The short answer
Use the lowest setting that works, always use a heat protectant, and let hair dry to about 80 per cent naturally before any dryer touches it.
- Towel gently and let it air dry most of the way. The most damaging moment is a hot dryer on soaking hair.
- Use a heat protectant every time, on every tool.
- Lowest effective temperature. Straighteners at 180C do most jobs; 230C is rarely needed and is where the damage is.
- Keep the dryer moving and hold it a good distance away.
- One pass with a straightener, not four. Repeated passes at low heat still add up.
What will ruin it
Never use straighteners on wet or damp hair unless the tool is specifically designed for it. The water inside the hair shaft boils, and that damage is structural and permanent.
Why it works
Heat breaks the hydrogen bonds that hold hair's shape, which is how styling works. Above a certain temperature it also damages the keratin itself, and hair has no living cells and therefore no repair mechanism.
This is general information, not medical advice, and it does not replace a GP, a pharmacist or NHS 111. Call 999 in an emergency. Full disclaimer.