What the summer sun does to your hair

A woman with long dark hair at a New Zealand beach at golden hour, with the KiriCare keratin hair serum on a rail in the foreground
Kiri10â„¢ Wellness Desk

What the summer sun does to your hair, and why keratin matters

New Zealand's peak UV runs about 40% higher than comparable latitudes up north. Here is what that does to a strand, what helps, and the one step in our system that sits exactly where the sun hits.

What the research says

Four numbers worth knowing before the first beach day

40%higher peak UV index in NZ than at comparable latitudes in the Northern Hemisphere
14%loss of tensile strength in untreated hair after six days of simulated sunlight
UV-Bworks on the cuticle, the outer six to ten layers of the strand, first
UV-Agoes deeper and makes free radicals through tryptophan in the keratin

Sources: NIWA, UV and ozone; Molecules 2025, 30:1182; Sebetic et al., Annals of Dermatology 2010. Full list at the end.

Why the sun is harder on hair here

Three things stack up over a New Zealand summer. The ozone above us is thinner than it is over Europe and North America. Our air is cleaner, so less UV is scattered before it reaches the ground. And the Earth is closest to the sun in early January, which is the middle of our summer rather than the middle of a northern winter. NIWA puts the result plainly: peak UV index here is roughly 40% greater than at comparable latitudes in the Northern Hemisphere, and summer readings regularly pass 12 on a scale where 11 is already "extreme".

Skin gets the warnings. Hair gets the same light, for the same hours, with no sunscreen.

Seven things the sun does to a strand

Hair is about 85 to 95% keratin, a protein. Sunlight is an oxidiser. The research on what happens when the two meet is decades deep, and it is remarkably consistent.

Seven things the summer sun does to your hair, with sources

Save this one for the beach bag. The seven points, with their sources, are written out below.

  1. Our sun is stronger

    Thinner ozone, cleaner air and a closer sun put NZ's peak UV index about 40% above places at the same latitude in the Northern Hemisphere.

    NIWA, UV and ozone.

  2. UV-B hits the surface first

    UV-B cannot penetrate far, so its damage is concentrated in the cuticle, the overlapping outer layers of the strand. It breaks down cuticle proteins and the disulphide bonds that hold keratin together, and the outer layers roughen before the inside is touched. Protein loss has been measured after only 56 hours of UV-B exposure.

    Sebetic, Sjerobabski Masnec, Cavka, Biljan and Krolo, Annals of Dermatology 2010, "UV damage of the hair".

  3. UV-A goes deeper and makes free radicals

    UV-A reaches the cortex. Tryptophan, an amino acid in keratin, absorbs the light and passes the energy on, oxidising cystine and other amino acids throughout the fibre. The result is cysteic acid where there used to be a cross-link, which is why sun-worn hair loses its mechanical strength.

    Fernandez, Martinez-Teipel, Armengol, Barba and Coderch, Journal of Photochemistry and Photobiology B 2012.

  4. The glue between the layers oxidises

    Lipids such as linoleic acid sit between cuticle cells and hold them together. Under UV they peroxidise, producing hydroxyl radicals that attack the lipids around them and open tiny holes in the cuticle. Once the cuticle is open, light reaches the pigment and protein underneath more easily.

    Tokyo University of Technology, Cosmetics 2018, 5(2):24, summarised by TRI Princeton.

  5. Colour fades, and lighter hair loses more

    Melanin and hair dyes bleach in sunlight. Melanin also happens to be the hair's own sunscreen, so hair with less pigment, including blonde, grey and lightened hair, loses more protein for the same exposure.

    Sebetic et al. 2010; TRI Princeton, "Hair UV protection claim support".

  6. Strength drops

    In a 2025 study, hair tresses spent six days in a xenon aging chamber at 35°C to simulate summer sun. Untreated hair lost about 14% of its tensile strength, and the keratin's denaturation temperature shifted, a sign that the protein structure itself had changed.

    Molecules 2025, 30:1182, "Performance and mechanism of hydrolyzed keratin for hair photoaging prevention".

  7. A keratin film on the surface helped

    In the same study, hair coated with hydrolysed keratin before the exposure showed only slight surface damage afterwards and held its strength better. The researchers propose a simple mechanism: the keratin layer on the surface takes some of the UV energy itself, and as it breaks down it leaves smaller keratin fragments that penetrate into the fibre.

    Molecules 2025, 30:1182.

The honest bit

This is about the strand, not the root. The sun is not why hair falls out. It is why the hair you already have can feel dry and straw-like, snap in the brush, tangle and lose its shine by the end of summer. If your hair loss is sudden, patchy or severe, that is a different thing entirely, and worth a visit to your GP.

