Hair Porosity: The Complete Science

Hair porosity is a measurement of damage state, not a hair type you were born with.

Specifically, it is the structural condition of three nested layers in every fibre of every hair: a one-molecule-thick surface lipid called 18-MEA (18-methyleicosanoic acid), a stack of overlapping cuticle scales, and a deep cortex cable of twisted protein. When all three are intact, your hair is low-porosity. When one or more is compromised by bleach, heat, dye, chlorine, UV, or years of mechanical wear, it is high-porosity.

The three things your hair really is

Picture a single fiber, magnified ten thousand times. The first thing you'd see, before any structure becomes visible, is a thin sheen. That's 18-MEA (18-methyleicosanoic acid), a single layer of branched fatty-acid molecules covalently glued to the surface. It's why a drop of water beads on healthy hair. It's also the most fragile thing about the fiber, structurally speaking. A single bleach treatment strips more than 80% of it.

Below 18-MEA: the cuticle scales. Five to ten of them stacked, each one a flat cell shaped like a guitar pick, anchored at the root end and tapered toward the tip. They overlap like fish scales, all pointing the same direction. When you run your finger from root to tip, you're going with the scales; it feels smooth. From tip to root, against the scales, it feels rough. The outermost edge of each scale, called the A-layer, is so densely crosslinked with disulfide bonds (about a quarter of its amino acids are cysteine) that it's almost a different material from the rest of the fiber.

And under the cuticle: the cortex. This is the part that decides how strong your hair is. It's a bundle of microfibrils, long protein cables, twisted around each other like a steel suspension cable, held together by hydrogen bonds, salt bonds, and disulfide crosslinks. When something pulls on your hair, this is what carries the load. When the disulfide crosslinks break, these cables start to fray.

This is what porosity describes: how compromised each of those three layers has become, and in what proportion. The lab measures it as pore surface area (BET gas sorption); the mirror reads out the consequences: slip, sheen, water uptake, color hold, breakage. Both are talking about the same fibre.

Common myths about porosity

You may have taken a porosity quiz that asked about your texture, your ethnicity, whether products "sit on top" of your hair. You may have been told that low, medium, or high porosity is something you were born with, like eye color. Two things to know.

First: virgin hair, across every ethnicity studied, sits in a narrow porosity range. Asian, African, Caucasian, Latin. The variation between groups at the virgin state is real but small. African hair averages slightly fewer cuticle layers; Asian hair holds the most integral lipid; cross-sectional shape varies across all groups. These differences matter, but they're swamped by the difference between virgin hair and damaged hair.

Second: the dramatic high-porosity states described in porosity-quiz culture (water that sinks instead of beads, color that fades in two washes, the gummy wet texture) almost always trace to a documented insult. Bleach. Heat above 180°C. Chlorine. UV. Repeated permanent dye. Relaxers. Years of mechanical wear on long hair.

For most people, "high porosity hair" describes the consequence of damage. Treat the cause; the porosity follows.

What happens to your hair in 60 seconds of bleach exposure

In 2008, three chemists named Yves Hessefort, Brad Holland, and Robert Cloud published a paper in the Journal of Cosmetic Science that should have changed how we talk about bleach. They put bleached hair into a gas-sorption instrument and measured its accessible surface area. Then they measured it again at 30 seconds. And again at 60 seconds.

In the first 60 seconds of bleach exposure, the fiber's surface area triples.

Triples.

Sixty seconds is shorter than the time it takes to wet your hair. In one minute of contact with persulfate and peroxide (the active chemistry in every bleach product, salon or drugstore), your hair gains three times the accessible pore surface it had a minute earlier. The fiber doesn't just acquire damage. It acquires a new geometry.

What's happening at the molecular scale: persulfate and peroxide are oxidizers. They attack three things simultaneously. They cleave the thioester bonds anchoring 18-MEA, dissolving the surface lipid into the bleach mixture. They oxidize cysteine residues (the amino acids that form disulfide bonds) into cysteic acid, which can no longer hold the crosslink. And they physically perforate the endocuticle, the softer inner layer of each cuticle cell. Marina Di Foggia and colleagues at the University of Bologna mapped all of this with Raman spectroscopy in 2021. The damage progresses with each additional bleach session and never reverses on its own.

This is why your colorist's "we'll just lift a couple of levels" is never just two levels of color change. It's two levels of color change plus the architectural rearrangement of every fiber on your head.

What scientists found in February 2025

There is one more layer of mechanism, and it just became public. In early 2025, Steve Breakspear, Burkhardt Noecker, and Crisan Popescu (KAO Germany cosmetic chemists who have studied hair for decades) managed something no one had done before. They physically separated the cuticle from the cortex of a single hair fiber and measured how much water each one absorbs.

