Steareth-20 is a derivative of the benign fatty substance stearyl alcohol. In cosmetics, it functions primarily as a surfactant, but is also used to make products more stable, especially if they contain active ingredients in an emulsion.
Steareth ingredients can be animal or synthetic. They are made by interacting the chemical intermediate gas ethylene oxide with stearyl alcohol, a process that forms an entirely new stable compound. The number that follows (for example 20) indicates the average number of ethylene oxide units used in the preparation.
There have been concerns about the safety of steareth ingredients because toxic 1,4-dioxane, a byproduct of ethoxylation, can be produced; however, this is eliminated through purification processes. The independent Cosmetic Ingredient Review Panel has determined that steareth-20 is safe as used in cosmetics in amounts up to 25%.