Hair porosity

Key takeaways

• All hair is porous. Water enters mainly through the non-keratin regions inside the fiber and swells it, so the claim that healthy hair "does not absorb water" is wrong. The pores are about 4 nanometres in radius, far too small to see Habe 2025.
• Damage is the main cause of high porosity. Oxidative bleach nearly tripled internal surface area within the first minute Hessefort 2008. Bleaching adds many small pores, and long bleaching merges them into larger cavities Habe 2025. Bleaching followed by acid straightening tripled total porosity compared with acid straightening on virgin hair Lima 2023. UV light, and UV plus air pollution, increased dye uptake Yang 2024. Even routine surfactant washing increased pore surface area in one industry study Kaushik 2022.
• Porosity rises from root to tip. In very long hair the cuticle abrades first, then 18-MEA, ceramides and structural proteins decline further out Thibaut 2010. Ends are almost always more porous than roots.
• Differences between hair types exist but are modest. East Asian hair took up the least water and resisted hydration changes most, European hair took up the most, and African hair let water diffuse fastest and had the most lipids Oliver 2020, Marti 2016, Daniels 2023. "Low porosity" as a fixed hair type is a consumer concept without validated cut-offs.
• The float test has no scientific validation. We found no peer-reviewed study comparing it with a lab porosity measure (searched 2026-10-10). Residue, curl shape, trapped air and timing all change whether a strand sinks.
• The lab version of "water beads up" is real: water-droplet absorption time and contact angle track loss of the water-repelling 18-MEA surface layer Wiesche 2011, Tanamachi 2010. At home, conditioner, oils and silicones confound it.
• Porosity is not the same as dryness. Bleached hair lost absorbed water about an hour faster than untreated hair under infrared microscopy Fuse 2022. Damaged hair takes water in and loses it faster.
• The most reliable self-assessment signal is history: bleach, lightening, relaxers and perms, frequent hot tools, long sun exposure and how long the ends have been on your head.

What porosity physically is

• Where water goes. Water enters mainly through the cell membrane complex between cells, the endocuticle and the protein matrix around the keratin filaments, and it is not spread evenly inside the fiber Kamath 2020. Internal lipids help organize water when hair is wet Ishihara 2021.
• The surface barrier. The cuticle surface is coated with 18-MEA, a fatty acid chemically bonded to the cuticle protein. It makes virgin hair water-repellent and slippery. Alkaline treatments strip it, making the surface water-attracting Jones and Rivett 1997. Without it, wet hairs stick together, tangle and dry in a disordered, frizzy arrangement Tanamachi 2010, and friction rises Weiand 2025.
• Swelling. Bleached hair took up more water and swelled more at alkaline pH, and less at pH 3 Malinauskyte 2020.

How labs measure porosity

None of these is available at home. The consumer-scale proxies are water behavior on the strand and treatment history.

What raises porosity

• Oxidative bleaching and lightening: the strongest and fastest effect Hessefort 2008, Habe 2025. It also breaks bonds and strips 18-MEA Fuse 2022, Wiesche 2011.
• Chemical straightening and perming: acid straightening with a flat iron (about 180 °C) denatured cortex filaments, and on bleached hair it tripled total porosity Lima 2023.
• UV and pollution: UV raised porosity within hours in lab tests and particulate pollution amplified it Yang 2024. Simulated three months of Brazilian summer sun damaged the cuticle and lowered elongation at break Ramos 2026. See UV and hair.
• Heat drying: rapid blow-drying from soaking wet created splits within cuticle layers Okamoto 2003 and pores in the medulla Nagase 2002; both make hair look whitish and less shiny.
• Washing and weathering: surfactants can extract proteins Kaushik 2022 (industry), and cumulative grooming abrades the cuticle from root to tip Thibaut 2010.
• An unhealthy scalp: in a 24-week randomized trial in people with dandruff, hair grown while the scalp was treated with zinc pyrithione had better surface water-repellency than hair grown on a placebo-treated scalp Schwartz 2018 (industry-funded).

Differences between hair types

Sources: Oliver 2020, Marti 2016, Cruz 2013, Daniels 2023. These are small lab samples of pooled hair from three broad groups, and damage history shifts porosity far more than ancestry does. The peer-reviewed work does not support the popular idea that tightly coiled hair is naturally high porosity, although coils do face more mechanical wear (see curl pattern and hair by ancestry).

Why the float test fails

The float test says a strand that floats is low porosity and one that sinks is high porosity. In reality, surface tension can hold a thin, water-repellent fiber at the surface, and damaged hair that has lost its 18-MEA layer wets more easily. Air trapped in coils, residue (oil keeps hair afloat; conditioner and gel make it sink), whether the strand is pushed under and how long you wait all change the result. Informal home experiments and chemists' explainers reach the same view: the test is unreliable. At best it hints at surface damage on perfectly clean hair. It should not be used as a porosity test.

Porosity, dryness and drying time

• At the fiber level, water left bleached hair about an hour sooner than untreated hair Fuse 2022. Damaged hair is more water-attracting and can hold more water at high humidity.
• At the head level, drying time is dominated by water held between fibers, which depends on length, density, curl and thickness. We found no study linking whole-head drying time to porosity, so "low porosity hair takes forever to dry" is unsupported. Thick, dense, long or coily hair dries slowly regardless.
• Damaged, water-attracting hair tangles and mats when wet and dries into a disordered, frizzy arrangement Tanamachi 2010. People experience this as dry, frizzy hair even though it may hold more water.

What it means for your hair

Coily hair accumulates mechanical surface damage faster, so its ends are often high porosity even if the fibers are not innately so. Mixed porosity on one head is normal: roots are usually lower and ends higher.

Myths and facts

• "Healthy hair doesn't absorb water." All hair absorbs water and swells.
• "The float test tells you your porosity." It is unvalidated and swayed by surface tension, residue, curl shape and technique.
• "Porosity is genetic and fixed." Fiber type changes water behavior slightly Oliver 2020, but bleaching, chemical treatment, UV and weathering dominate Hessefort 2008, Yang 2024, Thibaut 2010. Each new centimetre of growth starts low.
• "High porosity hair is always dry; low porosity holds moisture longer." Damaged fibers take up and release water faster Fuse 2022.
• "Cuticles open and close like doors." Alkaline products swell the fiber and lift cuticle edges Gavazzoni Dias 2015. Acidic pH did reduce water uptake in bleached hair Malinauskyte 2020, but "closing the cuticle" with cold water has no direct evidence here.

How to judge your own porosity

Use several signals together; history is the strongest.

Describe the result as low, normal or high, or mixed (roots lower, ends higher), and weight treatment history most. See how to work out your hair type at home.

Bottom line

Porosity is mostly a damage marker, not a hair type you are born with. Judge it from your treatment history, protect the ends, and ignore the float test. The lab evidence is consistent, but no at-home test has been validated against it.