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Définition porosité des cheveux.

What is hair porosity?

Often raised in hair diagnostics, the concept of hair porosity remains poorly understood. Yet it influences hair hydration, shine, and resilience. How does hair porosity work? Let’s explore this phenomenon.

Summary
Published June 10, 2025, updated on June 10, 2025, by Pauline, Chemical Engineer — 6 min read

Hair porosity: what is it?

Whether long or short, hair is an important part of our appearance and is sometimes considered a marker of good health. To care for it, it is important to understand its structure. It is organised into three main layers : the medulla, cortex, and cuticle. The medulla forms the core of the hair fibre, but its role remains unclear. The cortex, the intermediate and thickest layer, is rich in keratin organised into fibrils linked by disulfide bonds. These bonds give hair strength and elasticity. It is also in the cortex that pigments of melanin, responsible for hair colour. Finally, the cuticle, the outermost layer, is made of flat cells arranged as overlapping tiles, similar to fish scales. The cuticle plays a major protective role for the inner layers of hair.

Porosity refers to a hair fibre’s ability to absorb and retain water or other liquids, such as treatments or dyes.

Porosity depends on the condition of the cuticle and the intercellular lipid cement. It consists of ceramides, free fatty acids and cholesterol. The latter ensures cellular cohesion and limits the penetration of water and chemicals. When the cuticle’s lipid cement is damaged by UV rays or by chemical or thermal treatments, the hair fibre becomes more permeable and its porosity increases. The term “porosity” derives from the fact that one can observe micropores in the structure of porous hair fibres.

Illustration de la porosité des cheveux.
Hair appearance (A and C) and corresponding micrographs showing micropores in the cortex (B, scanning electron micrograph, and D, transmission electron micrograph)
Source: NAGASE S. Hair Structures Affecting Hair Appearance. Cosmetics (2019).
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Several external factors can influence hair porosity, including heat, bleaching and sun exposure. A study examined the effects of UV rays on hair porosity. For this, volunteers’ hair braids were exposed to a UV irradiator at an intensity of 7.2 mW/cm2 over varying durations. The results, shown in the figure below, indicate that porosity increases with UV exposure time. This is why it is important to protect your hair in the sun, first with a hat, and a UV protective spray.

Effets des rayons UV sur la porosité des cheveux.
Effects of UV radiation on hair porosity.
Source: JEONG E.-T. et al. Effects of ultraviolet radiation and particulate matter on the porosity of damaged hair. Journal of Cosmetic Dermatology (2024).

How can we determine whether our hair is porous?

We distinguish three levels of porosity, which some people use to build their haircare routine:

  • Low porosity : The cuticle is smooth and sealed, which limits water uptake and allows good retention. Treatment absorption is slower but efficacy is maintained.

  • Medium porosityThere is a balance between water absorption and retention. This state is favourable for healthy hair.

  • High porosity: the cuticle is raised or damaged, facilitating water entry but preventing retention. Hair tends to be dry, brittle and prone to frizz.

There are several methods to assess hair porosity, with varying rigour. The most popular among the general public remains the water glass test, in which a strand is submerged in a glass of water. If it floats for a long time, it is considered low porosity, whereas if it sinks, it is considered high porosity. Although this test provides an initial indication of hair porosity, it is imprecise, as it is influenced by hair diameter and residues on its surface. Observing hair can also serve as a useful indicator: hair that reflects light is likely to be low porosity. This is shown in the following figure, from a study on hair samples from 120 volunteers with varying porosity. Note that the Hue angle, used here as a measure, denotes the average tone of light reflected by the hair surface. Here, the higher the Hue angle, the duller the hair appears, which is associated with greater porosity.

Capacité de réflexion de la lumière des fibres capillaires selon leur porosité.
Light reflectance of hair fibres as a function of porosity.
Source: Nagase S. et al. Influence of internal structures of the hair fibre on hair appearance. Light scattering from the porous structure of the medulla of human hair. Journal of Cosmetic Science (2002).

During scientific studies, other reliable methods are used. These include scanning electron microscopy (SEM), which enables observation of cuticle scale arrangement. Fourier transform infrared spectroscopy (FTIR) is another technique for assessing hair porosity as it detects molecular changes in the lipid cement, a loss of protective lipids being linked to porous hair. These precise laboratory methods cannot be used by individuals. In this context, techniques such as the water glass test or visual inspection of hair prove useful.

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