Propylene glycol is much safer than ethylene glycol, but like other chemicals needs to be used with care. Ethylene glycol is the type found in common automotive antifreeze, is toxic, and has a sweet taste making it attractive to animals and even children. Most states now require a bittering agent to be added to ethylene glycol. The issue is that engine manufacturers require EG type antifreeze to be used and can void the warranty if there is a issue caused by the cooling system.
From Wikipedea
Propylene glycol is considered generally recognized as safe (GRAS) by the U.S. Food and Drug Administration, and it is used as an humectant (E1520), solvent, and preservative in food and for tobacco products, as well as being the major ingredient in the liquid used in electronic cigarettes. It is also used in pharmaceutical and personal care products.[4] Propylene glycol is a solvent in many pharmaceuticals, including oral, injectable and topical formulations, such as for diazepam and lorazepam that are insoluble in water, use propylene glycol as a solvent in their clinical, injectable forms.[7]
Like ethylene glycol, propylene glycol is able to lower the freezing point of water, and so it is used as aircraft de-icing fluid.[4][8] It is similarly used as automotive antifreeze.[9] Water-propylene glycol mixtures dyed pink to indicate the mixture is relatively nontoxic are sold under the name of RV or marine antifreeze. It is also used to winterize a vacant structure.
Ethylene glycol is moderately toxic with an oral LDLO = 786 mg/kg for humans.[8] The major danger is due to its sweet taste. Because of that, children and animals are more inclined to consume large quantities of it than of other poisons. Upon ingestion, ethylene glycol is oxidized to glycolic acid which is, in turn, oxidized to oxalic acid, which is toxic. It and its toxic byproducts first affect the central nervous system, then the heart, and finally the kidneys. Ingestion of sufficient amounts can be fatal if untreated.
Antifreeze products for automotive use containing propylene glycol in place of ethylene glycol are available, and are generally considered safer to use, as it possesses an unpleasant taste in contrast to the perceived "sweet" taste of toxic ethylene glycol-based coolants, and only produces lactic acid in an animal's body, as their muscles do when exercised.
Now here is the EPA's stand on disposal
Antifreeze Common Wastes & Materials | US EPA
