Hair Care Guide 10 min read

Healthy Hair Starts at the Root: The Complete Hair & Scalp Guide

Hair is more than what you see in the mirror. Its strength, shine and movement depend on a tiny internal structure, the chemical bonds that hold it together and the scalp it grows from. Drawing on the COSRX R&D Center's hair science series, this guide explains how hair works, what damages it and how to build a routine that repairs it from within.

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By COSRX R&D Center

Key ingredient: PEPTIDE-132™ (keratin-bonding peptide)
Best for: Damaged, coloured & heat-styled hair
Top benefits: Strength · Shine · Less breakage
Your routine: Shampoo → Treatment → Oil serum

Anatomy of hair: three layers and the glue between them

Every strand grows from a follicle in the scalp, but the part we style and brush, the hair shaft, is made of non-living, keratin-packed cells. Seen in cross-section, it is built in three layers, each with its own job. Understanding them is the first step to choosing care that actually works.

The structure of a hair strand

01

Cuticle: the shield

Thin, overlapping cells arranged like fish scales and coated with a natural lipid layer called 18-MEA. When the scales lie flat, hair feels smooth and reflects light; heat, humidity and pH changes make them lift and swell.

02

Cortex: the strength

The thickest layer and the core of the shaft. Twisted bundles of keratin fibres, held in a moisture-retaining matrix, decide strength, elasticity and bounce. Damage here means breakage and stiffness.

03

Medulla: the core

A soft, partly air-filled centre. It may be missing in fine hair and its role is still not fully understood, but it affects how perm and colour chemicals travel through the strand.

+

Cell membrane complex: the glue

An invisible network of ceramides, fatty acids and cholesterol that binds all the layers together and provides the channels through which moisture moves.

What hair is made of

Like skin, hair stays healthy only when its building blocks are in balance. Protein does most of the work, but small amounts of water and lipids make the difference between hair that bends and hair that snaps.

Hair by composition

65–95%

Protein (keratin)

Gives hair its shape and elasticity. Keratin-associated proteins (KAP) lock the fibres together for stability.

10–15%

Moisture

The water content of healthy hair, keeping it flexible and soft instead of brittle.

1–8%

Lipids

Reduce friction, prevent moisture loss and act as the natural adhesive between the hair layers.

~3%

Melanin

Determines hair colour and offers some protection from UV light. Trace minerals and vitamins reflect your nutrition.

What wears these building blocks down?

Colouring and perming, hot tools, UV exposure, ageing, frequent shampooing and changes in humidity all slowly strip protein, lipids and moisture from the strand. Because whole proteins are too large to enter the hair, effective repair delivers small peptides and amino acids that can reach the gaps inside, while lipids and humectants seal and smooth the surface.

The four bonds that hold keratin together

Keratin chains are linked by four kinds of chemical bonds. Think of a ladder: some bonds are the rails, others the rungs. How much damage your hair can take, and how it reacts to water, heat and chemicals, depends on which bonds are affected.

Strongest

Peptide bond

Links amino acids into long keratin chains, the rails of the ladder. Only severe, repeated heat or mechanical stress breaks it.

Strong

Disulfide bond

Connects chains side by side and sets your hair's shape and elasticity. Perms and relaxers break and re-form these bonds; once lost, hair stretches, snaps and won't hold a style.

Weak

Hydrogen bond

Breaks with water and heat and re-forms as hair dries. It explains humid-day frizz and why a blow-dry can temporarily change your hair's shape.

Weak

Ionic bond

Forms between oppositely charged amino acids, like magnets. pH swings from dyes and bleach disrupt it.

Sources: Journal of Oleo Science, 2003; Journal of Investigative Dermatology Symposium Proceedings, 2007; International Journal of Cosmetic Science, 2014.

How your hair type is decided

Whether hair grows straight or coily is decided before birth. The shape and angle of each follicle are set by your genes, and so is the number of follicles, which does not change after birth. What changes over time is how thick each strand is and how many strands are actively growing; hormones can also slightly reshape the follicle.

Four hair types

Straight

Vertical follicle

Nearly round cross-section. Hair grows smooth, and scalp oil travels easily down the strand, so roots can look oily sooner.

Wavy

Slightly tilted follicle

Oval cross-section and a soft S-shaped wave. It loves light hydration that won't weigh the wave down.

Curly

Angled follicle

Flatter cross-section and defined curls. Sebum struggles to reach the ends, so lengths need extra moisture and lipids.

Coily

Strongly tilted follicle

Very flat cross-section and tight coils with many bends where breakage starts. Prioritise gentle handling and rich conditioning.

The hair growth cycle

Each follicle works on its own timetable, repeating a four-phase cycle about 25 times in a lifetime. Inside the follicle, dermal papilla cells send growth signals and nutrients, matrix cells divide to produce keratin, and melanocytes add colour.

