What is a humectant?

An humectant is a hygroscopic substance, meaning it has the ability to attract and retain water. This could be moisture from the air, water present inside or outside the hair shaft, or even that contained within the stratum corneum. Humectants work due to their chemical structure, often featuring hydroxyl groups -OH, aldehydes R-COH, ketones R-CO-R' or carboxyls -COOH, which are capable of forming hydrogen bonds with water molecules. Therefore, it is the chemical structure of humectants that gives them their ability to capture moisture.

The primary purpose of humectants is to attract water into the stratum corneum, the outermost layer of the epidermis.

The water content in the stratum corneum controls the biomechanical properties of the skin, that is, its suppleness, elasticity, and firmness. Therefore, in order for the skin to maintain its tone for as long as possible, it is important that it is well hydrated and that the water content in the stratum corneum is sufficient. It should be noted that, although it is possible for humectants to attract water from the environment, particularly when the ambient humidity exceeds 70%, water is primarily drawn from the dermal tissues.

Indeed, the dermis, nourished by the blood network, is a rich reservoir of water. It contains approximately 80% water, while the stratum corneum is composed of only about 10 to 15% water. This difference in content creates a natural concentration gradient that promotes the migration of water from the deeper layers of the skin to the superficial layers. The humectants present in cosmetics applied to the skin's surface support this water transfer and contribute to maintaining a optimal hydration level in the stratum corneum.

Humectants are therefore favoured ingredients when formulating moisturising treatments. However, they are not sufficient on their own. Indeed, a humectant used alone draws moisture to the epidermis, but if the skin barrier is damaged, the humectant may exacerbate dryness. It is therefore important that humectants are combined with film-forming agents, which possess occlusive properties. These agents trap the attracted water in the stratum corneum by forming a protective veil on the surface of the epidermis. Ideally, a moisturising treatment includes a third category of ingredients: emollients. Emollient agents soften the skin and improve its barrier function by strengthening the cohesion of the lipids in the stratum corneum. The tighter ceramides hinder the evaporation of water, thus increasing the water content of the stratum corneum.

Examples of humectants used in cosmetics?

There are numerous humectants in cosmetics. These include glycerine, pyrrolidone carboxylic acid and its derivatives, propylene glycol, fructose, glucose, glycyrrhizic acid and its derivatives, histidine, hyaluronic acid, and its salts, hydrolysates of silk, keratin or soy, and even urea.