Vape Ingredients Part One: Propylene Glycol

By Karlhahn - Own work, Public Domain, this series, we’ll be exploring the ingredients used to make vape juice in-depth. Each part of this series will be a deep-dive on what the individual ingredient is, how it’s used in general, how safe it is to consume, and why it’s used in vape juice. For this first part of the series, we’ll get the scariest-sounding one (propylene glycol) out of the way first.

Propylene glycol is used as an ingredient in vape juice to carry the nicotine and flavoring as it vaporizes easily and smoothly, carries flavors well, and has a low vaporization temperature. The Dow Chemical Company is one manufacturer of food-grade propylene glycol (denoted by the USP abbreviation), their product page can be found here.  The FDA (Food and Drug Administration) recognizes it as “generally safe”, as indicated here. This moniker specifically relates to use in food products – i.e. it’s perfectly safe to eat.

Propylene glycol has an extremely wide range of uses, some involving consumption by humans or animals, some not:

Propylene glycol is not to be confused with ethylene glycol, which is an extremely toxic substance that most people are familiar with as antifreeze, or a component of engine coolant. They are completely different substances (albeit with some similar properties). Due to its property of lowering the freezing point of water, propylene glycol is also used in some antifreeze preparations (usually for marine applications), and as an aircraft de-icer.

For the application of propylene glycol that we are interested in (vaporizing and inhaling it), this article has some great information. It’s pretty dense, so we’ll extract the relevant bits and break them down. From the article:

  • Propylene glycol is metabolized into pyruvic, lactic, and acetic acids (all products of normal digestive and metabolic processes), and propionaldehyde (used in flavoring agents)
  • The levels of propylene glycol ingested via vaping or electronic cigarette use (at most 0.9g per day) is well below the level at which any toxicity occurs (1g/L of blood plasma)
  • The study this article is based on exposed rats to extremely high levels of propylene glycol via inhalation, and found that they experienced irritation of their eye and nose mucus membranes, but that even at the extremely high levels of exposure that irritation occurred, found that ” PG does not appear to pose a significant hazard via inhalation of either the vapor or a vapor/aerosol mixture”
  • Furthermore, exposing the animals to propylene glycol vapor at extremely high concentrations for extended periods of time, and comparing them to animals kept in a normal air atmosphere found that “the results of these experiments along with the absence of any observed ill effects when patients were exposed to 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.”

It’s worth noting that the animals did experience some bleeding from the nasal and ocular mucosa (nose and eye mucus membranes) after intensive exposure, but that this was attributed to propylene glycol’s hygroscopic effects (it dried out their sensitive tissues, essentially). This effect can be likened to people who experience nosebleeds when breathing in very dry environments, and the conclusion of the study in this regard is that “all the side effects were observed at PG exposure levels hundreds of times higher than the exposure levels of 4.29mg/kg/day to 6.43mg/kg/day which correspond to e-cigarette levels of PG”.

To conclude, propylene glycol has a multitude of uses, many of which involve direct consumption by humans and animals. Consumption via inhalation has been studied in rats and dogs, and has been found to be generally safe, especially at the exposure levels that vaping produces.


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