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? 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!) 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.”
