i believe the term is "GRAS" (generally recognized as safe). pretty much anything is harmful if you eat too much x.x i believe it's also used in the "non-toxic (not as toxic?)" types of antifreeze, as well as deicing airplanes.
going further off topic:
propylene glycol is one of the primary constituents of vape juice, inhaled on a daily basis by nicotine addicts. technically speaking, it's used as the flavor delivery mechanism in vape juice, as opposed to the other primary component, vegetable glycerin, which contributes more to the vapor itself and is a poor solvent for the flavorings used.