What it is
Steareth-100 belongs to the steareth series, made by reacting stearyl alcohol with a large number of ethylene oxide units, here on average about 100. The high degree of ethoxylation makes it strongly water-loving, so it is typically paired with a lower-ethoxylated steareth such as Steareth-2 to build stable oil-in-water emulsions. In sunscreens it helps disperse oil-soluble UV filters and emollients evenly into a creamy or lotion base.
Safety & warnings
Steareth-100 is generally well tolerated at the low levels used in cosmetics and is not considered a notable allergen. As with other ethoxylated (PEG-type) ingredients, manufacturing can leave trace 1,4-dioxane, which reputable suppliers reduce through purification. It is not regarded as significantly comedogenic.
Origin & sourcing
Synthetic. It is produced industrially by ethoxylating stearyl alcohol, a fatty alcohol that may be sourced from plant oils or petrochemical feedstocks.
Questions everyone asks
What does Steareth-100 do in a sunscreen?
It acts as a nonionic emulsifier and surfactant, helping water and oil phases combine into a smooth, stable lotion or cream and dispersing oily UV filters evenly.
Why is the number 100 in the name?
The number reflects the average count of ethylene oxide units added to stearyl alcohol; a higher number means a more water-loving ingredient.