We also will not tell you that any shampoo, conditioner or serum is a sunscreen for hair. The studies above measured a protective effect from a hydrolysed keratin film in the lab. That is a reason to put keratin on the surface over summer. It is not an SPF.

The summer habits that actually help

  • Cover it. A hat or a scarf does for hair what sunscreen does for skin. Between eleven and three, shade is the single most effective step on this list.
  • Wet it with fresh water before the sea or the pool. Hair that is already full of fresh water takes up less salt and less chlorine.
  • Rinse straight after. Never let salt or chlorine dry in. The combination of residue plus sun is harder on the strand than either alone.
  • Wash gently, condition every single time. Summer is not the season to skip conditioner.
  • Put something on the ends. A leave-in on damp mid-lengths and ends is the layer that sits where the light does its work.
  • Go easy on heat. Sun-worn hair has fewer cross-links to spare. Let it air dry where you can.
The step that sits where the sun hits

KiriCareâ„¢ Softness & Shine Keratin Hair Serum

KiriCare Softness and Shine Keratin Hair Serum

The serum is the fourth step in the Kiri10â„¢ system and the only one that stays on your hair after you rinse. It is a lightweight, water-based leave-in for mid-lengths and ends, powered by the Triple-Keratin Complexâ„¢ with argan, jojoba, castor and grapeseed oils, Pro-Vitamin B5 and Vitamin E. It smooths frizz and flyaways, adds softness and shine, and supports stronger, more resilient hair without the heaviness of a traditional oil.

In summer it earns its place as the post-wash protector: two or three drops through damp ends after every swim, more if your hair is long. It is the keratin layer on the surface, where the research says the sun does its work.

Shop the serum, $44 Get it free with Inside & Out
Inside & Out

The serum is free in your first box

Commit to 90 days of KiriGlowâ„¢, the capsule that builds the protein from the inside, and your first box includes the full-size KiriCareâ„¢ shampoo, conditioner and serum, worth $148, at no charge. The inside builds the keratin. The outside looks after it in the sun.

  • 1 month of KiriGlowâ„¢ Daily Keratin Capsules, $83
  • KiriCareâ„¢ Keratin Shampoo, free
  • KiriCareâ„¢ Keratin Conditioner, free
  • KiriCareâ„¢ Keratin Hair Serum, free
  • 180-day growth guarantee
Start Inside & Out, $83 a month

$83 a month plus shipping, three month minimum. The free trio is only while stocks last. Most people start to notice real changes from 90 to 180 days onward, so this is a routine worth sticking with.

Questions, answered

Does the sun make hair fall out? No. UV works on the hair shaft, the part you can see, not on the follicle. It makes the strand drier, rougher and weaker. Shedding and thinning have other causes.

Is the serum a sunscreen for hair? No, and we will not call it one. It is a keratin leave-in that sits on the surface of the strand. The lab evidence for hydrolysed keratin films under UV is encouraging, and covering up is still the first line of defence.

Does dark hair get a free pass? Not quite. Melanin absorbs some of the UV, so darker hair loses less protein than lighter hair for the same exposure, but the lipids and cuticle proteins are still affected.

What about KiriGlow in summer? Keep taking it. The capsule builds keratin from the inside on its own timeline. The serum looks after what you have grown from the outside.

Sources

  1. NIWA, UV and ozone: "the peak UVI in NZ is approximately 40% greater than at comparable latitudes in the Northern Hemisphere". niwa.co.nz/atmosphere/uv-and-ozone. Read 28 October 2026.
  2. Stats NZ, UV intensity: data to 2023. Read 28 October 2026.
  3. Sebetic K, Sjerobabski Masnec I, Cavka V, Biljan D, Krolo I. UV damage of the hair. Annals of Dermatology 2010 (Photoaggravation of hair aging, PMC2938585).
  4. Fernandez E, Martinez-Teipel B, Armengol R, Barba C, Coderch L. Efficacy of antioxidants in human hair. Journal of Photochemistry and Photobiology B 2012; and Photodamage determination of human hair, same journal, 2011.
  5. Performance and mechanism of hydrolyzed keratin for hair photoaging prevention. Molecules 2025; 30:1182 (PMC11902160).
  6. Tokyo University of Technology, lipid oxidation in hair under UV, Cosmetics 2018; 5(2):24, as summarised in TRI Princeton, Hair UV protection claim support 101.

Product statements on this page are taken from the KiriCareâ„¢ and Inside & Out product pages. The studies cited measured laboratory effects on hair fibres and are not claims about Kiri10 products.