What is it: Propylene glycol
The Agency For Toxic Substances and Disease Registry says "propylene glycol rarely causes toxic effects, and then only under very unusual circumstances" such as "excessively large or rapidly infused intravenous injections". The agency points out that it is less toxic than ethylene glycol (used in anti-freeze), but toxic it most certainly is. On the hand, the FDA says it is safe for use in deodorants, cosmetics and toothpaste.
So what's a girl to do? The FDA claim is based on the amounts of propylene glycol in cosmetics being so miniscule as to cause no harm. As far as I can discover, that seems to be the case. A rat study found it to be non-irritant. However, it should not be used on damaged skin. Even the oral toxicity of propylene glycol is very low. In fact, the body metabolizes propylene glycol into lactic acid (something that occurs naturally when we exercise).
Having said that, high levels of consumption results in the destruction of red blood cells. Humans exposed to high levels of propylene glycol mist experienced sore throats and eyes. 50 humans known to be sensitive to allergic reactions showed only mild irritation from contact with it. Propylene glycol has not undergone rigorous testing for links to cancer, so that potential hazard remains unknown.
So what's the bottom line? A large number of cats, mice, rats, dogs and rabbits have been donated to the cause of establishing that is safe in normal use. Just don't drink your anti-freeze (or indeed splash it on your skin) and remember the jury is still out on the cancer issue until there is more data.