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I’ve been straightening and curling my hair wet/damp for years, despite all the popcorn-sounding fizzes and crackles – but some science I’ve recently read have made me do a complete 180. It turns out wet and dry hair respond to extreme heat very differently – and wet hair cops it far worse, and not just because it’s more fragile.
First, a little refresher on hair anatomy. Hair has three main parts:
- the medulla – boring central core, actually non-existent in light and fine hair
- the cortex – middle layer, responsible for colour, texture and most of the strength of hair
- the cuticle – protective outermost layer, made of overlapping cells like roof shingles or fish scales, shiny
The cuticle lays flat, but water can get in between the gaps. When you wet healthy hair, it can actually absorb up to 30% of its own weight in water into the inner spongy cortex (more if it’s damaged hair).
An average hair straightener heats up to 185-230 °C. Curling irons are a little cooler, at 95-200 °C. They’re both well above the normal boiling point of water, which is 100 °C. (Temperatures in Fahrenheit are straighteners 365-446 °F, curlers 203-392 °F and boiling water 212 °F, for any readers from non-metric countries like the US, Liberia or Myanmar.)
What happens when the wet inside of hair gets heated well beyond its boiling point? Well, it’s not too dissimilar to popping corn…
When water turns from liquid to gas it expands. When it’s heated strongly it expands rapidly. Since it’s confined in the cortex by the cuticle, it has to bust out. That’s right – the water explosively evaporates, shattering whatever’s in its path, which happens to be… your hair.
(Hair does not usually look this spongy up close, in case you’re wondering – it should look like sliced playdough with a small spongy section right in the middle.)
It’s not only on the inside that the boiling water wreaks havoc – it’s got to get out somehow, and in the rush to leave it bangs up the outside of your hair cuticle pretty good too – instead of lying flat, the “shingles” or scales of your cuticle are now bumpy from being pummeled from the inside:
Don’t freak out too much – this probably happens mostly to the hair that’s right against the heating plates of your straighteners and curling irons (it’s doing the rest of your hair a solid, sacrificing itself for the greater good!). But if you’re doing this to your hair every day, the damage builds up.
Of course, dry hair doesn’t get away unscathed – it still causes microscopic cracks on the hair’s surface, denatures hair proteins, and decomposes pigment – but this happens in wet hair too. All this damage goes on to give you dull brittle hair, split ends, brassy colour, increased tangling and breakage.
What can you do? Well, obviously, you should straighten and curl your hair only when it’s dry! You can also invest in a heat protecting spray (most of these work, to some extent!).
M. Gamez-Garcia, Void and pore formation inside the hair cortex by a denaturation and super-contraction process occurring during hair setting with hot irons, J Cosmet Sci 2001, 62, 109-120.
S. B. Ruetsch & Y. K. Kamath, Effects of thermal treatments with a curling iron on hair fiber, J Cosmet Sci 2004, 55, 13-27.