A Critical Dimension of Hair Health
Hair porosity — the hair shaft’s capacity to absorb and retain moisture — is one of the most important yet most misunderstood dimensions of hair science. It affects how hair responds to humidity, how well products are absorbed, how hair holds moisture between wash days, and how vulnerable hair is to environmental damage. Yet it is invisible to the naked eye and entirely absent from common hair typing systems.
For CROWN’s research programme, porosity is a key measurement dimension. The CROWN Diagnostic uses impedance sensing to quantify porosity objectively, replacing the subjective “float test” and “spray test” that currently serve as the only available methods for most people.
The Science of Porosity
Hair porosity is determined by the condition and arrangement of the cuticle — the outermost layer of the hair shaft, composed of overlapping scales of hardened keratin protein.
Low porosity hair has tightly arranged cuticle scales that lie flat against the hair shaft. Moisture penetrates slowly, but once absorbed, is retained well. Low porosity hair may resist product absorption and take longer to become fully saturated when washed.
Medium (normal) porosity hair has cuticle scales that are moderately raised, allowing balanced moisture absorption and retention. This porosity level is generally associated with the easiest hair management and the widest range of product compatibility.
High porosity hair has cuticle scales that are significantly raised, damaged, or partially absent. Moisture enters the hair shaft quickly but is also lost rapidly. High porosity hair tends to frizz in humid conditions, absorbs products readily but may feel dry shortly after, and is more vulnerable to environmental damage.
What Determines Porosity
Porosity is influenced by both genetic and environmental factors:
Genetics. The baseline structure of the cuticle — the number of cuticle layers, their thickness, and their natural arrangement — is genetically determined. Different hair types have characteristic cuticle structures: research shows that Afro-textured hair tends to have fewer cuticle layers and a more uneven cuticle arrangement than straight hair, which can contribute to higher porosity.
Chemical treatment. Chemical relaxers, permanent dyes, bleaching agents, and keratin treatments alter cuticle structure, generally increasing porosity. The health risks of chemical straightening include damage to cuticle integrity, which permanently affects porosity until the treated hair grows out.
Heat damage. Repeated use of heat styling tools (flat irons, blow dryers at high temperatures) can damage the cuticle and increase porosity. The protein structure of hair is temperature-sensitive, and sustained heat application disrupts the bonds that maintain cuticle integrity.
Environmental exposure. UV radiation, chlorinated water, hard water minerals, and pollution all contribute to cuticle degradation over time, gradually increasing porosity.
Mechanical damage. Aggressive brushing, tight hairstyles, and friction from fabrics can abrade the cuticle, increasing porosity in affected areas. This is particularly relevant for Afro-textured hair, where the elliptical fibre shape creates natural stress points along the hair shaft.
Why Porosity Matters
Understanding porosity has practical implications for hair health and care:
Product selection. Low porosity hair benefits from lighter products that can penetrate the cuticle barrier; heavy oils and butters may sit on the surface without absorbing. High porosity hair benefits from heavier products and sealants that slow moisture loss.
Wash routine. Low porosity hair may need warm water to open the cuticle during washing and cool water to close it afterward. High porosity hair benefits from protein treatments that temporarily fill gaps in the cuticle.
Styling outcomes. Porosity affects how hair holds styles, responds to humidity, and interacts with styling products. Understanding porosity enables more effective and less damaging styling practices.
Damage assessment. Changes in porosity can indicate cumulative damage from chemical treatments, heat, or environmental exposure — serving as an early warning system for hair health.
Measuring Porosity
Current methods for assessing porosity are limited and subjective:
The float test. A strand of hair is placed in a glass of water; hair that floats is considered low porosity, hair that sinks quickly is considered high porosity. This test is unreliable — results are affected by product residue, air trapped in the cuticle, and water temperature.
The spray test. Water is sprayed onto a section of hair; how quickly the water is absorbed provides a rough porosity indication. This test is difficult to standardise.
The slide test. Running fingers along a hair strand from tip to root; roughness indicates raised cuticles and higher porosity. This test is entirely subjective.
None of these methods produces quantified, reproducible results suitable for research or clinical assessment.
The CROWN Diagnostic addresses this limitation through impedance sensing — measuring electrical impedance across a range of frequencies (100Hz–100kHz) to quantify moisture absorption rate and generate a reproducible porosity index. This objective measurement replaces subjective methods with data that can be tracked over time, compared across individuals, and aggregated into the CROWN Hair Commons for population-level research.
Porosity and Discrimination
Porosity connects to CROWN’s anti-discrimination mission in several ways. Hair with high porosity — whether from natural cuticle structure or from damage caused by conformity-driven chemical treatments — is more difficult to manage, more prone to frizz, and more likely to deviate from the smooth, sleek appearance that workplace grooming standards implicitly require.
Understanding porosity through objective measurement helps reframe “difficult” hair as hair with specific, quantifiable properties that require specific care — not as a deficiency requiring correction. This shift from subjective judgement to objective characterisation is central to CROWN’s mission: what can be measured can be understood, and what is understood can be properly valued.
The CDI research programme includes porosity as one of the physical hair dimensions correlated with discrimination experiences, enabling researchers to examine whether individuals with higher-porosity hair face more discrimination — and to quantify that relationship with precision.
