Heat can crack and fray hair by destroying the proteins in hair. The vibrancy will fade, tone will change, there will be a loss of pigment, it will lose shine, and hair will become dull. Heat damage will also cause split ends and breakage.
Straightening and curling irons can go 360-450°F or 185-230°C. Water boils at 212°F or 100°C. Hair can hold 30% of its weight in water. When there is excessive heat (like above water's boiling point) applied to hair that is still wet, the water will burst out into steam and greatly damage the cuticle.
Benefits of using heat: easier manipulation, can set styles with better results, and efficiency.
How to use heat safely:
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use a diffuser for indirect heat
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lower the heat setting
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use a hair dryer with excellent temperature control that does not go above 212°F or 100°C
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squeeze out all water by towel drying before using heat
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use the hover diffuse method
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use a heat protectant (like products with PVP/DMAPA acrylates copolymer, quaternium 70, hydrolyzed wheat protein, and especially silicones)
Unless you are using temperatures above water's boiling point (212°F or 100°C), heat damage is less of an issue
Before using irons feel if hair is still wet. If hair is cold when you touch it, it is still wet. Use irons on dry hair only.
If you have fine hair, use lower heat, heat protectant, and products with protein in them to strengthen hair