Hair care is another category where understanding the physical structure clears up most of the confusion, because the fibre is simple and the claims are not.

The structure

A hair fibre has an outer layer of overlapping scales called the cuticle, and an inner core called the cortex.

The cuticle protects and determines shine. When the scales lie flat, light reflects evenly. When they lift, hair looks dull and tangles.

The cortex carries the strength, the pigment and the structure that determines curl.

Damage means cuticle scales lifting or being lost, exposing the cortex. Once that happens the fibre cannot be repaired biologically, since hair above the scalp is not living tissue.

Which is why repair claims need reading carefully

Nothing restores damaged hair to its original state.

What products can do is smooth the cuticle temporarily, fill gaps with conditioning agents, and coat the fibre so it behaves better and looks better.

Some bond-building treatments do form new links within the fibre, which is a genuine chemistry rather than a coating, and the effect is real and partial.

The honest framing is management rather than repair, and the products that describe themselves that way are being straight with you.

Surfactants

Shampoo cleans by using molecules with a water-attracting end and an oil-attracting end, which lift oils into water so they rinse away.

Stronger surfactants clean more thoroughly and strip more, which is why they are appropriate for oily scalps and not for dry hair.

The sulphate-free category substitutes milder surfactants, which foam less and clean less aggressively.

Less foam is not less clean. Foam is a property of the surfactant, not a measure of cleaning.

Conditioner

Works largely through positively charged compounds that bind to hair, which carries a slight negative charge, particularly where damaged.

Which means conditioner deposits preferentially where damage is greatest, and it is why the ends benefit more than the roots.

Silicones smooth the cuticle and provide slip, which reduces mechanical damage from combing — a genuine protective function, not just cosmetic.

Some silicones build up and require a clarifying wash to remove, and others rinse out readily, which is the distinction behind most of the argument about them.

Porosity

How readily the fibre takes up and releases moisture, determined largely by cuticle condition.

High porosity hair absorbs quickly and loses it quickly, which is characteristic of damaged or chemically treated hair.

Low porosity hair resists absorption, which means products sit on the surface and it takes longer to wet through.

The concept is useful for choosing product weight, and the home tests circulating for measuring it are not reliable.

Heat

The main source of cumulative damage in most routines.

Water inside the fibre expands rapidly when heated, and above a certain temperature the internal structure is permanently altered.

Heat protectants work by distributing heat more evenly and by slowing the transfer, which reduces damage rather than preventing it.

Using lower temperatures matters more than any product, and most styling tools run considerably hotter than necessary.

The scalp

Increasingly treated as skin, which is correct and overdue.

Flaking has multiple causes — a yeast that most people carry, dryness, contact dermatitis, and conditions requiring medical treatment.

Which means a single anti-dandruff approach addresses only some of them, and persistent problems warrant a doctor rather than another product.

Growth claims

The area where scepticism is most warranted.

Hair grows from the follicle, and topical products face the question of whether anything reaches it in an active form.

A small number of compounds have clinical evidence for hair loss and are regulated as medicines.

Cosmetic products may improve the appearance and condition of existing hair, which can look like growth, and that is a different claim.

Anyone experiencing genuine hair loss should see a doctor, since several causes are treatable and some indicate other conditions.

Curl and why it behaves differently

The shape of the follicle determines the shape of the fibre, and a curved follicle produces a curved fibre.

Which means the cuticle scales cannot lie as flat along a curved fibre, so curly hair reflects less light and is more prone to moisture loss.

It also means natural scalp oils travel down a straight fibre easily and a curled one poorly, which is why curly hair is drier at the ends.

Which explains most of the differences in recommended routines — less frequent washing, heavier conditioning, and minimal mechanical disruption.

Combing and mechanical damage

The most underestimated source of damage in most routines.

Wet hair is at its weakest, since water swells the fibre and the internal bonds are temporarily disrupted.

Which means combing wet hair aggressively causes more breakage than almost anything else.

Working from the ends upward, with conditioner still in and a wide-toothed comb, reduces it substantially.

Colouring

Permanent colour works by lifting the cuticle, removing or altering existing pigment, and depositing new pigment in the cortex.

Which is inherently damaging, since the cuticle is disrupted every time.

Semi-permanent colour deposits on the surface without lifting, which is far less damaging and washes out.

Bleaching removes pigment and does the most structural damage, and hair that has been bleached repeatedly has lost integrity that no product restores.