Propanediol is one of those dubious, confusing ingredients that's found in countless personal products. Seriously: You can find it in a wide range of formulas, from face creams to eye shadows. The ingredient is also related to propylene glycol, a notorious skin irritant that's also commonly found in personal care products (the big one: traditional deodorants.)
Yet, propanediol is thought to be safer and generally less irritating, so it's frequently used as an alternative in clean cosmetics. But is it actually any better?
What is propanediol?
Propanediol, also known as 1,3-propanediol, is a colorless liquid that's naturally derived from corn glucose, or corn sugar. It can also be synthesized in a lab for use in personal products. Propanediol is water-miscible, which means it can fully dissolve in water. The two can create a uniform, consistent solution when combined.
In terms of chemical makeup, propanediol is an alkanediol, which consists of an alkane and a diol. A quick chemistry lesson: An alkane is a chain of carbons with hydrogens attached. A diol is any compound that has two alcohol groups. Finally, the prefix prop- refers to three carbon atoms in that chain. Prop + alkane + diol equals propanediol.
So, propanediol is a chain of three carbons with hydrogens, plus two alcohol groups attached. The location of each alcohol group matters, too. In this article, the propanediol we're referring to has one alcohol group on each end. That's why it's called 1,3-propanediol—because the alcohol groups are on the first and third carbons. (This info will come in handy later...promise!)
What is it used for in skin care?
It dissolves ingredients.
Propanediol is an excellent solvent, which means it effectively dissolves other ingredients. And while this might sound like a negative trait, it's essential for optimal application. "As a solvent, propanediol dissolves active ingredients—like salicylic acid—for easier delivery to the skin," explains board-certified dermatologist Kiran Mian, D.O. It's also colorless, odorless, and mixes well with other ingredients, so it's useful for enhancing a formula's composition in a minimally invasive way.
It makes products more fluid.
Spreadability is a major factor of a good, high-quality formula. After all, if a product is too sticky and tacky, it would be an absolute pain to apply. Propanediol helps by reducing a product's viscosity, says Mian, so it can flow in a manageable way. This allows you to effortlessly move around the formula and apply it with ease.
It adds moisture to the skin.
Propanediol "also acts as a humectant and emollient," says Mian, meaning it adds moisture to the skin and reduces moisture loss. This can help improve skin texture, adds Sobel, which explains why it's found in so many personal products. The ingredient's humectant and emollient properties also make it useful in cosmetics like makeup primer, foundation, and mascara.
Propanediol versus propylene glycol.
Propanediol has a synthetic cousin named propylene glycol1, or 1,2-propanediol. It's also easy to confuse the two, as they're both types of propanediol. But according to Mian, propylene glycol has been recently found to cause allergic reactions. It was even selected as the American Contact Dermatitis Society's "Allergen of the Year" in 2018. (Talk about an unfortunate award.) Basically: Propylene glycol is not a cool cousin.
Back to that mini chem lesson. Like 1,3-propanediol (the head honcho of this article), 1,2-propanediol is a chain of three carbons with hydrogens, plus two alcohol groups. Those two alcohol groups, however, are on the first and second carbons. That's why it's called 1,2-propanediol.
This slight structural change makes a difference. Compared to 1,3-propanediol, 1,2-propendiol is more likely to cause contact dermatitis, says Sobel. "It's common for many people to have an allergic reaction to this ingredient," he notes. Possible symptoms include itching, irritation, and redness.
Conversely, 1,3-propanediol is much gentler on the skin, explains Sobel. This makes it a great alternative for those who have developed unpleasant reactions to 1,2-propanediol. "Since it's a naturally derived ingredient, 1,3-propanediol doesn't cause much irritation or [reactions] compared to synthetic ingredients," says Sobel. It's "a generally safe ingredient that works well when used topically and mixed with other ingredients."
Are there any side effects?
Although propanediol is gentler than propylene glycol, it can still cause skin sensitivity, says Mian. A 2010 study also found that the possibility of skin irritation due to propanediol is minimal. However, its overall risk for skin reactivity is low, and side effects are uncommon. It is GRAS certified ("Generally Recognized As Safe") by the United States Food and Drug Administration (FDA) and considered safe for use both as a cosmetic ingredient and for ingested food products. It's also been approved by EcoCert (an internationally recognized natural and organic seal) and the Natural Product Association (a U.S.-based natural personal care certification).
Who shouldn't use it?
Avoid propanediol if you have a known allergy or sensitivity to the ingredient. Similarly, if you know you're sensitive to propylene glycol, it doesn't hurt to use caution with propanediol. Always do a patch test before slathering on a new product.
Propanediol is used as a solvent, viscosity reducer, humectant, and emollient. It's seldom associated with side effects, but in rare cases, it can cause irritation. If you're sensitive to propanediol (or its cousin, propylene glycol), double-check the ingredient label before using a product. On ingredient decks, propanediol may be listed as 1,3-propanediol, trimethylene glycol, or propane-1,3-diol.
Kirsten Nunez is a health and lifestyle journalist based in Beacon, New York. She has a Master of Science in Nutrition from Texas Woman's University and Bachelor of Science in Dietetics from SUNY Oneonta. Kirsten specializes in nutrition, fitness, food, and DIY; her work has been featured in a variety of publications, including eHow, SparkPeople, and international editions of Cosmopolitan. She also creates recipes for food product packaging.