Aqua/Water/Eau at a glance

  • The most widely used cosmetic ingredient
  • Functions as a solvent
  • Typically appears as the first ingredient in skin care products
  • Only a 10% concentration is needed in skin’s outer layers for softness and pliability

Aqua/Water/Eau description

Aqua/Water/Eau (also simply known as water) is the most frequently used cosmetic ingredient. Aqua/Water/Eau in skin care products is almost always listed first on an ingredient label because it is usually the ingredient with the highest concentration in the formula. Water’s primary function in skin care formulas is as a solvent. Despite claims of skin’s need for hydration and claims regarding special types of water, it turns out that water for skin may not be as an important ingredient as commonly thought. Only a 10% concentration of water in the outer layer of skin is necessary for softness and pliability in this part of the epidermis, known as the stratum corneum. Studies that have compared the water content of dry skin with that of normal or oily skin do not find a statistically significant difference in moisture levels between them. Further, too much water can be a problem for skin because it can disrupt the essential substances in skin’s surface layers that keep it intact, smooth, and healthy-looking. As an example, recall how skin on your fingers and toes “prune” when you soak in the bathtub or other body of water for too long.

Aqua/Water/Eau references

  • Skin Research and Technology, May 2015, pages 131-136
  • Skin Pharmacology and Applied Skin Physiology, November-December 1999, pages 344-351
  • Journal of Cosmetic Science, September-October 1993, pages 249-262

Peer-reviewed, substantiated scientific research is used to assess ingredients in this dictionary. Regulations regarding constraints, permitted concentration levels and availability vary by country and region.

Ingredient ratings


Proven and supported by independent studies. Outstanding active ingredient for most skin types or concerns.


Necessary to improve a formula's texture, stability, or penetration.


Generally non-irritating but may have aesthetic, stability, or other issues that limit its usefulness.


There is a likelihood of irritation. Risk increases when combined with other problematic ingredients.


May cause irritation, inflammation, dryness, etc. May offer benefit in some capability but overall, proven to do more harm than good.


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Not rated

We have not yet rated this ingredient because we have not had a chance to review the research on it.