I have training as a professional chemist having a BS Chem and a MS. From a purely chemical viewpoint propylene glycol would seem safer than ethylene glycol since it is not metabolized to oxalic acid in the human body. However, this chemical must be evaluated from a toxicological perspective not merely a chemical one.
Originally Posted by gainer
It appears that propylene glycol is not as safe as once believed. Current literature reports several cases of reverseable kidney failure after the IV administration of drugs containing propylene glycol. A tentative safe level has now been set at 25 mg/kg of body weight although a toxic dose has not been established.
Of all the solvents, other than water, that that we commonly use to prepare concentrated developers, propylene glycol is probably the safest. As with all chemicals do not ingest it and avoid breathing the vapor when heating it. Skin absorption does not appear to be a problem but avoid eye contact.