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What Are Propylene and Butylene Glycol, and Are They Safe?

May 20, 2018 By Michelle 16 Comments

Affiliate Disclosure: I receive a small commission for purchases made via affiliate links.

One of the common reader requests I get is for “toxic” ingredient breakdowns, so today I’m looking at two ingredients that are commonly on “avoid” lists: propylene and butylene glycol.

What are propylene and butylene glycol, and are they safe?

What are propylene and butylene glycol?

Glycols in chemistry are ingredients that contain two OH (alcohol) groups. Propylene glycol contains 3 carbon atoms, while butylene glycol is a little larger and contains 4 carbon atoms. In glycols, the alcohol groups are attached to different carbons.

Confusingly, the names “propylene glycol” and “butylene glycol” can refer to several slightly different substances, since there are a few choices of carbon atoms for the OH groups to be attached to.

Propylene glycol usually refers to propane-1,2-diol (formerly known as 1,2-propanediol). The less commonly used propane-1,3-diol is also sometimes called propylene glycol, but usually in cosmetics it’s called “propanediol”. Propanediol is become more popular since propylene glycol’s been on all these watchlists.

What are propylene and butylene glycol, and are they safe?

It’s a similar story for butylene glycol. “Butylene glycol ” usually means butane-1,3-diol, but sometimes it’s also used to refer to the related butane-2,3-diol.

What are propylene and butylene glycol, and are they safe?

What do propylene and butylene glycol do in products?

Alcohol (OH) groups on ingredients usually make them good humectant moisturisers that can hold onto water and keep your skin or hair hydrated. For example, glycerin has almost the same structure as propylene glycol, but with an additional alcohol group. Propylene and butylene glycol are both humectant moisturisers.

Propylene and butylene glycol are also commonly used in products as solvents. They’re good at dissolving ingredients that aren’t very water-soluble. This means you end up with a more effective product since dissolved ingredients can spread out on your skin better and penetrate. Additionally, they can have antimicrobial effects and boost the effectiveness of preservatives.

Propylene glycol is a bit more common in products than butylene glycol. Both ingredients are commonly used in a ton of products, such as serums, moisturisers, toothpaste, shampoos and cleansers. They’re often also the main ingredients (after water) in sheet masks. They have a slightly slimy, goopy feel.

You’ll also find propylene glycol used as antifreeze, and in foods (it gets metabolised into lactic acid after you eat it). A few skin conditions can also be treated with propylene glycol, including seborrheic dermatitis and ichthyosis.

What’s wrong with propylene and butylene glycol?

There are a whole bunch of reasons why people tell you to avoid propylene and butylene glycol – let’s take a look at them.

“They’re petroleum-derived”

The word “petroleum” is pretty scary for most people, since it makes you think of oil spills and toxic waste. But a lot of non-scary chemicals can be derived from petroleum too – for example, almost all plastics are made from petroleum. Where something comes from doesn’t tell you much about its toxicity.

Related post: Video: Are Natural Beauty Products Better?

“They’re used as anti-freeze”

An anti-freeze is a substance that decreases the freezing point of water. The “scary” anti-freeze that causes poisoning is ethylene glycol, which is much like propylene and butylene glycol, but with only 2 carbons.

What are propylene and butylene glycol, and are they safe?

The three substances are similar in lots of ways, since they all have two OH groups: they’re all colourless liquids and work well as solvents, and they all work as anti-freezes. But a slightly longer or shorter carbon chain can make a big difference in terms of toxicity. 10 mL of ethanol (e.g. in the form of a shot of tequila) is quite fun and enjoyable for most people, but if you take off a carbon you get methanol, which is super toxic – 10 mL can make you go blind.

Same deal with the glycols – ethylene glycol is far more toxic than propylene and butylene glycol.

“They’re so dangerous workers need lots of protection to handle them”

A few places warn that workers need to wear special equipment when handling these chemicals. These warnings come from the MSDS (material safety data sheets), which list the precautions for handling the raw material. But these warnings need to capture the worst case scenarios, and are for very large, highly concentrated amounts of the substance.

The MSDS information sounds scary even for otherwise safe substances. For example, sodium chloride (table salt) gets scary phrases like:

  • A self contained breathing apparatus should be used to avoid inhalation of the product
  • May affect behavior (muscle spasticity/contraction, somnolence), sense organs, metabolism, and cardiovascular system.
  • May cause adverse reproductive effects and birth defects in animals, particularly rats and mice (fetotoxicity, abortion, musculoskeletal abnormalities, and maternal effects (effects on ovaries, fallopian tubes)

Related post: What’s wrong with SLS?

