What It Means That Hair Is Dead
The living part of a hair is only the root, hidden in the follicle beneath the skin. Everything you see and touch is no longer alive and has no way of repairing itself.
So damage is permanent. Products can fill, smooth and coat the surface, which makes hair visibly and tangibly better, but they do not rebuild the structure in any literal sense.
Why this is good news
Because it points the effort in the right direction. Hair health is decided at the root and in how you treat it day to day, not in a product promising repair after the fact.
The Three Layers
A hair is built from three layers, one inside the next. On the outside is the cuticle, beneath it the cortex, and in the middle of thicker hairs sits the medulla.
Almost everything you do to your hair affects the first two layers. The cuticle determines how it looks; the cortex determines how strong it is.
The Cuticle: The Protective Sheath
The cuticle is made of flat cells arranged like roof tiles, pointing from root to tip. When they lie flat, the surface is smooth, reflects light and hair looks shiny.
When they lift, the surface turns rough, light scatters and hair looks dull. A lifted cuticle also catches on neighbouring strands, which is what produces tangling and a coarse feel.
What lifts it
An alkaline environment, heat, mechanical friction and chemical processes. Which is why hot water, brushing wet hair hard and frequent high-heat drying leave hair dull, even if you have never coloured it.
The Cortex: Where the Strength Is
The cortex makes up most of the strand and consists of keratin fibres bonded to one another. Those bonds determine strength and elasticity. There is more on the protein itself in our article on hydrolysed keratin.
This is also where the pigment that gives hair its colour sits. Lightening works precisely by opening the cuticle and breaking down pigment in the cortex, which explains why it is the most damaging process.
When the bonds in the cortex are disrupted, hair loses elasticity: it stretches and does not spring back, and breaks more easily when wet. That is damage no conditioner undoes.
The Medulla
The medulla is the hollow core and is present mainly in thicker hairs. In fine hair it is often absent entirely, which is not a problem.
For practical care it matters little: products do not reach it and do not affect its state. If you see a promise about acting on the medulla, treat it as advertising.
Porosity, and Why It Explains So Much
Porosity describes how easily hair takes in and gives up moisture, and depends mainly on the state of the cuticle. Low-porosity hair has a tightly closed cuticle: it is slow to wet and slow to absorb products.
High-porosity hair absorbs quickly but also loses quickly. It usually results from lightening, frequent heat or years of mechanical strain. It needs more frequent but lighter care rather than one heavy mask a month.
What Follows From All This
That prevention is worth more than repair. Lower drying heat, combing from the ends upward, a milder shampoo and fewer chemical processes do more than any repair product. What that looks like in practice is set out in our hair care routine.
And that growth is the only genuine way to replace hair. It grows roughly a centimetre a month, so badly damaged lengths are solved with a haircut rather than a product.
The Growth Cycle
Every follicle passes through three phases: active growth, a short transition, and rest, after which the hair falls and a new one begins to grow in its place.
The growth phase lasts years and the resting phase months. So at any given time some hairs shed daily, and that is not a cause for concern. The follicle stays in place and produces a new hair.
When a change matters
If shedding increases sharply and continues for months, if hair visibly thins in particular areas, or if the scalp shows where it did not before, that warrants an appointment rather than a change of shampoo.
Why Everyone Hair Differs
The shape of the follicle determines whether hair is straight, wavy or curly. A round follicle gives a straight strand, an oval one wavy, and a strongly flattened one curly.
That has a practical consequence. On curly hair, sebum from the scalp struggles to travel down the bends to the ends, so it is almost always drier along the lengths and needs more softening.
Wet Hair Is at Its Most Fragile
When a strand takes on water it stretches more easily and breaks under less force. That is exactly why combing straight out of the shower causes more breakage than the same motion on dry hair.
The practical consequence is simple: blot lightly with a towel rather than rubbing, and comb with a wide-tooth comb from the ends upward rather than from the roots down.
