UV and sun damage to hair

Key takeaways

• Sunlight bleaches and weakens hair, but the exact mechanism is still debated. A 2006 review concluded that how different wavelengths change hair structure is poorly understood, and that effects on different hair types are controversial, partly because melanin is poorly understood. Nogueira 2006
• Free radicals are involved. In white hair, UV produced sulfur and carbon radicals, and under wet conditions UVA produced hydroxyl radicals. Copper ions boosted hydroxyl radical formation when the hair was wet. Millington 2020 The authors work at Procter & Gamble with an Australian collaborator.
• Light hair loses colour more. UVA and visible light faded colour only in the blond hair among the tested tresses, which the authors attribute to its low content of the photostable pigment eumelanin. The same study found UVA and visible light increased combing work in Afro-textured hair, suggesting cuticle damage. Bloch 2019
• Antioxidant pretreatment shows lab promise, not proven benefit. N-acetylcysteine cut free-radical signals in white hair under UV but did not stop yellowing in the photoyellowing test. Millington 2020
• UV protection claims on hair products are mostly lab claims. An experimental lipid-carrier cream reduced protein and colour loss under UV and heat in vitro, and vitamin E gave a slightly greater protective effect. Prasertpol 2020
• Coloured or bleached hair, with less natural melanin or more oxidised protein, is the most plausible to suffer (reasoning, not directly tested in these studies).

The science

Melanin absorbs UV and is destroyed in the process, which is why sun lightens dark hair. In melanin-poor hair, UV acts on keratin, especially cystine, tryptophan and cuticle lipid. Millington found that radical chemistry differs between dry and wet hair, so a sunny beach day with wet hair differs from dry sun. Only about 0.3% of terrestrial sunlight is UVB, so many lab UVB studies are an imperfect model of real sunlight. Millington 2020

What it means for your hair

• Blond, gray, white and bleached hair is the most photosensitive and the most prone to colour fading. Bloch 2019
• Dark, high-eumelanin hair is more photostable but still gets drier.
• Curly and coily hair showed higher combing work after UVA and visible exposure in the Bloch study.
• Coloured hair fades in the sun (see bleach and colour damage).

Myths and facts

• Myth: hair SPF sprays are as proven as skin sunscreens. We found no human or standardised data.
• Fact: hats and covering hair are the most reliable physical protection (common sense, not tested in these studies).
• Fact: sun lightening is not harmless "highlights". It comes with oxidative changes.

Bottom line

Wear a hat or scarf in strong sun, especially with bleached, coloured or light hair. Rinse chlorinated or salty water out after sun (see swimming and chlorine). Leave-in conditioners may add some surface protection, but treat claims as unproven.