The results were surprising.

At low humidity, cuticle and cortex absorb water identically. But above 65% relative humidity (anywhere from a humid summer day to a hot shower), the cuticle starts holding much more water than the cortex. At 95% RH, the cuticle sits at 34.33% water by weight. The cortex underneath: 21.5%. The two layers, in other words, swell at different rates whenever the air gets humid.

Now imagine what happens at the boundary between them. The cuticle expands. The cortex doesn't expand as fast. The two surfaces shear against each other. Microscopically, slightly, but every cycle, all day, every day.

What stops this shear from causing damage? The 18-MEA layer. The branched fatty acid we started with, the one bleach strips off in 60 seconds. 18-MEA acts as a lubricating gliding interface between the cuticle and the cortex. Take it away, and the two layers grind against each other with every hot shower, every humid morning, every wet-dry cycle.

This is the structural mechanism of hygral fatigue in bleached hair. It's why bleached hair gets worse with use, not just worse with age. The chemistry that strips 18-MEA also delivers the conditions that make the loss matter. The full paper is Breakspear, Noecker & Popescu, Int J Cosmet Sci 47:639–651 (2025).

What the home tests actually measure

You've probably heard about three home tests for porosity: the water-drop test, the strand-float test, and the wet-stretch test. They circulate on TikTok, in salon literature, in beauty magazines. Each one measures something. None measures porosity directly. The laboratory methods that do that cost tens of thousands of dollars and destroy the sample.

The water-drop test. Place a drop of water on a clean, dry mid-length strand. If it sits on the surface for 30+ seconds, your 18-MEA is intact. If it absorbs in under 10 seconds, the surface lipid is gone. This is the cleanest of the three: it measures one specific structural feature.

The strand-float test. Float a clean shed strand on a glass of water. Two to four minutes. If it floats, the cuticle is hydrophobic enough to repel water. If it sinks fast, water has penetrated. This is the noisiest of the three. Product residue on the strand floats; thick coarse hair sinks under its own weight; results vary by water temperature. Read it as a rough signal, not a verdict.

The wet-stretch test. Stretch a clean wet strand gently. A strand that stretches before snapping and then partially retracts has cortex elasticity: disulfide bonds still doing their job. A strand that snaps immediately has lost both. This is technically a tensile test, not a porosity test, but it correlates tightly with porosity in damaged hair.

Read together: if all three fail, you have cuticle and cortex damage. If only the water-drop fails, your 18-MEA is gone but the cortex underneath is still mostly intact, common after one bleach session, before damage compounds. The longer walk-through is at Hair Porosity Test: 3 Methods That Actually Work.

What actually repairs porosity

Here's where the science gets specific and the marketing gets vague. The peer-reviewed repair literature, when you read it across journals, splits cleanly into three classes. Almost every consumer "bond builder" or "molecular repair" product fits into one of them.

Class 1: Surface lipid replacement. Replaces the 18-MEA you lost. Restores surface hydrophobicity, slip, combability; the water-drop test passes again. Mechanistically protective against the hygral-fatigue mechanism Breakspear's group described. Examples: quaternized 18-MEA derivatives; plant-triglyceride nano-emulsions (Lai et al. 2025); coconut oil, one of the few oils with documented penetration into the cuticle scale itself.

Class 2: Polyphenol-quinone non-covalent crosslinking. Polyphenols from the catechol family (EGCG from green tea, certain tannins) oxidize to quinones under mild conditions, then react with cysteine and lysine residues to form non-covalent crosslinks inside the cortex. Yoshida, Maruyama, and Yamauchi published the clearest peer-reviewed demonstration in 2023: radial swelling of bleached hair dropped from 45% back to 26%, almost to virgin baseline. Real chemistry, partial recovery.

Class 3: Thiol-reactive click-chemistry covalent crosslinking. Small allyl- or alkyne-functional molecules react with the free thiol groups that bleach exposed (the cysteines that lost their disulfide partner) to form new covalent C-S bonds. The reaction class is click chemistry, the same family that won Barry Sharpless, Morten Meldal, and Carolyn Bertozzi the 2022 Nobel Prize in Chemistry. The new bonds form at the same 2.2–2.4 ångström length scale as the native disulfide bonds they replace, a geometry Breakspear's group also nailed down. This is the chemistry behind ANATOMY's molecular reconstruction system.

ANATOMY's two patented molecules:

• Aminalyl S, a smaller allyl molecule (INCI: Allylammonium Succinate).
• Pro-amino X, an alkyne (INCI: Aminopropyldimonium Hexynoate).

