Propanediol has a synthetic cousin named propylene glycol, 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.”