Heat damage: What flat ironing does to your hair
In a groundbreaking study on how heatstyling affects tightly curled hair, scientists measured the difference in curl before and after flat ironing at 220°C (428°F) and 185°C (365°F).
The first article in this series breaks everything down, from how they chose the curl types to how they worked out the safe temperature to straighten curly hair. Here's a quick recap: The researchers found that after flat ironing 50 times at 185°C, curls sprang back to their original selves once washed and airdried. At 220°C, that didn't always happen. In fact, several of the tight curls transformed into bigger curls and even loose waves that were almost straight. 
That overall loss of curl indicated the hair was damaged by the higher heat level. Whereas at 185°C, curl reversion - the technical term for when your curls come back looking the same after heat styling - was pretty much reassured.
But they weren't done. Heat damage: What temperature is safe to flat iron your hair?
Lack of curl reversion is a strong indication that hair is damaged. But curl reversion doesn't mean that your hair is not damaged. Remember, even out of the tresses straightened at 220°C, there were some strands that did revert to their natural curl.
To check for actual damage, the researchers had to use some standard laboratory tests. They took 50 strands from each of the hair tresses they had flat ironed. First they used a tensile apparatus, which applied stress onto the strands until they broke. The break stress - the amount of pulling it took for the strand to break - was used to determine whether the hair's tensile strength had changed. If it had, that meant the hair was damaged. 
For the hair flat ironed at 185°C, nothing had changed. The hair fibres didn't break any easier than before straightening and they hadn't lost any of their original stiffness.
But the tensile testing showed that the hair straightened at 220°C had lost its natural stiffness and started to break more easily. Its break stress was less than half of hair that hadn't been straightened. 
Then they broke out the DSC machine - a gadget that scans and analyses heat to detect changes in proteins.
Just like the curl loss and poor tensile strength indicated, the DSC analysis showed that the internal structure of the hair ironed at 220°C was damaged. The equipment revealed that there were fewer bonds in the hair’s molecular framework. The internal structure of the hair flat ironed at 185°C, though? It was unaffected. Its internal structure was intact. Why hair straightened at higher temperatures gets heat damaged
When hair is exposed to high temperatures, this causes the hair protein to permanently change shape or denature, like what happens when you cook an egg.
If the heat level gets high enough, it starts to break apart the disulphide bonds and the alpha keratin backbone that give your hair its internal structure. This doesn't happen at 185°C because there isn't enough energy to break those bonds. Why do looser curls mean your hair is heat damaged?
The disulphide bonds that hold your hair together and give it its natural structure are also responsible for something else - shape memory.
When hair that has been safely heat-straightened gets wet, it returns to its natural curl pattern - that's because the water activates its shape memory. 
That's what happened to the hair that was flat ironed at 185°C. Because it wasn't heat damaged, its ability to revert to its natural curl pattern stayed intact. As soon as it got wet, the shape memory kicked in and it curled right back up again.
Heat damaged hair has lost some of its internal structure, which takes away that shape memory. So when it gets wet, the strands are unable to revert to their original curl pattern. That's what happened to the hair that was straightened at 220°C. The structural damage from all that heat meant that many of its disulphide bonds were broken. 
Without those disulphide bonds - the same bonds that get broken by a relaxer when you chemically straighten your hair - the hair experienced an overall loss of curl.
This is what caused the "shift to lower curl types" or the change from tight curls to loose curls and waves which was observed by the research team. Avoid heat damage: turn down the heat
So the moral of the story? To avoid frazzled, permanently straightened, heat damaged hair, be cautious with the temperature when you flat iron. Resist the temptation to turn the dial up past 185°C. There are other, better ways to get your hair straighter and smoother.
And remember: the heat protection from staying at 185°C has only been confirmed up to 50 passes - cumulatively. Bear that in mind if you're a 2+ passes to straighten kinda girl. It all adds up even if you space hair straightening out. And if you colour your hair, you might want to keep the temperature even lower. Heat damage on dyed hair can change its colour. But what if you straightened your hair on max before you saw this post and now wetting won't bring your curls back? Check out this post for a checklist on whether you need to take drastic action next.
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