Flat Iron Heat: What 450°F Means
Where hair starts to change.
Some flat irons go as high as 450°F. That tells you how hot the tool goes, not what’s safe for your hair.
What Happens Inside the Hair
In a lab study, the protein inside hair melted at about 205°C (401°F) when hair was dry, and at about 155°C (311°F) when it was damp. 450°F is 232°C, past both. Heat protectant helps, but it doesn’t make high heat safe.
Heat on damp hair can leave tiny bubbles inside the strand, which makes it weak and brittle.
What to Do
- Protect before any heat.
- Using a flat or curling iron? Make sure your hair is fully dry first. If it hisses or steams, stop.
- Start low on the dial. Go up only if the style won’t hold.
- Fewer passes.
- Lightened or damaged hair? Stay at the low end of the dial.
Related: Breakage · Highlighted · Blow drying
Sources: Cao J, Leroy F. Biopolymers 2005;77(1):38-43 (re-checked 17 Oct 2026) · Detwiler SP et al. Journal of the American Academy of Dermatology 1994;30(1):54-60 (bubble hair; re-checked 17 Oct 2026) · Gummer CL. British Journal of Dermatology 1994;131(6):901-3 (checked 17 Oct 2026) · American Academy of Dermatology, Tips for healthy hair (updated 12 Aug 2024, checked 17 Oct 2026) · American Academy of Dermatology, 10 hair care habits that can damage your hair (no date on page; checked 17 Oct 2026)
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