Ingredient Spotlight: Propylene Glycol

Photo credit TJ Cosgrove @ Flickr | https://www.flickr.com/photos/explosivofilms/8355301288/

Before anyone asks, no, I don’t add any propylene glycol to my products directly.  I do, however, plan on adding flavor oils to some of my newer products such as lip balms, whipped body butter, and other body products where there is a danger of it being ingested.

Isn’t propylene glycol a dangerous chemical?

The biggest complaint I hear about propylene glycol is that propylene glycol is an antifreeze.  Who wants to eat antifreeze or spread it on their skin?

Actually, ethylene glycol is what most antifreezes use. Ethylene glycol crystallizes as it is metabolized [source] and this crystallization is the main reason ethylene glycol is toxic to humans.  Propylene glycol does not crystallize when metabolized.

Instead of thinking we are spreading antifreeze on our skin, however, we should realize that propylene glycol is a food-grade product that is also used in antifreeze. There are plenty of examples of food-grade chemicals being used in non-food applications, which should serve as a good reminder that not all chemical-sounding names are bad!

  • Table salt, also known as sodium chloride, is also found in various coolants and antifreezes, among other things.
  • Water, which has the intimidating chemical name dihydrogen monoxide, but is more accurately called oxidane, is famously used as an example of how chemical names can be scarier than the actual chemical is.  (Yes, water is technically a chemical.  Everything is!)
  • Table sugar is a disaccharide, and it has the intimidating chemical formula C12H22O11.  This formula encompasses many things, including sucrose and maltose.  You can’t avoid consuming C12H22O11 if you eat fruits or vegetables.  Too much sugar is a bad thing, yes… but a little sugar is good for you too!

When propylene glycol is used in antifreeze, it’s actually labeled “non-toxic antifreeze”.  That makes sense–the freezing point of liquid propylene glycol is -74.2°F or 59°C, so if I wanted something to not freeze in cold temperatures, using some propylene glycol would work quite well.  Another thing I could do would be add salt, for example.  There are a lot of options for lowering the freezing point of something.  Propylene glycol is one of them.

What is propylene glycol, then?

Propylene glycol is a vicinal diol, a member of the diol family of chemicals.  Diols are pretty similar to alcohols chemically [source] and in fact “diol” just means there are two alcohols.  It’s a humectant (keeps things moist) and mixes with anything (water, oil, other alcohols), so it’s a great carrier for flavorings.  This is why you will find propylene glycol in some Bath Geek ingredients; it’s a component of the flavor oils.

Can it be an allergen?

Unfortunately, yes, some people are allergic to glycols.  This is why I have added it to the allergen filters.  You should be able to choose “glycol-free” and this will exclude all the products that have propylene glycol in them (there are only a couple).  Remember that I make unfragranced and unscented versions of everything Bath Geek carries, and so it is easy enough to avoid when shopping at Bath Geek.

Is it safe?

If you are not allergic to propylene glycol, yes.  In very small amounts (like the tiny amounts you will find in Bath Geek products), propylene glycol is considered harmless.  It is metabolized into lactic acid and pyruvic acid, which are then metabolized into carbon dioxide and water.

To make it even better, propylene glycol is not biocumulative (it fully metabolizes and doesn’t stay or accumulate in the body), and it is fully metabolized within 48 hours.  The elimination half life of propylene glycol is about 4 hours, meaning 50% of what is consumed is gone (metabolized) within 4 hours.  In comparison, it takes 6 to 8 hours to digest a meal.

The World Health Organization has recommended a maximum consumption of 25 mg/kg/day. [Source] In relatable terms, that is 1.8 g/day for a 75 kg (165 lb) human.

How much propylene glycol is Bath Geek products?

Miniscule to trace.  I’m not just being hyperbolic.  My flavor oils are used at very low concentrations (0.1-0.5% of the overall formula).

Assume that we’re looking at a 0.15 oz lip balm good for 50 or so applications.  (I’ve honestly never thought to track how many times I can use my lip balm, so if someone knows how many applications are in a stick of 0.15 oz lip balm, PLEASE do let me know!)

0.15oz divided by 50 applications = 0.003 oz.  That is three thousandths of an ounce.

The flavor oil I use is 0.1% of the formula.  0.1% of three thousandths of an ounce is 0.000003 oz.

I don’t know what % of the flavor oil is propylene glycol, but I’m sure it’s less than 100%.  Not all of it will be absorbed, either.  Overall, that is an extremely small dose.  As long as you don’t have an allergy to propylene glycol (or any glycols), I think you are safe.  Again, if you are allergic to glycols, just choose “glycol-free” when searching for products!

[Edited to add, 10:55pm: All products have been labeled (or not, as the case may be)!]

Why not just use essential oils instead of flavor oils?

It’s a long answer, but I’ve written about my stance on essential oils here.  I’d have to use higher levels of essential oils than I do flavor oils.  I’d rather make unscented products than rely on essential oils, whether or not those oils are food-grade.  Allergies are one thing, unexpected medical side effects are another.

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