Heat damage and heat protectants

Key takeaways

• Any hair will form heat damage if heated enough. In the classic bubble-hair experiment, the hair of every one of 16 volunteers developed bubbles when subjected to enough heat, and the bubbles contained gas. Detwiler 1994
• Bubble hair typically comes from heated tools on wet hair. A review of 11 case reports found most patients were middle-aged women who used a heated tool to dry or style wet hair, and the treatment was to stop. Do not flat-iron or curl damp hair. Li 2023
• Keratin degrades at high temperatures. In a Brazilian lab thermal-analysis study of cut virgin hair (no styling tool), water left the fibre between 25 and 170 C, sulfur-containing gases appeared from 200 C and the denaturation temperature was 237 C. Electron microscopy and spectroscopy showed protein loss that differed between Asian and Caucasian hair (3 tresses). Lima 2019
• Hair type changes who suffers. A comparison of Caucasian and Afro-textured hair found bleaching, dyeing and UVA or visible light raised combing work in Afro-textured hair, a sign of cuticle damage. Bloch 2019
• Evidence for heat protectants is thin. We found no human trial and few peer-reviewed lab comparisons of retail sprays. One 2020 academic lab study showed an experimental lipid-nanoparticle cream reduced protein loss and colour loss after UV and heat versus a cream base and commercial products, but it is not a retail product. Prasertpol 2020

The science

Water in the fibre vaporises first, so damp hair pressed with a hot iron can boil water inside the cortex, producing the bubbles of bubble hair. At higher temperatures the keratin itself denatures and releases decomposition gases, and heat disrupts the cuticle surface. Lima 2019

Bubble hair is an acquired shaft defect with air-filled cavities. It is diagnosed by microscopy and is reversible only by growing out the damaged length. Li 2023

Typical retail heat protectants contain silicones, polymers and conditioners that coat the fibre (see silicones). The idea is a lubricating film and slower heat conduction. We found no independent comparison of retail protectants, so do not assume a protectant lets you use 230 C safely.

What it means for your hair

• Fine hair heats through faster, so use lower settings.
• Coily or tightly curled hair (3C-4C) is fragile, so extra heat is risky. Stretching with low-heat methods is safer (reasoning).
• Bleached, relaxed or high-porosity hair has already lost cystine and protein, and heat on top compounds the damage.
• Low-porosity, undamaged hair tolerates occasional heat better.

Myths and facts

• Myth: a heat protectant makes high heat safe. Unproven.
• Myth: ceramic or tourmaline plates protect hair. We found no verified study; the claims are marketing.
• Fact: heat on wet hair is the clearest harm.
• Fact: hair responds to heat dose, which is temperature times time.

See also hair drying.

Bottom line

Dry hair almost fully (air or low heat) before ironing, use the lowest effective temperature with one or two passes, and keep tools moving. Replace an overheating dryer; Detwiler's case came from a faulty one. A protectant is fine if you like the slip it gives, but keep temperatures low anyway. Take heat breaks and trim burned ends (see split ends).