Ideally this is a vegetable glycerin mixed with grain alcohol, both of which are natural. Usually it is a synthetic petrochemical mix used as a humectant. It has been known to cause allergic reactions, hives and eczema. When you see PEG (polyethylene glycol) or PPG (polypropylene glycol) on labels, beware—these are related synthetics. The natural versions are fine and perfectly safe, not so their synthetic counter parts.