Propylene Glycol VS Propanediol

Last week, I was asked a REALLY good question. Julie asked me this on Facebook:

"Is corn-derived Propanediol the same thing as propylene glycol? My dermatologist says to avoid all propanediol, since it's simply a synonym for PG. BUT, many of the natural and organic skincare lines say that corn derived propanediol is a different thing entirely. Who is right? I'm not willing to risk my health to find out, but I'm definitely curious!"

I've heard about propanediol being another name for propylene glycol, but I didn't know the answer to her question. I REALLY wanted to know the answer, though. So, I did a little bit of research and found some interesting info. Part of the reason propylene glycol allergy is so frustrating and confusing is because there are a zillion names for it and then a bunch of mixed messages about what's what.

Propanediol is commonly used in personal care products... just like propylene glycol. Since propylene glycol has been seeing a bit of a backlash from consumers, companies are starting to remove propylene glycol from their beauty / personal care products.

But what shall they replace it with?

Enter Propanediol... Toms of Maine and Beauty Counter proudly use propanediol as a propylene glycol alternative. They say that it's completely different from PG... That this substance is made by fermenting corn. Yet, in the details supplied to Beauty Counter's consumers... PG is clearly listed as a synonym for propanediol. I got that link from an email Julie forwarded to me. In that same email Beauty Counter states they will never work with propylene glycol... but you list PG as a synonym of propanediol?

That's not confusing at all...!

These companies state they're using the type of propanediol made from corn and they probably are. It can also be made other ways... and still called the same thing.

It gets even trickier: there's more than one type of Propanediol.

First we have the kind that beauty companies seem to be saying is different: 1,3-propanediol. The other is 1,2-propanediol. Both of these use propylene glycol as a synonym. 

So, how is 1,3-propanediol made? Fermenting corn is one way. This is a newer method. Per the U.S. National Library of Medicine's website, this is the other method:

"One commercial route to 1,3-propanediol starts from acrolein. The addition of water under mild acidic conditions gives 3-hydroxypropionaldehyde with high selectivity. Preferentially buffer solutions with a pH 4-5 or weak acidic ion exchange resins are used as catalysts. Further hydrogenation of this aqueous solutions gives 1,3-propanediol. There is an alternative route via hydroformylation ofethylene oxide and subsequent hydrogenation of the intermediate 3-hydroxypropionaldehyde." 

You can also do it by reducing ethyl glycidate with lithium aluminum hydride.

When it comes to 1,2-propanediol, there are a few different ways to create it: Again from the U.S. National Library of Medicine, here they are:

"Propylene glycol (PG) is produced commercially by the hydration of propylene oxide (PO)."

"Manufactured by treating propylene with chlorinated water to form the chlorohydrin, which is converted to the glycol by treatment withsodium carbonate solution. It is also prepared by heating glycerol with sodium hydroxide."

"Preparation of levorotary propylene glycol from hydroxyacetone by yeast reduction."

Here's where it gets really interesting... both propanediols have the same exact molecular formula: C3H8O2.

Buuut their molecular STRUCTURES are different:

1,2-Propanediol     (image source: PubChem)
1,3-Propanediol     (image source: PubChem)


What does all of this mean? It means that these two propanediols have different boiling points, melting points and densities... they really do behave differently. But at the same time, when it comes down to it... they're made of the same exact kinds of molecules. 

So... if we're allergic to one are we allergic to the other? The beauty / personal care companies sure don't want us to think so.

In my opinion, this all boils down to one question:

Do our allergies depend on the molecular formula of the substance, the molecular structure or both?

This is definitely not a question I'm qualified to answer. So, I've reached out to several doctors for answers so we can get to the bottom of this! I'm hoping to have their answers in a blog post this week. Stay tuned for the doctors' opinions!

sources: Toms of Maine, Herban Cowboy, US National Library of Medicine (1), US National Library of Medicine (2)