Butylated hydroxytoluene, a potent synthetic antioxidant that also has health concerns when consumed orally. The amount of BHT uses in cosmetic products is typically 0.01-0.1%, and does not pose a risk to skin, nor does it penetrate skin far enough to be absorbed into the bloodstream.
References for this information:
2008 CIR Compendium, Cosmetic Ingredient Review, 2008, pages 43-44
Mechanisms of Ageing and Development, May 2002, issue 9, pages 1203–1210