What it is
Water is the universal solvent of cosmetic chemistry, dissolving the many ingredients that prefer water over oil and serving as the continuous phase of lotions, essences, and gels. Because it usually makes up the largest share of a formula, it is typically the first ingredient on the label. The water used in cosmetics is not tap water but specially treated, purified and deionized water from which nearly all mineral ions have been removed, which helps keep the finished product stable over its shelf life.
Safety & warnings
Water is among the most broadly tolerated cosmetic ingredients and is not a recognized sensitizer. Worth noting for context: water hydrates skin from within, but very prolonged contact with plain water, such as long baths, can actually have a drying effect on the skin surface, which is one reason finished formulas pair water with humectants and emollients rather than relying on water alone.
Origin & sourcing
The water used in cosmetics is technically processed rather than used as drawn. Manufacturers purify and deionize it to remove minerals and contaminants before it is blended into a formula, which is why labels list it as purified or deionized water.
Questions everyone asks
Why is water almost always the first ingredient listed?
Ingredients are listed in order of how much is present, and water usually makes up the largest portion of a water-based product, so it appears first.
Is the water in my sunscreen just tap water?
No. Cosmetic manufacturers use purified and deionized water with minerals removed, which helps the product stay stable over its shelf life.
Does water in a product moisturize my skin?
It acts as the carrier for moisturizing ingredients. Plain water alone can even be drying on prolonged contact, so formulas pair it with humectants and emollients.