What it is
Aqua is simply water (H2O, two hydrogen atoms covalently bonded to one oxygen) listed under its INCI name. Because it is a polar molecule, water is an excellent solvent for substances that dissolve in water rather than oil, and it is the medium in which many cosmetic ingredients are dispersed or dissolved. In most lotions, creams, and sunscreens it is the largest single component and the continuous phase of the emulsion, so it is usually listed first on the ingredient label.
Safety & warnings
Water is inert and non-irritating as a formula ingredient and is not an allergen. Worth noting for context: applying plain water to skin, as in long baths, can actually be drying, which is why cosmetic water is combined with humectants and occlusives rather than used alone. In a finished product it simply serves as the carrier for the other ingredients.
Origin & sourcing
The water used in cosmetics is not tap or spring water; it is purified and deionized so that nearly all dissolved mineral ions are removed. This processing keeps the finished product more stable and consistent over time.
Questions everyone asks
Why is water listed first on my sunscreen?
Ingredients are listed in order of amount, and water is usually the largest component and the base that the other ingredients are mixed into.
Is the water in cosmetics special?
It is purified and deionized to remove dissolved minerals, which keeps the product stable, unlike ordinary tap water.
Does water hydrate skin on its own?
Plain water on the skin can actually be drying, so it is paired with humectants and occlusives that hold moisture in.