Propylene glycol has many functions such as a solvent or solubilizing aid. It can be irritating to the skin with studies showing it can provoke allergic reactions in people with eczema and other skin allergies. Extended exposure to this chemical has also been linked to the development of asthma and allergic reactions in children. Having a small molecular weight, propylene glycol can be readliy absorbed through the skin. It has also been shown to cause liver abnormalities and kidney damage in laboratory animals.
The EWG score is a hazard score ranging from 1-2 (low hazard), 3-6 (moderate hazard) and 7-10 (high hazard) published by the Environmental Working Group. Their data is sourced from the Skin Deep® database and studies published in open scientific literature.