The first two undergo thiol-ene mono-adduct chemistry: one new bond per reaction. Pro-amino X undergoes thiol-yne bis-adduct chemistry: two new bonds per reaction. Independent LC-MS quantification of the bis-adduct selectivity: 81.5%. Independent single-fibre tensile testing at SGS Proderm in Schenefeld, Germany (bleached hair) measured force-to-break of 15.2 cN baseline and 35.8 cN after a single treatment. A 135% increase. The E-modulus reduction of about 15% means the fiber is also more flexible, not just brittle-strong.

Three granted patents cover the chemistry.

A pragmatic protocol

If you suspect your hair is high-porosity or damaged, here's what actually works, in order of importance.

1. Stop adding insult. Six weeks without bleach. No flat iron above 180°C while the cuticle is repairing. Skip chlorine pools or rinse immediately after. Cuticle damage naturally clears in about 8 weeks (Ahn & Lee, 2002, tracking a single permanent-dye treatment through TEM imaging for 8 weeks). Chemistry can accelerate that, but only if you stop adding new insult.
2. Chelate. If your tap water has meaningful mineral content, get a chelating shampoo with sodium phytate (plant-derived, biodegradable) or EDTA. Porous hair picks up calcium, copper, and iron from water; chelators clear them. The underrated first step.
3. Reconstruct internally. A Class 3 system that delivers small reactive molecules where they need to go. Apply, leave, rinse; repeat on the cycle the protocol specifies.
4. Restore the surface. Class 1 lipid replacement, ideally in the same product as Class 3, so both fronts are covered simultaneously.
5. Maintain. Even with optimal chemistry, restoration takes weeks of consolidation. The maintenance phase is just not adding any further insult.

That's the entire protocol. Steps 3 and 4 are what the two products below do, in one system. Step 1 is your discipline. Step 2 is a chelating shampoo. Step 5 is time.

Frequently asked

How do I know my hair's porosity? Use the three home tests together. A water drop that beads and sits means a sealed, water-repellent surface (lower porosity); one that soaks in fast means the surface is stripped (higher porosity). A shed strand that floats is less porous than one that sinks. A wet strand that stretches and springs back has an intact core; one that snaps is damaged. Read all three, not one. Full method: Hair Porosity Test.

What is the worst hair porosity? There is no inherently bad porosity, but very high porosity is the most troublesome: the strand has lost its surface seal and broken its core bonds, so it drinks and loses water fast, fades color, tangles, and snaps. It is also a damage state, which means it can be repaired, not a permanent type.

Does ethnicity determine hair porosity? Only slightly. Texture, scale-layer count, and natural oil content vary between people from birth and give virgin hair a small natural porosity range. But that variation is minor next to the difference damage makes. Dramatic high porosity is almost always caused by bleach, heat, or wear, not by ethnicity.

Is hair porosity a hair type or hair damage? Damage, almost always. Virgin hair across every ethnicity sits in a narrow porosity range; the dramatic high-porosity states people describe trace to bleach, heat, chlorine, UV, color, relaxers, or years of wear. Treat the cause and the porosity follows.

Does bleach increase hair porosity? Rapidly. The first minute of bleach roughly triples the strand's accessible surface, strips more than 80% of the protective oil coating, and breaks the bonds in the core. Any visible lift means measurable porosity.

Can high porosity hair be repaired? Yes, to behave like healthy hair again: rebuild the core bonds, reseal the surface, and clear mineral build-up. It will not become chemically identical to never-damaged hair, but it can be restored to test stronger and look and feel healthy. Only new growth is truly undamaged.

References

• Hessefort, Y., Holland, B.T. & Cloud, R.W. J Cosmet Sci 59:303-315 (2008). link
• Breakspear, S., Noecker, B. & Popescu, C. Int J Cosmet Sci 47:639-651 (2025). doi
• Di Foggia, M. et al. Data in Brief 38:107439 (2021). doi
• Ahn, H.J. & Lee, W.-S. Int J Dermatol 41:88-92 (2002). doi
• Yoshida, M., Maruyama, R. & Yamauchi, A. J Cosmet Sci 74:143-157 (2023). link
• Lai, N.H. et al. Dermatol Res Pract 2025:5385312. doi
• Robbins, C.R. Chemical and Physical Behavior of Human Hair, 5th ed. Springer (2012).
• 2022 Nobel Prize in Chemistry, Sharpless, Meldal & Bertozzi. nobelprize.org
• ANATOMY / SGS Proderm, Schenefeld, Germany. Single-fibre tensile testing, study 22.0088-96 (2022).