“They can corrode stainless steel containers – imagine what they do to your face!”

The MSDS also says that the glycols can corrode steel containers, which has led some people to say that “if they can corrode steel, imagine what they can do to your skin!” Luckily our skin isn’t steel… since water also corrodes steel.

“They’re penetration enhancers”

Since propylene and butylene glycol are penetration enhancers that can help other ingredients enter your skin, many “toxic ingredient” lists say that they’ll increase penetration of other toxic ingredients into the bloodstream. This is technically true, but:

  1. this means they’ll also help actives that you want in your skin penetrate, and
  2. a lot of really unexciting things are also penetration enhancers, like water on your skin from cleansing.

Related post: All About Cleansing & How to Choose a Gentle Cleanser

“They can cause irritating and allergic reactions”

Here’s the actual legitimate issue with propylene and butylene glycol: in high concentrations, they can be irritating, and very rarely they can cause allergic reactions.

The Cosmetic Ingredient Review, who investigate cosmetic ingredients, have found that both propylene glycol and butylene glycol are safe when used in products that are designed to be non-irritating – in general, this means that propylene glycol can be used in products at up to 50% concentration (although most products will contain less than 20%), while butylene glycol can be used pure without many problems.

Propylene glycol is a bit more irritating than butylene glycol. Unsurprisingly, irritation with propylene glycol has been found to be worse when the product’s applied and then covered, and on broken skin as well.

It’s also possible to have a true allergy to propylene and butylene glycol, where your immune system gets triggered by them, but it seems to be extremely rare. Being allergic to propylene glycol doesn’t necessarily mean you’ll be allergic to butylene glycol.

Related post: Should You Be Avoiding Parabens? The Science

Are propylene and butylene glycol worth worrying about?

Even the EWG, which usually is pretty scaremongery and chemophobic, only rates propylene glycol at 3 on their “danger scale” and butylene glycol at 1.

If you’re sensitive to propylene or butylene glycol, you’ll notice that products with large quantities might make your skin itchy and irritated, and you’ll want to avoid those. But otherwise, they’re very safe ingredients.

This post contains affiliate links – if you decide to click through and support Lab Muffin financially (at no extra cost to you), thank you! For more information, see Disclosure Policy.


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Filed Under: Analysis, Science, Skincare Tagged With: active ingredients, analysis, controversial ingredients, science, skincare

« Product Rant: SPF Sunscreen Drops
Surfactants Are Everywhere, aka Stop Being Terrified of Chemicals »

Comments

  1. Saurs says

    May 20, 2018 at 12:56 pm

    Thank you for this, as always.

    It’s one of my pet peeves with one- or two-ingredient serums and oils* (particularly the overly precious sort that successfully market themselves at insultingly high price points using appeals to nature): the absence of surfactants and adjuvants and preservatives, the agents that make well-rounded skincare products stay on your face rather than pill up or slide off, that help disparate active ingredients blend together and have a sensible shelf-life post-blending, and that deliver those active ingredients where they’re actually effective.

    As you say, part of the antipathy for these helpful “chemicals” is the success of scare-mongering and part of it is just straight-up scientific illiteracy posing as Boot Strapped, Contrarian Free Thinking. It’s kind of depressing. As you may have seen, Paula Begoun was briefly interviewed by The Cut last week, and while some of that conversation was frustrating to read (Paula was not picking up what the interview was laying down half the time), she made a similar point as you have, in the past, about the decline of parabens and the consequences of pointlessly demonizing them.

    *which I like and I use because they have a place in my regimen, but I try not to be credulous about doing so

    Reply
  2. LindaLibraLoca says

    May 20, 2018 at 5:28 pm

    I don’t mind both in my regime, as I am not allergic to them, but get so many questions when I list them in a review. Thank you for this article, I´ll just link to you int the future now.

    Reply
  3. Tess says

    May 25, 2018 at 12:59 am

    I love propylene glycol as a humectant. Where I live the pharmacy carries a very basic moisturizer containing 20% propylene glycol, and this is actually the first hand recommendation to use during acne treatments such as isotretinoin. That’s why I first started using it and have continued after treatment because it has such a plumping effect on my skin.

    Reply
  4. Miki says

    May 28, 2018 at 10:36 am

    DAMN, drop the mike. This was such a great post! As always, thank you for your epic, educational candor. I love it. Really good information.

