Propylene glycol
The acute oral toxicity of propylene glycol is very low, and large quantities are required to cause perceptible health effects in humans; in fact, propylene glycol is three times less toxic than ethanol. Propylene glycol is metabolized in the human body into pyruvic acid (a normal part of the glucose-metabolism process, readily converted to energy), acetic acid (handled by ethanol-metabolism), lactic acid (a normal acid generally abundant during digestion), and propionaldehyde (a potentially hazardous substance). According to the Dow Chemical Company, The LD50 (Lethal Dose that kills in 50% of tests) for rats is 20 g/kg (rat/oral).