Propylene glycol is a chemical related to alcohol. Unlike many similar chemicals, it is not toxic in its normal uses and concentrations and may be used in applications where exposure to humans or other living things is a consideration. It mixes well with other liquids (including water), has a very low freezing point (and can be used as an anti-freeze, although it is more expensive than toxic products), and has no odor or color (although it has a sweet taste).
It is used in a wide range of applications, including many where it may be deliberately ingested by humans or will be exposed to the skin such as emulsifiers, moisturizers, perfumes, massage oils, antibacterial solutions, food coloring and food flavoring. When applied to the skin or ingested, it is very well tolerated by the body and is generally regarded as safe as, when it is metabolized by the body it is broken down into normal metabolic by-products like lactic acid. As such, the toxic dose is very high.
However, it does pose more of a problem as an aerosol. Propylene glycol is often used in aerosol products because most ingredients dissolve well in it and the propylene glycol evaporates very quickly. Although it is harmless to the skin, if taken into the lungs it can cause severe irritation in some individuals, particularly children. As such, it is only recommended for use in well ventilated areas. It can also be a problem when used as a solvent for intravenous drugs, although such complications are very rare.