Propylene Glycol is a compound typically found in the liquid nicotine cartridges of e-cigarettes. While it is generally considered to be safe, propylene glycol's presence as a compound in the liquid nicotine cartridges has been somewhat controversial.

Used in cosmetic products, food coloring, cake mixes, sodas, and many other products, propylene glycol is generally considered safe when ingested orally. It has a very low toxicity. In other words, one would have to ingest large amounts of propylene glycol for any harm to result.

What is not known, however, is the safety of propylene glycol when it is inhaled. When vaporized, propylene glycol has an appearance and consistency similar to smoke, even though it is not real smoke. That is the reason why propylene glycol is contained in liquid nicotine cartridges. It emulates the smoke found in real cigarettes.

Because it is used extensively to make "fake smoke", there have been many anecdotal reports that propylene glycol, when vaporized, often causes irritation to the eyes or throat. Also, there is some research that shows that propylene glycol should not be inhaled.

However, there is no definitive scientific evidence as of yet that shows that propylene glycol is harmful.

On the other hand, there is no evidence that shows propylene gylcol is completely safe either. Nor is there research that shows how safe inhaling propylene glycol is compared to inhaling real smoke, although there is a general consensus that inhaling propylene glycol is probably safer. Accordingly, more research on the health effects of propylene glycol being inhaled is currently being done. Hopefully, science will provide an answer soon.