The four phases

2–6 years

Anagen · growth

85–90% of your hair. Cells multiply quickly and the strand grows longer and gains its colour. Typically 3–5 years for men and 4–6 years for women.

2–3 weeks

Catagen · transition

About 1% of hair. The follicle shrinks, separates from the dermal papilla and pigment production stops.

2–4 months

Telogen · rest

11–13% of hair. The follicle pauses and the strand waits to be released.

Shedding

Exogen · release

A new hair growing underneath pushes the old one out. This is a normal, healthy part of the cycle.

How much shedding is normal?

Losing up to about 100 hairs a day is normal. If you consistently notice more than that, in the shower, on your pillow or in your brush, your scalp needs attention, and a dermatologist can help find the cause. Shedding in autumn, after a summer of sun, heat and sweat, is also common and usually temporary.

Your scalp is skin, but not like your face

It's tempting to treat the scalp like facial skin, yet it behaves very differently. Its density of follicles, oil glands and nerves means it needs care designed for it.

Scalp vs. facial skin

100,000–150,000

Hair follicles

Packed into roughly 650–700 cm² of scalp, far more than anywhere on the face.

17 vs 28 days

Faster renewal

Scalp cells turn over in about 17 days versus about 28 on the face, so build-up forms faster and cleansing matters more.

Most oil glands

Sebum hotspot

The scalp has one of the highest concentrations of sebaceous glands in the body, so oil balance is key.

Microbiome

A living ecosystem

Microbes such as Malassezia, Propionibacterium and Staphylococcus live here naturally. Problems begin when their balance is disturbed.

The scalp also has more nerves and blood vessels than facial skin, which is why it can sting, itch or feel tight so easily.

What is your scalp type?

A healthy scalp is mildly acidic (pH 5–6), holds around 15–20% moisture, has a thin, even layer of sebum, sheds cells normally and hosts a balanced microbiome. When one of these slips, a distinct scalp type appears. Find yours below.

Balanced

Normal scalp

Comfortable, with no tightness or flakes, and hair stays fresh for a day or two. Keep it that way with a gentle, pH-friendly shampoo.

Dry

Too little sebum

Tightness, fine white flakes and itching. Left alone it can turn sensitive. Choose moisturising, gentle cleansers and avoid very hot water.

Oily

Too much sebum

Hair clumps and looks greasy soon after washing (hyperseborrhoea). Excess oil can clog follicles and lead to inflammation. Cleanse regularly with a lightweight formula.

Sensitive

Easily irritated

Stinging, burning, redness or tightness, whatever your oil level, because the scalp is rich in nerves. Choose soothing, fragrance-light products.

Dandruff

Abnormal flaking

Affects over half of adults worldwide. Oily scalps shed large, yellowish, greasy flakes linked to Malassezia; dry scalps shed small white flakes. Often comes with itching and odour.

Hair loss

Shedding & thinning

Shedding, thinning or weakening strands. Some loss is seasonal, but persistent loss deserves a dermatologist's assessment.

Why your scalp changes

Your scalp isn't fixed: it shifts with hormones, seasons and lifestyle. These are the main triggers, inside and out.

▶ Sebum imbalance

Hormonal changes raise or lower oil production. Too little and the scalp dries out; too much and the oil oxidises, feeds unwanted microbes and hardens into plugs. Problem scalps also lose barrier-supporting lipids while inflammatory lipids build up.

▶ Microbial imbalance

Malassezia yeast feeds on sebum. On a weakened scalp it multiplies, and its by-products trigger the abnormal flaking we call dandruff. Microbes rarely start the problem, but they speed it up.

▶ Nutrition

Vitamins reach follicles through the blood or directly from products: niacinamide (B3) strengthens the barrier and fights oxidative stress, panthenol (B5) stimulates follicle cells, biotin (B7) is essential for keratin, and vitamin E neutralises lipid peroxides.

▶ Pollution

Hair and scalp can hold 1.5–2 times more impurities than the surrounding air. Fine dust slips past the scalp barrier, causes oxidative stress in follicles and leads to itching, dullness and shedding. Strengthen the scalp's defences rather than simply washing more often.

▶ UV exposure

The scalp is the skin closest to the sun. Repeated UV exposure creates reactive oxygen that ages scalp cells and follicles and can bring on early shedding. Cover up on bright days and use antioxidant-rich care.

▶ Seasons

Summer heat increases sweat and sebum, and poor airflow under the hair encourages microbes. Cold, dry winter air pulls moisture out, so dandruff and flaky scalps often flare. Use lighter, purifying products in summer and more hydrating ones in winter.

▶ Stress, hormones and headwear

Stress and hormonal shifts, such as after childbirth, can unsettle the scalp. Constant pressure and trapped heat from hijabs, caps and hats add stress too. Let your scalp breathe when you can, and make sure your hair is completely dry before covering it.

