What it is
Sodium hydroxide is an inorganic ionic compound (NaOH), also known as lye or caustic soda, that is a potent alkali. In cosmetics it is used in very small quantities as a pH adjuster and buffering agent, raising the acidity of a formula to a target range so the product is stable and comfortable on skin. During formulation it typically reacts with and neutralizes acidic components, so in the finished sunscreen it is no longer present as free caustic alkali. It carries no scent, color, or conditioning role; its job is purely to fine-tune pH.
Safety & warnings
In its pure, concentrated form sodium hydroxide is highly corrosive, but in skin care it is used at very low levels and is largely consumed in neutralizing other ingredients, so the finished product is formulated to a mild pH and is considered safe. Concern attaches to the raw material, not to a correctly pH-balanced sunscreen, where only trace residual amounts remain.
Origin & sourcing
It is produced industrially, most commonly by the electrolytic chloralkali process, in which electricity is passed through brine (sodium chloride solution) to yield sodium hydroxide, with chlorine gas as a main byproduct.
History
Early forms were prepared by soap makers, with soap-making procedures documented in medieval Arabic texts. Industrial-scale alkali production grew through the Leblanc process patented in 1791, and by the twentieth century electrolysis of sodium chloride had become the primary method of making sodium hydroxide.
Questions everyone asks
Isn't sodium hydroxide (lye) dangerous?
Pure lye is corrosive, but in sunscreens it is used in tiny amounts and reacts with acidic ingredients during manufacturing. The finished product is balanced to a mild, skin-friendly pH.
What is it actually doing in my sunscreen?
It is a pH adjuster. Formulators add a small amount to raise acidity to a target range so the product stays stable and feels comfortable, not to provide any skin benefit on its own.
Will it irritate or dry my skin?
In a properly pH-balanced formula only trace amounts remain after it neutralizes acids, so it is not expected to cause the irritation associated with the concentrated raw chemical.