The Truth about Hair Porosity

Porosity is one of the most widely discussed, and misunderstood, concepts in hair care. You may have heard terms like low porosity, high porosity, or even seen floating “porosity tests” that suggest your hair type determines how porous your strands are. But the science behind porosity is more nuanced than that.

 I'm sharing the science of hair porosity, debunking common myths, and explaining what truly influences how your hair absorbs and retains moisture, including the crucial role of the F-layer, cuticle structure, hair type, and environmental or chemical damage.

 What Is Hair Porosity, Really?

 Porosity refers to the hair’s ability to absorb and retain water and substances. It’s not a fixed trait, it exists on a spectrum and is influenced by:

• The condition and number of cuticle layers
• Surface chemistry of the hair (such as the presence or absence of the F-layer)
• Cortex structure
• Hair fiber diameter and density

 While it’s often described as low, medium, or high, these are functional categories—not rigid definitions.

 The F-Layer: The Unsung Hero of 'Low' Porosity Hair

A key player in hair porosity is the F-layer (also known as the 18-MEA layer), a thin, lipid-based coating that naturally covers the outermost layer of the cuticle.

• Composed of 18-methyleicosanoic acid, this fatty acid makes virgin hair hydrophobic, meaning it repels water.
• It helps maintain the hair’s smoothness, reduce friction, and protect the hair from excessive moisture uptake or protein loss.

When the F-layer is intact, water and product absorb more slowly (than your hair at higher porosity), this is typical of true low porosity hair.

 When the F-layer is stripped (through chemical or mechanical damage), the hair becomes hydrophilic (water-attracting), and behaves as high porosity, even if the cuticle structure hasn’t been fully eroded.

Take, for example, an individual who undergoes a full scalp blonde colour every four weeks. Newly emerging hair begins as low porosity, with an intact cuticle and F-layer. However, repeated chemical processing gradually degrades this protective F-layer and compromises the cuticle and cortex, resulting in high porosity hair. This is why regrowth often feels oilier, it's not due to increased sebum production, but rather because the virgin hair is less absorbent. The scalp’s natural oils sit on the surface instead of being absorbed, unlike chemically treated hair, which is more porous and readily takes in these oils.

 Types of Hair Porosity Explained

 1. Low Porosity

• Cuticles are flat and tightly packed.
• F-layer is usually intact. (Depended on chemical, environmental, and mechanical disruption)
• Water beads on the surface or absorbs slowly. (hair diameter plays a role here)
• Can be naturally occurring in coarse or highly textured hair.
• May resist deep conditioning, color absorption, and prone to buildup.

 2. Medium (Normal) Porosity

• Cuticles are slightly raised but flexible.
• Good moisture balance, absorbs and retains water and products well.
• F-layer may be partially intact.
• Common in healthy, untreated hair. 

3. High Porosity

• Cuticle is raised, chipped, or missing sections.
• F-layer often degraded or absent.
• Absorbs moisture quickly but loses it just as fast.
• More prone to frizz, dryness, and breakage.
• Caused by chemical treatments, harsh products, heat styling, or UV/environmental damage.

Hair Type and Porosity, Do They Always Go Together?

 It’s a common myth that certain hair types (like textured or curly hair) are always low porosity. In reality, porosity is not dictated only by curl pattern or ethnicity, it’s based on hair structure and surface integrity.

 

That said, certain hair types tend to behave a certain way:

Hair Type	Typical Traits	Natural Porosity Tendency
Finer hair	Fewer cuticle layers, small diameter	May absorb water quickly even when undamaged due to fewer cuticle layers, even at lower porosity
Coarse/ thick/textured hair	More cuticle layers, compacted structure	Naturally lower porosity due to more cuticle layers, absorbs water slower compared to low porosity with less cuticle layers
Colour-treated or bleached hair	damaged cuticle and lost F-layer	High porosity regardless of original hair type
Aged or long hair (ends)	Exposed to repeated wear	Increased porosity over time

 

 Can Porosity Change Over Time? Absolutely.

 Hair that starts out with low porosity can become high porosity due to:

• Chemical processing (bleaching, relaxing, perming)
• Heat styling (flat irons, curling wands)
• Harsh shampoos or surfactants
• Environmental exposure (UV rays, wind, salt water, pollution)
• Mechanical damage (rough brushing, towel drying, tight styles)

Even if the hair’s cuticle appears intact under magnification, the loss of the F-layer can make the hair functionally more porous.

 

All Hair is Low Porosity Until Damaged, Myth or Fact?