Meet PEPTIDE-132™: bond care that works from within

Most conditioners only smooth the surface. COSRX set out to repair the structure inside. Using AI-driven molecular modelling, the COSRX R&D Center designed PEPTIDE-132, a patented tripeptide made of three amino acids chosen for how they interact with hair.

Because it is small enough to pass through the cuticle, PEPTIDE-132 reaches the cortex and binds to weakened keratin like a puzzle piece filling the gaps, reinforcing the strand from within while the rest of the formula smooths the surface.

384.9 Da

Small enough to enter

Below the 500-dalton threshold, so it can pass the cuticle and reach the cortex.

+11.1%

Stronger strands

Increase in tensile strength measured on artificially damaged hair.

Fewer breaks

Brushing test

Significantly fewer broken strands than the water-treated control after 8–16 hours of repeated brushing.

Cortex

Proven penetration

Lab cross-sections show the peptide entering and accumulating inside the hair cortex.

Results from COSRX in-house laboratory tests on hair samples.

Your 3-step bonding routine

The PEPTIDE-132 line is designed to be layered, so each step builds on the one before.

Step 1 · Cleanse with Peptide 132 Ultra Bonding Shampoo

Massage into a wet scalp with your fingertips (not your nails) for one to two minutes, let the lather run through the lengths, then rinse well. With PEPTIDE-132, collagen and 18 amino acids, it cleanses and soothes the scalp while strengthening damaged hair.

Step 2 · Repair with Peptide 132 Bonding Treatment

Squeeze out excess water and apply from mid-lengths to ends, avoiding the scalp. Leave on for a few minutes, then rinse. It repairs broken bonds and restores softness, shine and elasticity to heat- or colour-damaged hair. Dry or very damaged hair can use more treatment than shampoo.

Step 3 · Seal with Peptide 132 Bonding Oil Serum

Warm one or two pumps between your palms and smooth over damp or dry lengths and ends. Vitamin E and seven botanical oils seal split ends, tame frizz and add a glossy shine without residue. Use it before heat styling, or to refresh ends during the day.

Bonding Shampoo Bonding Treatment

Bonding Oil Serum Get the 3-step kit

Everyday habits for healthier hair and scalp

Wash the scalp, not the lengths. Focus shampoo where oil and build-up form; the rinse cleans the lengths.
Use lukewarm water. Hot water strips lipids from the scalp and cuticle; finish with a cooler rinse.
Dry your scalp completely. A damp scalp under a hijab or cap is an ideal home for microbes. Blow-dry roots on a cool or medium setting.
Turn the heat down. Use the lowest effective temperature on straighteners and curlers, and apply a protective oil first.
Detangle gently. Start at the ends with a wide-tooth comb and work upwards, especially when hair is wet and most fragile.
Space out chemical services. Colour, bleach and perms break disulfide and ionic bonds, so give hair time and bond care between appointments.
Protect from the sun. Cover your hair and scalp during long hours outdoors, especially in summer.
Adjust with the seasons. Lighter, purifying care in hot months; extra hydration in cold, dry months.
Eat for your hair. Enough protein, iron, B vitamins and vitamin E supports keratin production from within.

Q. How often should I wash my hair?

A. It depends on your scalp. Oily scalps usually feel best washed daily or every other day; dry or sensitive scalps may prefer every two to three days. Always wash after heavy sweating.

Q. Is it normal to lose hair every day?

A. Yes. Up to about 100 hairs a day is part of the normal growth cycle. If you consistently lose more, it's worth checking with a dermatologist.

Q. Can I use bond-repair products on coloured or bleached hair?

A. Yes, that's exactly what they are for. Chemical services break internal bonds, and PEPTIDE-132 helps reinforce keratin from within.

Q. Should I apply the treatment to my scalp?

A. No. Shampoo is for the scalp; apply the treatment and oil serum from mid-lengths to ends so roots don't feel weighed down.

Q. Does wearing a hijab cause hair loss?

A. A hijab on its own is not the cause, but constant tight pulling, trapped heat and covering damp hair can stress the scalp and strands. Keep styles loose, make sure hair is fully dry and let your scalp breathe at home.

Q. When should I see a dermatologist?

A. If you notice sudden or patchy hair loss, persistent itching, pain, redness or thick scaling, please consult a dermatologist.

References

[Reference] COSRX R&D Center hair science series (2025–2026); Journal of Oleo Science, 2003; Journal of Investigative Dermatology Symposium Proceedings, 2007; International Journal of Cosmetic Science, 2014.

Written with the COSRX R&D Center. This guide is for general hair and scalp care education and is not medical advice. For persistent hair loss or scalp conditions, please consult a dermatologist.

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