What Is Propylene Glycol?



Propylene glycol is an organic compound. It is a colorless, nearly odorless, clear, viscous liquid with a faintly sweet taste that is very low in toxicity. It is a derivative of propylene oxide, which, in turn, is a derivative of propylene, and is used as an ingredient in many household products and cosmetics. It is also used widely in medicine, such as in asthma inhalers. Propylene glycol is mainly used in e-cigarettes for its temperature regulating, water retaining, and dissolving/ bonding properties.

Note that although propylene glycol is safe for human consumption, it is metabolized in the body as alcohol, and can potentially cause adverse reactions in users with sensitivities.

Why Is Propylene Glycol Used In E-Liquid?

E-liquid contains vegetable glycerine (VG), which is quite thick. Propylene glycol is used to help “thin” the e-liquid and make it simpler to fill tanks and vaporize. Because of PG, e-liquid is more easily absorbed by the cotton inside of an atomizer. The low density of PG also prevents any thick residue from building up on the heating element of the atomizer.

What Else Is Propylene Glycol Used For?

To give you an indication of the wide variety of products propylene glycol is used in, take a look at the following list:

  • Toiletries and cosmetics, like soap, deodorant, lotions, shampoo, lipstick and lubricants
  • Food, like salad dressings, soft drinks, cake mixes, fat-free ice cream and popcorn
  • Other household items, such as non-toxic automotive antifreeze
  • Asthma inhalers and fog machines at clubs/ concerts

Has Propylene Glycol Been Medically Tested and Studied?

PG is licensed for inhalable medicines and injectable medicines by all authorities, and licensed for long-term, high-volume inhalation under all employee health regulations (it is the main ingredient in theatre/ disco fog). However, note that individual tolerance may vary and that allergies to PG have been reported. Because PG is a powerful humectant, it can have a drying effect on your mouth when used excessively. It will also make the vapor feel harsher on the throat. We therefore recommend consuming water while vaping. In rare case, it may cause irritation of the throat. To avoid this, a 20-minute break for each 15 minute vaping session is recommended.

On the other hand, PG may also be a powerful deterrent against pneumonia, influenza, and other respiratory diseases when vaporized and inhaled, according to a study by Dr. Oswald Hope Robertson. In 1942, decades before the e-cigarette was invented, a study was conducted by Dr. Robertson of the University of Chicago’s Billings Hospital on the inhalation of vaporized propylene glycol in laboratory mice. A more in-depth article was printed in the November 16, 1942 issue of TIME Magazine. For more information about this test, read the abstract below.


O.H. Robertson
Clayton G. Loosli
Theodore T. Puck
Henry Wise
Henry M. Lemon and
William Lester, JR.
Author Affiliations

From the Department of Medicine, the Douglas Smith Foundation for Medical Research and the Bartlett Memorial Fund of the University of Chicago and the Commission on Air-Borne Infections, U. S. Army Epidemiological Board


With a view to determining the safety of employing the vapors of propylene glycol and triethylene glycol in atmospheres inhabited by human beings, monkeys and rats were exposed continuously to high concentrations of these vapors for periods of 12 to 18 months. Equal numbers of control animals were maintained under physically similar conditions. Long term tests of the effects on ingesting triethylene glycol were also carried out. The doses administered represented 50 to 700 times the amount of glycol the animal could absorb by breathing air saturated with the glycol.

Comparative observations on the growth rates, blood counts, urine examinations, kidney function tests, fertility and general condition of the test and control groups, exhibited no essential differences between them with the exception that the rats in the glycol atmospheres exhibited consistently higher weight gains. Some drying of the skin of the monkeys’ faces occurred after several months continuous exposure to a heavy fog of triethylene glycol. However, when the vapor concentration was maintained just below saturation by means of the glycostat this effect did not occur.Examination at autopsy likewise failed to reveal any differences between the animals kept in glycolized air and those living in the ordinary room atmosphere. Extensive histological study of the lungs was made to ascertain whether the glycol had produced any generalized or local irritation. None was found. The kidneys, liver, spleen and bone marrow also were normal. 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.

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