 Some experts argue that all newly grown hair is low porosity, because it has intact cuticles and F-layer. While this is biologically true, it doesn’t account for:

• Structural variation in hair fiber diameter and cuticle layer count
• Genetic differences in hair surface properties
• The fact that even virgin fine hair can absorb water quickly due to its lower barrier 

So yes, all new hair technically has intact cuticles and F-layer, but that doesn’t mean it behaves like textbook “low porosity” in every individual.

Porosity is not binary. Two people with healthy virgin hair can have completely different porosity behaviors based on thickness, surface chemistry, and hair type. Example: Finer hair in zone 1 (new growth), that is virgin, low porosity will absorb moisture quicker than hair that's thicker in diameter with more cuticle layers, that's why float tests are not accurate as they can not be comparable in real life hair.

Signs of Each Porosity in Hair Type

 

Porosity Level	Fine/Low Density Hair	Thick/High Density Hair
Low porosity	Absorbs water slowly at roots, may feel slippery or coated, prone to buildup, but water may not bead visibly.	Water beads up on the surface, takes a long time to dry, very resistant to moisture penetration.
Medium porosity	Easy to hydrate, holds style well, light products absorb well.	Also absorbs well, can tolerate richer products, less prone to frizz.
High porosity	Quickly absorbs water, may feel rough or dry, tangles easily, drys fast.	Absorbs water instantly, feels rough, often lacks shine, frizzes, needs layering of moisture and sealing.

 

How to Support Your Hair Based on Porosity + Strand Type

Hair porosity isn’t consistent from root to tip. The roots are typically the newest and most protected, meaning they are lower in porosity, while the mid-lengths and ends have experienced more wear, exposure, and manipulation, and therefore tend to be higher in porosity.

 This means you often have to treat your hair in sections, based on both porosity and strand diameter (fine vs thick). Your hair's porosity is only comparable to your new hair to distal ends, you can't compare your porosity to someone else's. 

 

Low Porosity Hair Care

Fine Low Porosity Hair

 

• Lightweight products only—thicker products can weigh it down or cause buildup.
• Use occasional heat (steam or heat cap) to help deep treatments absorb.
• Clarify regularly to prevent product film.
• Scenario: Avoid oils near the scalp; instead, use a spray-on leave-in on damp hair and rinse with warm water to lift the cuticle slightly.

 

 Thick/Coarse Low Porosity Hair

 

• Heavier but emollient-rich treatments (e.g., shea butter-based masks) may be suitable if applied with heat.
• Focus on hydration and softening, not sealing.
• Scenario: Weekly deep condition with heat. Water beads on strands? Use a humectant pre-spray or leave-in before applying masks.

 

Medium Porosity Hair Care

 

Fine Medium Porosity Hair

 

• Maintains moisture well—balance hydration and protein to prevent becoming over-soft.
• Scenario: Use water-based leave-ins and lightweight oils on ends, and only use masks or protein treatments once every few weeks.

 

Thick Medium Porosity Hair

 

• Responds well to richer products and moderate protein.
• Scenario: Apply mid-weight masks once a week, avoid very heavy oils on the scalp, and layer a butter or oil on ends to seal in moisture.

 

High Porosity Hair Care

 

Fine High Porosity Hair

 

• Prone to breakage, tangling, and protein loss.
• Needs protein, hydration, and sealing—but lightweight!
• Scenario: Use a strengthening conditioner (with hydrolyzed protein) mid-shaft to ends. Mist roots only with leave-in. Finish with a drop of oil on ends.

 

Thick/Coarse High Porosity Hair

 

• Can tolerate heavier oils, masks, and sealing products.
• Needs both protein and moisture, ideally in alternating routines.
• Scenario: Use a leave-in cream on damp hair, layer with oil or butter on ends, and follow a bond-building or protein mask every 1–2 weeks.

 

Tips for Multi-Porosity Hair

 

It’s extremely common to have:

 

• Low porosity at roots
• Medium along mid-lengths
• High porosity at ends

 

This calls for a zoned approach:

 

• Use lighter products at the root to avoid suffocating the scalp and causing buildup.

• Apply mid-weight hydration and leave-ins through the mid-lengths.

• Use rich oils or sealing creams only on the ends, especially if they’re split or chemically treated.

 

Final Thoughts

 

Hair porosity is a dynamic characteristic, influenced by both genetics and external factors. It isn’t solely determined by your hair type, nor can it be tested reliably with DIY methods like the float test. Understanding the science behind cuticle structure, the F-layer, and surface behavior allows for better, more personalized hair care.