I did some research about propylene glycol and am still a little confused about just how "dangerous" it really is. I'll give you my perceptions up front: it seems it is deemed safe by regulatory groups but is also potentially irritating and is an undesired ingredient by many people.
So what is it exactly? Propylene Glycol is a cosmetic form of mineral oil. In the skin and hair, propylene glycol works as a humectant, which causes retention of moisture content of skin or cosmetic products by preventing the escape of moisture or water.
Propylene glycol is commonly found in: makeup, shampoo, deodorant, detangler, styling mousse, cleansing cream, mascara, soap, skin cream, bubble bath, baby powder, conditioner, toner, after shave, baby wipes, tire sealant, rubber cleaner, antifreeze, stain removers, fabric softener, degreaser, paint, adhesive and wallpaper stripper.
Now, just because it is found in a wide variety of things doesn't make it bad. Take water for instance. I'm sure water is used as an ingredient in many things, some you'd want to ingest and some you wouldn't. Doesn't make the water itself a bad thing.
Some websites claim that the Material Safety Data Sheet (any ingredient has one) "warns users to avoid skin contact with propylene glycol as this strong skin irritant can cause liver abnormalities and kidney damage." Other websites say the opposite, such as Tom's of Maine, which says, "MSDS information can be found on the Internet, but rarely are viewers given any background or context on how to read one." They go on to say they feel confident that their use of propylene glycol is perfectly safe. They also admit that it comes from natural gas, and they'd rather it came from a renewable resource, but when they tried a more natural substitute consumers hated the product so they switched back.
One resource which I respect is the Cosmetic Database from Environmental Working Group. It rated Propylene Glycol a 4 out of 10 (a caution level). They report that "Propylene glycol is practically non-toxic when taken orally, i.e. added to food. However, it has been found to provoke skin irritation and sensitization in humans as low as 2% concentration, while the industry review panel recommends cosmetics can contain up to 50% of the substance."
So, maybe it is safe in small doses. It seems like it may irritate my skin, but that doesn't mean it will irritate yours. I guess the choice to use it depends on your comfort level of synthetic ingredients. I'm choosing to avoid it in deodorant because it seems to bother me, but it is likely in some other products I frequently use.