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Définition porosité des cheveux.

What is hair porosity?

Often mentioned in hair assessments, hair porosity remains misunderstood. It affects hydration, shine, and hair strength. How does 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, the hair are an important part of our appearance and some see them as a sign of health. To care for them, it’s important to understand their structure. This structure is organized into three main layers : the medulla, cortex, and cuticle. The medulla forms the core of the hair fiber, but its role remains unclear. The cortex, the intermediate and thickest layer, is rich in keratin organized into fibrils linked by disulfide bonds that give hair its strength and elasticity. The cortex contains pigments of melanin, responsible for hair color. The cuticle, the outermost layer, consists of flat cells arranged like overlapping tiles, similar to fish scales. The cuticle plays a major protective role for the inner layers of hair.

Porosity refers to a hair’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. Composed of ceramides, free fatty acids, and cholesterol, it ensures cell cohesion and limits water and chemical penetration. When the cuticle’s lipid cement is altered by UV exposure or by chemical or thermal treatments, the hair becomes more permeable and its porosity increases. The term "porosity" comes from the observation of micro-pores in the structure of porous hair.

Illustration de la porosité des cheveux.
Appearance of hair (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 affect hair porosity, including heat, bleaching, and sun exposure. A study examined the effects of UV radiation on hair porosity. In this study, volunteer hair braids were exposed to a UV lamp at an intensity of 7.2 mW/cm2 for different durations. The results, shown in the figure below, show that porosity increases with UV exposure time. This is why it’s 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 ultraviolet radiation on hair porosity.
Source: Jeong E.-T. et al. Effects of ultraviolet rays and particulate matter on hair porosity in damaged hair. Journal of Cosmetic Dermatology (2024).

How can we tell if our hair is porous?

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

  • Low porosity: the cuticle is smooth and sealed, limiting water entry and promoting retention. Treatments penetrate more slowly but are retained.

  • Medium porosityIt balances water absorption and retention. This configuration supports hair health.

  • High porosityThe cuticle is lifted or damaged, allowing water to enter and preventing its retention. Hair is dry, brittle, and prone to frizz.

There are several methods to assess hair porosity, with varying degrees of rigor. The most popular among the general public remains the water glass test, during which a strand is placed in a glass of water. If it floats for a long time, it is considered low‐porosity; if it sinks quickly, high‐porosity. Although this test provides an initial indication, it is imprecise because it is influenced by hair diameter and any surface residues. Observing hair can also serve as an indicator: hair that reflects light should have low porosity. The following figure, from a study on the hair of 120 volunteers with varying porosity, illustrates this. Note that the Hue angle here measures the average hue of light reflected by the hair surface. A higher Hue angle corresponds to duller hair, which associates with increased porosity.

Capacité de réflexion de la lumière des fibres capillaires selon leur porosité.
Light reflectance of hair fibers based on their porosity.
Source: Nagase S. et al. Influence of internal structures of hair fiber 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 also used. These include scanning electron microscopy (SEM), which allows one to observe the arrangement of cuticle scales. Fourier-transform infrared spectroscopy (FTIR) is another technique useful for assessing hair porosity because it detects molecular changes in the lipid cement. Loss of protective lipid compounds is associated with porous hair. However, these precise laboratory methods cannot be used by individuals. In this context, techniques such as the water glass test or a visual examination of the hair prove useful.

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