We often describe hair as dehydrated, just as we would speak of skin lacking water. However, given that hair is composed of dead material, is it biologically correct to talk about hair hydration? And if so, is it possible to regulate it and hydrate the hair? Read on to discover the answers to these questions.
Is it truly possible to hydrate one's hair?
Published on August 19, 2025, by Pauline, Chemical Engineer — 7 min of reading
What role does hydration play in hair?
Once they have emerged from the hair follicles, hair is not alive. It consists largely of inert keratin filaments, devoid of any metabolic activity. However, this does not prevent hair from containing water, nor does it prevent water from playing an important role in its appearance, elasticity and resistance to breakage.
By way of reminder, each hair fibre comprises three superimposed layers: the medulla, the innermost layer; the cortex, accounting for nearly 80% of the fibre; and the cuticle, the external protective layer. The cortex is composed of keratin and also contains the melanin pigments that give hair its colour. It is also the main reservoir for water. The cuticle, meanwhile, is formed of flat, overlapping cells resembling tiles, and their cohesion is essential to limit water loss but also prevent water ingress. When these cells are disorganised, often following alteration of the intercellular lipid cement, the hair becomes more porous: it retains water less effectively and absorbs it more readily.
Structure of hair fibres.
Source: BONTOZOGLOU C. & al. In vivo human hair hydration measurements by using opto-thermal radiometry. International Journal of Thermophysics (2018).
This is why the verb “hydrate” is not necessarily the most appropriate to use when talking about hair. The purpose of hair care is not to force water deep into the fibre. When water penetrates a porous hair strand, it causes a swelling of the cuticle, further disrupting the arrangement of its overlapping scales. This cycle of swelling and contraction, repeated through washing and drying, weakens the fibre, increases the risk of breakage and promotes the development of split ends. In fact, this is why it is not advisable to let one’s hair to air dry, as moisture lingers longer. The hazard that water poses to hair fibres is illustrated by the images below, captured with an atomic-force microscope. They clearly show the lifting of the cuticle when the hair is submerged. Without the application of a conditioner to smooth and reseal it, the cuticle remains raised and vulnerable to external aggression.
Atomic force microscopy image of a hair in air (a) and in water (b).
Source: BALDESCHWIELER J. D. & al. Atomic force microscopy of human hair cuticles: A microscopic study of environmental effects on hair morphology. The Journal of investigative dermatology (1995).
It is preferable to refer to ‘conditioning’ your hair or ‘protecting it against dehydration’ rather than ‘hydrating’ it.
How to "hydrate" your hair?
Hydrating, or rather conditioning one’s hair, involves preventing water from entering and leaving the hair. To achieve this, the cuticle scales must be properly smoothed. This is notably achieved through the systematic application of a conditioner after each wash. Indeed, using a conditioner flattens the cuticle scales along the hair shaft thanks to the cationic surfactants it contains. This treatment also enhances the adhesion of the cuticle scales to the hair shaft, thereby improving the hair’s ability to reflect light and reducing friction between the hair fibres.
It is also important to maintain the cuticle’s lipid barrier. This is where emollients and occlusive agents come into play. Vegetable oils, such as coconut oil or apricot oil, rich in fatty acids, can form a film on the surface of the fibres, thereby reducing water evaporation. Some studies have even shown that lauric acid, one of the main components of coconut oil, can penetrate the hair, reducing cuticle swelling caused by water and the stress this imposes on the hair fibres.
Other studies have investigated the effect of nanoemulsions on the hydration of textured hair samples provided by a woman. Some hair fibres underwent a bleaching process, with or without subsequent recolouring. For information, a nanoemulsion is a mixture of two or more immiscible liquids in which one is finely dispersed in the other, forming droplets at the nanometre scale. Specifically, three nanoemulsions were studied: a control nanoemulsion, a version enriched with 7.5% coconut oil and a third containing 7.5% coconut oil and 0.1% wine lees extract. These were applied to the three hair types and their hydration was then observed.
| Healthy hair | Bleached hair | Hair that has been bleached and subsequently recoloured | |
|---|---|---|---|
| Control nanoemulsion | 7.3 | 7.3 | 7.2 |
| Nanoemulsion containing 7.5% coconut oil | 7.2 | 7.3 | 8.3 |
| Nanoemulsion with 7.5% coconut oil and 0.1% wine lees extract | 7.6 | 7.7 | 9.6 |
Comparison of the "hydrating" effect of different nanoemulsions (arbitrary units).
Source: GOMES A. & al. Nanoemulsion with wine lees: a green approach. Anais da Academia Brasileira de Ciencias (2024).
Whilst cosmetics play an important role, prevention must not be overlooked. Limiting heat sources (straighteners, curling tongs), avoiding products containing harsh surfactants, protecting one’s hair from UV rays... are all beneficial habits to adopt to prevent your hair from becoming damaged.
Sources
BALDESCHWIELER J. D. & al. Atomic force microscopy of human hair cuticles: A microscopic study of environmental effects on hair morphology. The Journal of investigative dermatology (1995).
BONTOZOGLOU C. & al. In vivo human hair hydration measurements by using opto-thermal radiometry. International Journal of Thermophysics (2018).
SANJEEVA MURTHY N. & al. Structure of intermediate filament assembly in hair deduced from hydration studies using small-angle neutron scattering. Journal of Structural Biology (2019).
KAMATH Y. & al. Structure of intermediate filament assembly in hair deduced from hydration studies using small-angle neutron scattering. Journal of Structural Biology (2019).
GOMES A. & al. Nanoemulsion with wine lees: a green approach. Anais da Academia Brasileira de Ciencias (2024).
CAO Y. & al. Excellent hydration lubrication based on Zwitterionic Poly (N-vinylpyrrolidone-co-methacrylic acid) for hair combing improvement. Surfaces and Interfaces (2024).
HAYASHI T. & al. Analysis of the hydration water on the surface of human hair using a combination of infrared absorption vibrational spectroscopy and multivariate curve resolution. Surfaces and Interfaces Analysis (2024).
