Moisturizing (humectant),Fragrance,Solvent,Viscosity controlling
Propylene glycol is a clear, thick, odorless liquid of synthetic origin that is frequently used in skincare products.
Its main function is to stabilize cosmetic products exposed to varying temperatures. It is also a moderately strong humectant moisturizing ingredient with similar properties to glycerin, in that it stays on top of the skin and attracts water.
The recommended concentration of Propylene Glycol should not exceed 50% in leave-on cosmetic products. Almost none of it is absorbed into the skin and the small fraction that does is quickly converted to lactic acid. There is a big fuss over the safety of Propylene Glycol, so let's look at the current data from dermatology. The biggest concern appears to be the skin irritation potential of PG.
The famous Mayo Clinic recently published a massive report on their sensitivity patch testing from 1997 to 2016, including a staggering 11,738 patients. From this report, it is obvious to see that PG is a very safe compound, as only 0.85% of tested patients were sensitive and only 0.35% had irritant reactions - which were dependant on the concentration (5% PG caused fewer reactions than 20% PG).
Similarly, another summary report included 45,138 patients who were tested with 20% PG in water between 1992 and 2002. Out of these, 2.3% were sensitive and 0.6% had irritant reactions.
The safety committees agree that PG seems to exhibit very low sensitization potential, and the risk for sensitization to PG on unbroken skin seems to be very low. To further support this, PG is also approved as a food additive.
There is another interesting property of PG that is sometimes utilized in skincare products: it has the ability to enhance the penetration of otherwise poorly penetrating compounds into the skin. It has been shown in many experiments to be a penetration enhancer for water-soluble ingredients.
Although for this effect to happen we would need higher concentrations of PG than those used in skincare. Therefore, if this effect happens in your PG-containing skincare product, it is usually very mild and definitely nothing to worry about.