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?

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.

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 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.

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.

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