Beauty

Hair porosity: the test and what it changes in your care

Porosity is shorthand for how easily water and product cross the cuticle, the 6 to 10 overlapping layers of keratin scale that wrap the fibre, and in hair care it is a practical idea rather than a clinical measure: no dermatology body defines it, and the low, medium and high labels rest on no validated scale. What laboratories can measure is damage, and the gaps are small: in one 2025 series, tresses conditioned at 55 percent relative humidity gave up 9 percent of their weight as water on heating when virgin, 10 percent when bleached and 11 percent after bleaching and acid straightening. The popular float test settles none of this, because trapped air, surface tension and any conditioner left on the strand decide whether it sinks.

What porosity actually describes

A hair is a dead fibre: the cortex carries the strength and about 90 percent of the weight, and the cuticle wraps it in flat cells that overlap like roof tiles. A 2023 review puts that wrapping at roughly 6 to 10 cells thick; a dermatology overview counts 6 to 8 scales in Asian hair, slightly fewer in Caucasian and fewer still in African hair, and links that thinner cuticle to African hair breaking more easily. Each visible scale is 5 to 10 micrometres long.

Porosity is the shorthand for how easily water and dissolved product get past that wrapping. The outer skin of the cuticle carries a bound layer of 18-methyleicosanoic acid, a branched fatty acid held on by thioester links, which makes healthy hair water repellent and lets fibres slide past each other. Strip it and the surface turns hydrophilic: it wets faster, swells more, tangles more. Hair also sits at an isoelectric point near pH 3.67, and bleached hair carries more negative sites, which is why positively charged conditioning agents cling to it harder.

The three popular categories, low, medium and high, are a convenience. No dermatology body defines porosity, no standardised test produces a reading, and no study has checked the three labels against a laboratory measure. They name behaviour you can observe: whether water beads or soaks in, how long drying takes, whether a leave-in vanishes in 10 seconds or sits on top.

Why the glass of water test fails

The instruction is to drop a clean strand into a glass and read porosity from whether it floats for 2 to 4 minutes or sinks. Keratin is denser than water, so a properly wetted, residue free strand sinks whatever its cuticle is doing. What holds a strand up is trapped air and surface tension, and what keeps water off it is the film conditioners, oils and silicones leave, one of their stated jobs being to mimic the 18-methyleicosanoic acid layer. A well conditioned undamaged strand and a badly rinsed damaged one behave the same in the glass.

Laboratory methods exist, and each measures something narrower than the word suggests. A halogen moisture analyser weighs the water driven off a sample: in one drying study, tresses held at 82 percent relative humidity for 7 days gave an untreated moisture content of 4.6 percent, and 30 wash and dry cycles lowered it in every treated group, with no difference reaching statistical significance. Thermogravimetry does the same on a temperature ramp under nitrogen, separating the water that leaves between 30 and 150 degrees Celsius from the protein breakdown above 200 degrees. Electron microscopy grades lifted and cracked scales directly.

Read the size of those numbers before buying a routine around them. Between virgin hair and hair bleached then acid straightened, the water driven off differed by 2 percentage points, 9 percent against 11 percent. That came from tresses conditioned at 22 degrees Celsius and 55 percent relative humidity, not from anyone's head, and it is nothing like the three step ladder the word implies.

What bleach and heat do to the cuticle

Bleaching is the quickest route to a porous fibre. In that 2025 study, tresses were treated for 30 minutes at room temperature with an alkaline product at pH 10.5 in 20 percent hydrogen peroxide with ammonium persulfate. The chemistry oxidises melanin, breaks disulfide bonds and attacks the thioester links holding 18-methyleicosanoic acid to the surface, so the scales lose their lipid, lift and crack, and what comes out is weaker, rougher and more absorbent.

Heat is dose dependent, and the doses have been measured, again on tresses. Washed and dried 30 times over 30 days, they cracked along the cuticle at 47 degrees Celsius with the dryer 15 cm away, and showed holes and blurred scale borders at 95 degrees at 5 cm; both that hottest group and the group left to dry in room air measured lighter after only 10 rounds. Blow drying is thought to raise the fibre itself to around 80 degrees. Irons run far hotter: the straightening protocol used 10 passes at 180 degrees, and the same group reported that an iron at 200 degrees raised measured porosity.

Past a point the fibre stops behaving like hair. Thermogravimetry put the onset of decomposition at about 210 degrees Celsius for virgin and bleached hair and about 200 degrees once hair had been acid straightened, and in air the straightened sample broke down from 175 degrees. One finding cuts the other way: the only group with damage to the intercellular lipid layer was the one left to dry in room air, which the authors put down to long contact with water rather than heat, so hours in a wet towel is not the gentle option it sounds.

