As with skin, hair is sensitive to UV rays. But how does sunlight affect hair? Continue reading to learn more.
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Sun exposure: what are the effects on hair?
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Published June 23, 2025,
updated on June 23, 2025,
by
Pauline, Chemical Engineer
—
6 min read
How does the sun affect hair?
Sun exposure can alter hair color in small ways. In summer, it is common to notice lighter highlights, even a subtle natural lightening that gives hair a luminous appearance. This phenomenon, sought for its aesthetic effect, suggests a certain summer lightness. Some people appreciate the sun’s effect on blonde, chestnut, or brown hair to achieve golden or copper highlights without dye. Yet this change is not trivial: it reflects chemical and structural alterations in hair fibers. As a reminder, hair consists of three cylindrical layers: the medulla, the innermost layer; the cortex, which provides fibers with elastic properties and contains the melanin that gives them their color; and the cuticle, the external protective layer, made of flat keratin cells arranged like tiles.
When hair is exposed to sunlight, the fiber absorbs UV radiation. Hair proteins such as keratin contain amino acids like tryptophan, tyrosine, phenylalanine, and cysteine, some of which are sensitive to UV rays. For example, tryptophan absorbs UVB at 280 nm and participates in photodegradation. When exposed to these rays, it converts to N-formylkynurenine, a yellow compound absorbing at 420 nm. This conversion is one of the markers used to assess sun-damaged hair. Under UV exposure, disulfide bonds—essential for fiber cohesion, strength, and shape (straight, wavy, curly, or coily hair)—break. This reduces mechanical resistance and makes hair brittle.
In addition to interacting with hair fiber proteins, sun rays trigger the formation of reactive oxygen species, which constitutes a second photodegradation pathway. Produced under UVA exposure, these free radicals attack amino acids and contribute to oxidation of natural melanin or artificial pigments in dyed hair. This process can cause color loss or even yellowing in blonde or white hair. Sun-yellowed hair shows considerable pigment degradation. White hair is vulnerable because it lacks melanin, which, beyond its visual role, has a protective function. It absorbs UV and neutralizes free radicals but degrades while doing so. This explains why hair tends to shift to a lighter shade during summer. Hair is prone to dryness. Sun exposure leads to hydration loss in curly or coily hair, which is naturally drier.
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Lipids account for a portion of hair weight.
A recent study examined the lipid content of hair fibers and how it changes after sun exposure. Lipids form a hydrophobic barrier that limits water loss, maintains cuticle cell cohesion, and contributes to hair shine. A reduction in these lipids can lead to hair appearing dry from sun exposure, a common issue during summer.
The study was conducted on ten blonde hair strands divided into two groups. One group was exposed to visible light and UV rays (340 nm and 420 nm), and the other was not. The irradiation conditions (36 °C, 85 % relative humidity, six cycles of ten hours) were chosen to simulate a typical summer exposure in Miami. Samples were washed before analysis to remove sebum residues. The method, lipidomics by liquid chromatography–mass spectrometry (LC–MS), enabled a broad comparative identification of hair lipids. Although this method is not quantitative, it allows comparing the relative abundance of different lipids between groups.
Of the 3,521 detected signals, 791 lipids across 32 distinct classes were identified. A comparative analysis of UV-exposed and unexposed hair revealed that more than half of the lipid classes were significantly altered by UV exposure. Among the classes most affected by UV exposure, several decreased in abundance, while others increased. The main results are shown in the table below. They suggest that UV-induced oxidation leads to selective degradation of the lipids constituting the hair fiber.
| Lipid class | Hair exposed to UV radiation | Hair not exposed to UV radiation |
|---|---|---|
| Fatty acid esters of vitamin A | 119 978 ± 5 190 | 186,925 ± 8,708 |
| Sterol esters | 3,356 ± 256 | 5 052 ± 820 |
| Diacylglycerol | 230,279 ± 13,289 | 316,483 ± 26,952 |
| Phosphatidylethanolamine | 23,805 ± 1,330 | 27 965 ± 1 305 |
| Acylcarnitine | 2,158 ± 324 | 393 ± 188 |
| Sphinganine | 2,447 ± 480 | 4,227 ± 902 |
Relative abundance (arbitrary units) of different lipid classes in hair fibers with and without UV exposure.
Source: Willicut RJ et al. Exposure of hair to UV and visible light causes widespread changes in the hair lipidome. International Journal of Cosmetic Science (2022).
How to protect hair from UV damage?
To prevent hair yellowing, loss of shine, and dehydration from sun exposure, we recommend protecting hair during the summer. For this, the use of a hat or a headscarf is an effective measure between 11 AM and 4 PM. If you prefer not to wear coverings, use protective hair products containing UV filters.
Beyond direct photoprotection, we recommend using a hair after-sun treatment on summer days. Often rich in plant oils, this treatment restores cuticle lipids, reinforces intercellular cohesion, and increases shine. Aligned cuticle scales reflect light, enhancing hair brightness. Most after-sun treatments include antioxidants to help limit UV-induced oxidation.
Sources
KROLO I. & al. UV damage of the hair. Collegium Antropologicum (2008).
VELASCO M. V. & al. Effects of solar radiation on hair and photoprotection. Journal of Photochemistry and Photobiology: Biology (2015).
WILLICUT R. J. & al. UV and visible light exposure to hair leads to widespread changes in the hair lipidome. International Journal of Cosmetic Science (2022).
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