Propylene Glycol

The Principal Ingredient of the Electronic Cigarette


By John. B.Tech.,C.Eng., M.I.Chem.E., M.Inst.E.

What is it? | Properties | Uses | Safety | Testing


What is it?

Propylene glycol (PG) is a type of organic (carbon based) compound known as a diol, or double alcohol. The OH group, an oxygen atom joined to a hydrogen atom, is what makes an organic compound into an alcohol. Normal alcohols, such as ethyl alcohol (the alcohol contained in all alcoholic drinks) have one OH group; diols have two. It is colourless, has little odour and is a clear liquid.



l propylene glycol molecule

Propylene glycol.


PG is a colourless, clear, oily liquid with little odour. It has a boiling point of 188C and is miscible with water. It is stable under normal conditions of use, and is readily biodegradable in soil and in air.


"It is almost certain that you will have been using products containing PG all your life."

Propylene glycol has a huge number of applications, many of which involve consumption by humans, including medicines and food products. It is used as a solvent in many pharmaceuticals, whether to be taken by mouth or intravenously, including diazepam (eg. valium) or by application to tissue. Applications in the food industry include use as a humectant to keep foods moist (labelled as E1520) as an emulsifying agent and as a solvent for food colours and flavourings.

It is also employed in antibacterial lotions and handwashes, as a non-toxic antifreeze in drinking water systems, as a carrier in electronic cigarettes and fragrance oils and as a main ingredient in deodorant sticks. It is commonly found in make-up, shampoo, soap, conditioner and many other similar products. It is also used in baby wipes. These are just a few of the applications and there are many more not involving human consumption. It is almost certain that you will have been using products containing PG all your life.


""There are no adverse health effects via inhalation."

Quote: "There are no adverse health effects via inhalation". (Material Safety Data sheet - Mallinckrodt Baker Inc.).

This is obviously the most important fact for "Vapers" ("smokers" of electronic cigarettes). Other tests have found PG to be "relatively non-toxic", this means that it takes very large quantities and/or concentrations to cause any harmful effects. Ingestion of a sizeable amount (over 100ml) may cause some gastrointestinal upset or temporary central nervous system depression. It is a mild irritant; if there is eye contact it may cause transitory stinging and watering.

Even for people handling the neat chemical there are no first aid methods needed for inhalation or ingestion. Simply remove to fresh air or give water to drink, respectively. For skin contact, wash with soap and water; for eye contact flush with water. In other words this is a pretty safe substance and has been passed as "generally regarded as safe" (GRAS) by the US Food and Drug Administration, FDA. Dow Chemical Co. say the product is "essentially non-irritating to the skin and mildly irritating to eyes". PG shows no evidence of being a carcinogen or of being genotoxic.


"...rats were given feed containing up to 5% PG over a period of 104 weeks (2 years) and they showed no apparent ill effects."

The toxicology database for PG has been reviewed and agreed upon by the OECD SIAM process. In one study, rats were given feed containing up to 5% PG over a period of 104 weeks (2 years) and they showed no apparent ill effects. Since 1942, it has been included in New and Non-Official Remedies as a proper ingredient for pharmaceutical products and is listed in the US Pharmacopoeia.

This is an important and widely used additive and as such will have been tested exhaustively and has been found to be safe. There are websites which will tell you the opposite. These are either sponsored/controlled by companies with a vested interest, eg. health foods, "natural" cosmetics, etc. and /or people who deliberately or through ignorance misinterpret the results, for example saying it is toxic without mentioning it is only mildly toxic in extremely high doses.

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