Care that follows from the reading

If water beads, drying takes hours and leave-ins sit on the surface, treat the hair as low porosity. Use thin liquids and milks on damp rather than soaking hair, and clarify when films build up, near every 4 weeks for most people, an interval that is salon convention rather than a measured optimum. What the literature does set is contact time: about 5 minutes for a rinse out conditioner against 20 to 30 minutes for a deep one.

If hair soaks in seconds, dries fast, swallows product and still feels rough, treat it as high porosity. Layer a leave-in, seal it with an oil or cream, and alternate protein and moisture instead of stacking either. Protein hydrolysates below about 1000 daltons are small enough to diffuse into a damaged fibre, but the familiar interval of one treatment every 4 to 6 weeks is convention: the evidence describes the size of the molecule, not the frequency of use, and the spacing exists because hair treated more often tends to feel stiff. Oils are argued to work through swelling rather than nourishment, an oil film lowering the water the fibre takes up and with it the repeated swelling and drying that reviews name hygral fatigue. Take that loosely: hygral fatigue is a term in those reviews rather than a measured effect, and the finding that coconut oil cut protein loss where mineral and sunflower oils did not comes from one 2003 laboratory study on tresses, run by the research arm of a hair oil manufacturer.

Two habits matter more than any category. Wet hair is soft and extensible: virgin hair stretches 30 percent when wet without harm, changes become irreversible between 30 and 70 percent, and it fractures at 80 percent, so detangle with fingers or a wide tooth comb rather than dragging a brush through soaking hair. And lower the heat. No source names a safe setting, and 10 passes at 180 degrees Celsius were enough to damage tresses in the study above, so take fewer passes at the lowest setting that works and hold the dryer 15 cm away. The Hair Porosity Check turns five of these observations into a reading without any glass of water.

When the problem is the scalp and not the cuticle

Porosity describes the fibre you already have, and you cannot repair it. A National Library of Medicine reference text puts scalp growth at about 0.35 mm a day, near 1 cm a month, and a cuticle may have to survive up to 6 years, so hair can be intact near the scalp and bare of scales near the tip. Everything a conditioner does is cosmetic and temporary, filling and coating until the next wash, and scissors are the only method that removes damage.

Shedding is a separate question. About 10 to 15 percent of hairs are resting at any moment, and the same reference text puts normal shedding at roughly 100 to 200 hairs a day. Thinning at the temples, patchy loss, breakage along a tight parting or hairline from braids and ponytails, redness, scaling or itch belong with a dermatologist rather than a conditioning routine, and so does sudden shedding after illness, childbirth or a new medicine. If that is the real question, start with the guide to hair loss in women.

A 4 week observation check instead of the glass of water

Frequently asked

Does the float test measure anything?

No. Keratin is denser than water, so a properly wetted strand with no residue sinks whatever its cuticle is doing, and the result is actually decided by trapped air, surface tension and any conditioner, oil or silicone left on the fibre. Those films are exactly what conditioners are formulated to leave, since one of their stated jobs is to mimic the 18-methyleicosanoic acid layer and restore water repellency.

Can I change my hair porosity?

You can change how the fibre behaves, not repair it, because hair is dead tissue that cannot be renewed biologically. Conditioners fill and coat the damaged cuticle until the next wash, and new growth arrives at about 1 cm a month, which is why cutting is the only way to remove a porous length.

Is high porosity just a nicer word for damage?

Largely, yes. Bleached and chemically treated hair is more porous than virgin hair and binds conditioning ingredients more strongly because its isoelectric point is low, near pH 3.67, so it carries more negative sites. Fine, undamaged hair can still wet quickly, which is why the label describes behaviour rather than a diagnosis.

How often should I use a protein treatment?

There is no measured interval. Every 4 to 6 weeks for bleached or heat styled hair, with moisture in the weeks between, is salon convention rather than a trial result, and what the evidence describes is the size of the molecule: hydrolysates below about 1000 daltons can diffuse into a damaged fibre. Used more often, protein treatments tend to leave hair stiff rather than stronger.

What temperature is safe on a straightener?

There is no safe number, only a smaller dose. Thermogravimetry places the onset of decomposition at about 210 degrees Celsius for virgin hair and about 175 degrees for acid straightened hair in air, and 10 passes at 180 degrees were enough to damage tresses in the same study, so use the lowest setting that does the job, take fewer passes and never iron soaking hair.

