EWG Risk Score:
What is it?
Propanediol is a clear, thick liquid used in formulas to control viscosity. Also known as 1,3 propylene glycol, it is an isomer of the more widely-used chemical, propylene glycol.
Is it a risk?
Propanediol does not appear to pose any significant risks when used diluted in a cosmetic formula, based on current research. However, the manufacturing of propanediol does pose potential environmental and health concerns. It can be produced from acrolein, a compound found in cigarette smoke that may contribute to lung cancer. (Source) While this wouldn't pose a risk in the finished product of propanediol, there may be exposures to workers and the environment, especially if produced in countries with less-than-stringent working and environmental regulations.
You may also see propanediol created from "natural" sources. The most widely-distributed form is made by DuPont Chemical, created from (most likely GMO) corn syrup and a genetically-modified strain of e coli bacteria. (Source)
It can also be produced from fermenting glycerol with genetically modified bacteria. (Source)
Can be a skin allergen. (Source)
Propanediol as a finished ingredient doesn't pose significant health risks unless you have an allergy to the ingredient. However, when you see propanediol in a product marketed as "natural," think GMOs because it has most likely been created with either genetically-modified bacteria or from GMO corn (or both.)