What happens to your hair when it is lightened

What is going on under the foils

You are in the chair with foils in, and from the outside nothing much seems to be happening. Inside each strand, a chemical reaction is changing your hair for good. Knowing what that reaction does makes the rest of the appointment make sense: why your stylist asks so many questions first, why toning comes after, and why some hair needs more than one visit.

Start with where color lives. A hair has an outer layer of overlapping cuticle scales, which a 2015 review in the International Journal of Trichology compares to shingles on a roof, and an inner cortex. A 2024 microscopy study notes that keratin, the protein hair is made of, is colorless on its own, so your natural shade comes from melanin granules packed inside the cortex. Lightening is the process of breaking those granules down.

Step one: the alkaline mix opens the way

Lightener is not one ingredient. A 2018 paper on bleaching chemistry describes cosmetic bleaching as hydrogen peroxide and persulfate salts working under alkaline conditions, with ammonia or monoethanolamine supplying the alkalinity. Each part has a job.

The alkaline part gets the mix inside. The trichology review explains that hair is sensitive to changes in pH and that alkaline solutions swell the fiber and open the cuticle scales. The same review notes that alkaline chemical treatments strip a thin natural lipid layer from the surface, the layer that normally helps hair repel water.

That is the first reason lightened hair behaves differently afterward. Its surface has changed before its color has.

Step two: melanin breaks down and warmth shows

Once inside, the peroxide and persulfate oxidize the melanin. The 2024 study describes the reaction as irreversible: the pigment is broken down, not stored somewhere to come back later. A study of progressively bleached hair found extensive breakdown of melanin granules even after the mildest bleach treatment it tested.

The color does not fade in a straight line from dark to light. The 2024 study notes that as the color-carrying parts of the pigment are destroyed, brown hair moves from red toward yellow. Manufacturer education pages describe the same thing from the chair: everyone's hair has warmth within it, and when the natural color is removed, the underlying gold or red tones become more visible.

That warmth is normal, and it is why toning follows lightening. A toner deposits the opposite color on the wheel to neutralize it, as what toner does explains. It is also why a blonde drifts warm again as toner fades, which why blonde hair turns brassy walks through cause by cause.

What else the lightener reaches

Peroxide is not selective. The bleaching study above concluded that it quickly reaches the cortex and causes oxidative damage across the fiber in addition to the wanted loss of melanin. It oxidized the hair's proteins, in particular converting disulfide bonds, the sulfur links that help give hair its strength, into cysteic acid. The amount of protein that leached out of the hair rose as bleaching became more severe.

Under the microscope, the 2024 study found that excessively bleached hair had brittle, torn cuticle scales, places where the cuticle had separated from the cortex, and small pores left behind where melanin granules had dissolved. Untreated hair in the same study had a clean surface with intact scales.

None of this makes lightening reckless. It makes lightening a trade, and the size of the trade depends on how far the hair goes and how carefully it is done. Some lighteners and add-ins are sold with bond-protecting claims; what the evidence says about them is in purple shampoo vs bond builder vs hair mask. The Salon 11:11 booking menu also offers an Olaplex Moisture Treatment as an add-on to blonding services, which it describes as an intensive hydration treatment.

Porosity goes up

Put a lifted cuticle, a stripped lipid layer and new pores together and you get hair that is more porous: it takes in water and product faster and lets them go faster. The trichology review states that bleached hair is more porous than virgin hair and is penetrated more rapidly by conditioners, dyes and anything else applied to it. A 2008 lab study that measured pores directly found that oxidative bleach increased the number of pores in hair within the first minute of bleaching.

You notice this as hair that tangles more easily, drinks up conditioner and lets toner wash out sooner. It also means lightened hair takes color faster on the next visit, which your stylist plans around. What porosity is, and how to care for hair that has a lot of it, is in what is hair porosity.

Why your history and condition set the limit

Every lightening session adds to the last, so what your hair has already been through matters as much as where it is now. The American Academy of Dermatology suggests choosing color within about three shades of your natural shade, and notes that lightening more than three shades requires higher volumes of peroxide, which causes more hair damage. The 2024 study adds that repeated bleaching further reduces the hair's natural lipids.

Colorist education describes a strand test as the way to see how much lift the hair can take and how its elasticity and porosity feel before the full service. At Salon 11:11, every color service starts with a consultation, where a strand test is something to discuss, especially before a big lift or when your history is unclear. Justice's own consultation covers your hair goals, your color history and how much upkeep works for you, and the blonding consultation post shows how that conversation goes. These are the things worth telling her:

• Any box dye, henna or salon color in the last few years, even if you are not sure what it was.
• Previous lightening, including old highlights that have grown down into your lengths.
• Keratin, smoothing or relaxing treatments, and perms.
• How your ends feel now: stretchy when wet, rough, or snapping when you brush.
• How light you want to go and how often you want to come back. If you are still deciding, should you go blonde is the honest fit check.