Heat styling: what it does to hair

Hair holds its shape through bonds within its keratin structure, plus water bound inside the fibre. Heat temporarily breaks hydrogen bonds, which is precisely why styling works: the shape resets as the hair cools and reforms them.

Push the temperature higher and the damage stops being temporary. Above roughly 150 to 180 degrees Celsius, keratin begins to degrade structurally. The cuticle cracks and lifts, the fibre loses protein and moisture, and none of that reverses.

Applying a hot iron to hair that is still damp can flash the trapped water into steam inside the shaft. The steam expands, forming cavities along the fibre. Under a microscope it looks like bubbles in the strand, which is where the name comes from.

Hair damaged this way becomes brittle and snaps easily, and there is no fixing it. This is the single strongest argument for making sure hair is genuinely dry before a straightener touches it.

Most irons go far hotter than anyone needs. Start low and increase only if the style genuinely will not hold.

• Fine, light-coloured, bleached or already damaged hair: stay at the low end, around 150 degrees Celsius or below.
• Normal, healthy hair: around 170 to 180 is usually sufficient.
• Coarse, thick, resistant hair: may need higher, but treat 200 as a ceiling rather than a default.
• Never at maximum simply because it is faster.

One pass at a moderate temperature does less harm than three passes at a low one. Slow, deliberate sections beat repeated attempts.

They form a film that slows heat transfer and distributes it more evenly, and many contain silicones or polymers that also reduce friction as the iron moves.

They reduce damage. They do not eliminate it, and they are not a licence to raise the temperature. Applied to damp hair they also need to be dried through before ironing, for the reason above.

1. 01Rough dry to at least 80 percent before any hot tool, and fully dry before a straightener.
2. 02Use a protectant and let it dry through.
3. 03Set the lowest temperature that actually holds the style.
4. 04Work in clean sections, one steady pass each, rather than repeatedly going over the same piece.
5. 05Keep the dryer moving and at a distance rather than held on one spot.
6. 06Build in heat-free days. Consecutive daily styling is where cumulative damage compounds.
7. 07Oil the mid-lengths and ends on non-styling days to reduce friction generally.

• Ends that feel rough, dry and straw-like regardless of conditioning.
• Curls or waves that no longer form properly in sections you style most.
• Hair that stretches a long way and then snaps rather than springing back.
• White dots along the shaft where the cuticle has worn through.
• Increasing frizz that conditioner does not settle.

Heat-damaged hair cannot be restored, only managed and eventually cut. Recognising it early is what limits how much you lose.