Also known as lye, sodium hydroxide is a highly alkaline ingredient used in small amounts in cosmetics to establish and hold the pH of a product.
ItŠ—Ès also used as a cleansing and denaturing agent. In high concentrations, itŠ—Ès a significant skin sensitizer.
References for this information:
Acta dermato-venereologica, March 1995, pages 97-101