consumers are still warned of the possible side effects they'll experience as they transition from smoking to vaping. Temporary numbing of the taste buds and dryness in the mouth normally occur. To overcome these, drink a lot of water and space your vaping sessions responsibly.