What it is
BHT (butylated hydroxytoluene) is a synthetic phenolic antioxidant used widely in both foods and cosmetics. It works as a chain-terminating antioxidant, donating a hydrogen atom to peroxy radicals and converting them to more stable hydroperoxides, which interrupts the autoxidation that would otherwise cause oils and other ingredients to degrade. In cosmetic formulas it is typically used at very low levels, on the order of 0.01-0.1%, to protect the product rather than to act on the skin.
Safety & warnings
According to the Cosmetic Ingredient Review, BHT is used at low levels in cosmetics and does not penetrate the skin enough to be absorbed into the bloodstream, and is considered safe for use in cosmetics at those concentrations. Much of the public controversy around BHT comes from studies of oral intake rather than topical use. As with many additives, individual sensitization is possible, so people who react to a product should patch test and consult a clinician.
Origin & sourcing
BHT is manufactured synthetically. Industrially it is made by reacting p-cresol with isobutylene using an acid catalyst, which attaches two tert-butyl groups to the aromatic ring of the cresol.
Questions everyone asks
What does BHT do in a sunscreen?
It is an antioxidant that slows oxidation of the oils and other ingredients, helping keep the formula stable over its shelf life.
Is BHT absorbed into the body from skincare?
The Cosmetic Ingredient Review notes that at the low levels used in cosmetics it does not penetrate far enough to be absorbed into the bloodstream.
Is the BHT in cosmetics the same as the food additive?
Yes, it is the same compound, known as E321 in food, used here as an antioxidant at low concentrations.