Propanediol has a synthetic cousin named propylene glycol, or 1,2-propanediol. It is also easy to confuse the two, as they are two types of propanediol. But according to Mian, it was recently discovered that propylene glycol causes allergic reactions. He was even chosen as American Contact Dermatitis Society “Allergen of the Year” in 2018. (Talk about an unfortunate price tag.) Basically: Propylene glycol is do not a nice cousin.
Let’s come back to this mini chemistry lesson. Like 1,3-propanediol (the big head of this article), 1,2-propanediol is a chain of three carbon atoms with hydrogens, plus two alcohol groups. These two alcohol groups, however, are on the first and second carbons. That’s why we call it 1,2-propanediol.
This slight structural change makes the difference. Compared to 1,3-propanediol, 1,2-propendiol is more likely to cause contact dermatitis, Sobel said. “It is 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, says Sobel. This makes it a great alternative for those who have developed nasty reactions to 1,2-propanediol. “As it is a naturally occurring ingredient, 1,3-propanediol does not cause much irritation or [reactions] compared to synthetic ingredients, “says Sobel. It is” a generally safe ingredient that works well when used topically and mixed with other ingredients. “