In line at a shop the other day, a woman turned to me and told me that propylene glycol was in anti-freeze and ranch dressing! "I'm not eating anti-freeze," she said. I have no idea why she told me this. It wasn't like I was telling her about my love of cosmetic chemistry or holding a bottle of ranch dressing, but I think it's indicative of how people feel about this ingredient. So let's take a look at this much maligned humectant to see if it deserves this kind of derision!Propylene glycol (aka 1,2-propanediol) is a polyol or poly-alcohol like glycerin. It's a clear, colourless, viscous liquid that tastes a little sweet (again, like glycerin). It can be used as an excellent
humectant, a gelling agent in
deodorant sticks, and a
solubilizer of fragrances. It can be added to your surfactant based systems (like body washes and shampoos) to increase clarity (although it can reduce the viscosity), and it's added to our other products to act as an a freezing point lowering ingredient. It inhibits fermentation of products and it may be an effective fungicide and anti-microbial (although both of these things apply more to food than cosmetic products). It's edible, although you might find it has a bitter aftertaste. It's also a penetration enhancer, which means it can bring your actives into your skin a little easier than products that don't contain it.
Propylene glycol is a very effective humectant - possibly more effective than glycerin at certain humidities and temperatures - and this is why we see it in so many products. It's soluble in glycerin, water, and alcohol - so it's a water based ingredient we can add to our heated water phase of our products. Its suggested use is between 1% and 5%, although you can use it up to 50%. It can be an irritant at higher levels, so it's best to use it around 2% to 3%.
So what's with all the concern about this product?Yes, it can be found in anti-freeze and ranch dressing and it serves the same purpose - all the glycols (including glycerin) reduce the freezing point of water (probably through disruption of hydrogen bonding), so that's a bonus for products that might end up being in the mail or a courier depot centre in Alaska for a while! There are many ingredients we use that can be found in other products, but being a possible inclusion in anti-freeze doesn't make propylene glycol a bad thing. (You could use glycerin in anti-freeze for the same purpose, but it isn't used that way because it would gunk up your car!)
It is a penetration enhancer, meaning it can bring things into your skin more easily than a product without it, and this is one of the reasons it is used in pharmaceutical ointments and unguents. If you want something to penetrate your skin, you need to do some work.
Skin penetration isn't an easy thing - our skin is designed to keep stuff out, so we have to use some modifiers to get things through those layers. Adding a penetration enhancer can increase the efficacy of many of our ingredients - if they can get under our skin, they can moisturize from the inside out! A lot of things are penetration enhancers - urea, IPM, oleic acid, linoleic acid - and they don't get the same bad rap as propylene glycol.
Looking at some web sites I would normally avoid because they make me go ARGH, it is regularly noted that propylene glycol is an irritant. This is true. The
CIR Review Panel has suggested it be used at levels lower than 50% due to potential irritation. And people with sensitive or damaged skin might find it irritating at lower levels. This is why there's the suggestion to use it at 1% to 5% in our products.
And it can be
derived from petroleum, although there's a new product on the market,
Zemea, which is derived from corn sugar. Zemea is much more expensive than conventional propylene glycol, but it has the same characteristics and is used in the same way.
I'm not trying to convince you to use propylene glycol - I just like to see what all the fuss is about and whether it is justified. In this case, I'm really not getting the hype! It seems like the same arguments could be made for glycerin (the anti-freeze part), IPM (the penetration enhancing part), just about any ingredient we use for irritancy, and many ingredients for the derived from petroleum part. Join me tomorrow for a look dipropylene glycol!
