How Hot Is Too Hot When It Comes to Flat Irons?


Image: Imaxtree

If you prefer to tame your wild, unruly hair by flat ironing it into submission, there’s no need to risk the unwanted side effect of fried split ends. Contrary to popular belief, hotter isn’t better when it comes to hair straighteners. In fact, excessive heat can do more harm to your hair than good. 

“At 450 degrees, the inside part of the hair, called the cortex, melts and oxidizes resulting in weak, frizzy and difficult to style hair. Also, the color of the hair will change and become darker. Scientists call this process ‘denaturation,’ which occurs when you break the disulfide bonds in hair,” explains Dr. Tim Moore, a scientist and ghd’s chief technology officer.

But setting the flat iron at too low of a setting isn’t great, either. “In order to style hair, you need to heat the hair above the glass transition phase temperature of hair,” continues Moore. Above this temperature, hair becomes shapable, a bit like hot glass. And just like hot glass, when you cool hair back down again, it will maintain the shape. This process is completely safe and won’t damage hair. “However, if you flat iron below 365 degrees, the temperature is not high enough to be above the glass transition phase temperature, so the styling performance is very poor and you are just drying out the hair,” maintains Moore.

So what is the best temperature to flat iron your hair for optimum results, while keeping it healthy and shiny? Researchers at ghd’s research and development lab in Cambridge, England have discovered that 365 degrees is the ideal temperature for flat ironing. “365 degrees is above the glass transition phase temperature, so you get really good styling, but it is not too high to cause permanent damage to hair,” says Moore.

It is important, though, that your flat iron maintains the 365 degrees throughout the styling process.  Inexpensive plates don’t heat up evenly, which means that they smooth your hair in some areas and not in others. Look for ones that heat uniformly, so you can straighten an entire section of hair in just one pass ­— that way you’ll limit the heat exposure to your hair and won’t end up frying it. Also, if parts of the sectioned hair reach different temperatures, then the shine is reduced. “Thus, for the best shine and long lasting style, you need even temperature across the plates,” adds Moore.

To get the best styling benefit, you need to apply enough pressure to your hair to get good thermal contact between it and the plates, but not too much, which would cause you to pull your hair as you move the iron through it. “Pulling hot hair is bad for the hair and will reduce shine. You want to gently caress the hair with the plates, not pulling the hair, but still providing good thermal contact,” advises Moore.

So remember, a flat iron is not something to skimp on. Splurge on a high-quality styling iron — it’s worth every penny.


ghd Platinum Professional Styler, $249 at Sephora; Image: Courtesy of ghd