    Reply
  5. Lauren says

    June 1, 2018 at 1:56 am

    Haha, oh O-chem days. Wish more people would read this and stop saying stupid things like “ingredients you can’t pronounce” and “loaded with chemicals.”

    Re: DIY setting sprays–I take the extra “juice” from sheet mask packets and mix them with rose water ($2/bottle at international markets) and a little bit of vegetable glycerin. I’d like to try high-end setting sprays but my stingy side can’t deal with paying that much for fancy water.

    Reply
  6. Stop Itching Skin says

    June 10, 2018 at 5:54 am

    Thanks for the article, I really appreciated it.

    Reply
  7. Carol says

    December 4, 2018 at 4:39 am

    Sorry for the question ages after you posted this: what is the difference between these two and pentylene glycol? I see that one all the time now.

    Reply
  8. Veer says

    February 7, 2019 at 2:33 am

    Thank you for the article! It is a nice read!

    In the text is mentioned that people can be allergic to propylene glycol while not having any reaction to butylene glycol. Could this also be the other way around, that there is a allergic reaction or a sensitivity to butylene glycol but not to propylene?

    Reply
    • Michelle says

      February 8, 2019 at 10:13 am

      I’m not sure, but I would guess so!

      Reply
  9. Jeff says

    February 9, 2019 at 4:13 pm

    I feel like Lab Muffin has become less science driven and more affiliate driven. The science part is skewed and favours affiliate products rather than telling it how it actually is. I find it really difficult to read and actually take what you say seriously anymore.

    Very sad!

    Reply
  10. suki says

    February 26, 2019 at 12:21 pm

    Jeff, my dude, if you were trying to make some kind of bullshit point about Michelle’s audacity in conducting herself EXACTLY as her subbies need her to in order to review stuff they’re trying to decide whether or not to buy, or her outrageous conduct that is EXACTLY the SAME frickin way that most every goshdamn blog online conducts business, that NO ONE ELSE seems to take issue with, well, this was a particularly dumb-ass post on which to do so.
    This is an educational post on humectants, dude! Tell me, please, WHERE in this post does she slut herself for PR? I am disappointed in you, Jeff, I don’t know if I can trust your agenda..who do YOU have loyalties to?hmmm?
    I suspect you’re probably having some personal issues. It happens to the best of us, but for frocks’ sake, don’t take it out on my girl..she is one of the only voices on the internet any of us can trust, & you saying she’s a sellout really only reflects badly on you, Jeff.
    Muchlove, suki

    Reply
  11. Victor says

    June 4, 2019 at 4:23 am

    Very educational and eye-opening. Thanks!

    Reply
  12. tama says

    April 18, 2020 at 6:21 pm

    Hi,

    I’m actually one of the few who’s allergic to butylene glycol (it gives me scaley rashes and flakey skin). I’ve been looking into cross related ingredients that I could also be allergic to recently and I was wondering if you could give me any insight on whether 1,3 butandiol and 1,4-butandiol were concerns to me. In general, should I be wary of butane like ingredients as well.

    I’ve been testing my skin lately and I seem to be irritated to polyisobutene as well.

    Any thoughts?

    Reply
    • Michelle says

      April 27, 2020 at 3:30 pm

      Polyisobutene is very very different from butylene glycol – it’s actually a polymer, so it’s much larger than butylene glycol. If you’re sensitive to both, it would be for different reasons.

      1,3-butanediol and 1,4-butanediol are much more similar to butylene glycol so I think it’s more likely that you’d be sensitive to them for the same reason – you might not be sensitive to them, but I would be cautious when introducing a product that contains a lot of them.

      Reply
  13. Elle says

    October 20, 2020 at 8:17 am

    Hi! Thanks for the great article!! I’m one of the few people who are allergic to propylene glycol. I read an article a few months back stating that propylene glycol is the number one, common allergy among atopic dermatitis (Eczema) sufferers. I just find that so interesting seeing that propylene glycol is in almost all prescribed topical creams to treat this condition.

    Reply
  14. dMarie says

    October 30, 2020 at 5:17 pm

    I’m also allergic to propylene glycol and have a tough time keeping it out of my life. I appreciate your thorough explanation and have bookmarked this post for reference. So many medical practitioners don’t seem to know that this is a real allergy – I’m hoping that your article helps.

    Reply

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Michelle
Hi! I'm Michelle, chemistry PhD and science educator, and I'm here to help you figure out which beauty products are worth buying, and which ones aren't using science!

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