My hair is falling out. Is that a porosity problem?

No. Around 10 to 15 percent of hairs are resting at any time and one reference text puts normal shedding at roughly 100 to 200 hairs a day, but thinning, patches, a receding hairline or an itchy, scaly scalp are medical questions. See a dermatologist rather than changing conditioner.

Sources

• Fernandes C, Medronho B, Alves L, Rasteiro MG. On hair care physicochemistry: from structure and degradation to novel biobased conditioning agents. Polymers. 2023;15(3):608 · Open access review, read in full for the structure and weathering sections. Source of the cuticle at about 6 to 10 overlapping cells, the visible scale length of 5 to 10 micrometres, protein at 65 to 95 percent of hair weight and the cortex at about 90 percent, fibre diameters of about 50, 80 and 100 micrometres, the 18-methyleicosanoic acid layer bound by thioester links and removed by alkaline treatment, the cuticle lasting up to 6 years, the loss of scales from scalp to tip, and the contact times of about 5 minutes for an instant conditioner against 20 to 30 minutes for a deep conditioner. The review gives no figure for water uptake or swelling and does not use the term hygral fatigue.
• Gavazzoni Dias MFR. Hair cosmetics: an overview. International Journal of Trichology. 2015;7(1):2 to 15 · Peer reviewed dermatology overview, read in full. Source of the cuticle at 6 to 8 scales in Asian hair and fewer in Caucasian and African hair with the thinner cuticle linked to breakage, the isoelectric point of pH 3.67 and the higher affinity of bleached hair for conditioning agents, the 1000 dalton threshold for molecules that can diffuse into hair, and the wet stretching figures of 30, 70 and 80 percent. It names hygral fatigue in one clause without data, and the coconut oil result it reports is a 2003 laboratory study on tresses from the research department of a hair oil manufacturer, which is why both are flagged as weak evidence in the text. The review sets no interval for protein treatments.
• Lee Y, Kim YD, Hyun HJ, Pi LQ, Jin X, Lee WS. Hair shaft damage from heat and drying time of hair dryer. Annals of Dermatology. 2011;23(4):455 to 462 · Controlled laboratory study on hair tresses, read in full. Source of the five groups and 30 wash and dry cycles over 30 days, the measured temperatures of 47, 61 and 95 degrees Celsius at 15, 10 and 5 cm, the electron microscopy findings of cracks, lifting and holes, the colour change after 10 treatments in both the 95 degree and the ambient dried groups, the untreated moisture content of 4.6 percent after 7 days at 82 percent relative humidity with lower values in all treated groups but no statistically significant differences, and the bulging of the intercellular lipid layer seen only in the naturally dried group.
• Lima CRRC, Bandeira ACC, Martins TS, Otubo L, Oliveira CLP. Thermal induced changes in cuticle and cortex to chemically treated hair. Biopolymers. 2025;117(1):e70071 · Open access physics study using X-ray scattering, thermogravimetry and electron microscopy, read for methods and results. Source of the bleaching protocol at pH 10.5 in 20 percent hydrogen peroxide with ammonium persulfate for 30 minutes, the acid straightening protocol with 10 flat iron passes at 180 degrees Celsius, the dehydration mass losses of 9, 10 and 11 percent measured between 30 and 150 degrees under nitrogen on tresses air dried for at least 48 hours at 22 degrees Celsius and 55 percent relative humidity, the decomposition onsets of about 210, 200 and 175 degrees, the melting and denaturation range of 220 to 250 degrees reported in the literature, the estimate that blow drying raises the fibre to about 80 degrees, and the group's earlier finding that acid straightening with an iron at 200 degrees raised measured porosity.
• Hoover E, Alhajj M, Flores JL. Physiology, hair. StatPearls, National Library of Medicine, updated 30 July 2023 · National Library of Medicine reference text, read in full. Source of the growth rate, which it gives as about 0.35 mm a day, 1 cm a month or 15 cm a year; only the first two are used here, because 0.35 mm a day works out at nearer 13 cm a year and the annual figure is a rounded convention. Also the source of shedding at roughly 100 to 200 follicles a day, the statement that 10 to 15 percent of hairs are in the resting phase at any time, and the clinical sections on pattern loss, traction alopecia from tight braids and ponytails, alopecia areata, scarring alopecia and telogen effluvium.

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This guide is for general information. It does not replace advice from a dermatologist, physician or qualified practitioner, or an examination.