Hair colour and hair treatment

Five articles on hair colour and hair treatment: the chemistry, the numbering convention, the allergy warnings, and what the cancer studies actually found.

Each entry gives the belief in a reader's own words and the short answer the article defends. The evidence status links to the rubric that defines it.

The claim: “A 7.4 is the same copper whichever brand you buy — the numbers are a standard code.”
The short answer: Nobody owns the chart. The level/tone convention is industry practice, not a standard — no ISO, no annex, no legal definition — so the tone digits genuinely differ between manufacturers.; There Is No Standard Behind the Hair-Colour Number Chart · Settled

The claim: “Purple shampoo strips the yellow out of bleached blonde hair.”
The short answer: Lightening does not remove warmth, it exposes it. A violet shampoo deposits a direct dye that sits on the fibre and washes out; it never removes the yellow underneath. Only further lifting or an oxidative toner changes the pigment itself.; Purple Shampoo Does Not Remove Brass. It Covers It. · Settled

The claim: “If hair dye brings me out in a rash, a PPD-free box is the safe one.”
The short answer: Probably not. The EU files toluene-2,5-diamine — the usual PPD substitute — under the same phenylenediamine warning block, at the same 2% free-base cap after mixing. The regulation's own labelling choice contradicts the marketing.; “PPD-Free” Is Not the Same as Hypoallergenic · Settled

The claim: “Hair dye causes cancer. The WHO classified it.”
The short answer: Four different studies asked four different questions and headlines fuse them. IARC classes working as a hairdresser as Group 2A and personal use of hair colourants as Group 3, not classifiable. The straightener finding is not a dye finding.; Hair Dye and Cancer: Four Findings Usually Quoted as One · Contested

The claim: “A reconstruction treatment or an argan-oil mask repairs damaged hair.”
The short answer: No. Cuticle loss and cortical protein loss do not grow back. An oil can lubricate, reduce swelling on wetting and lay a surface film — real effects, all of them cosmetic and temporary. “Repair” and “nutrition” are marketing words, not regulated ones.; Argan Oil Does Not Repair Hair — and “Repair” Is Not a Regulated Word · Contested

Why five of ten

Half of this site is about hair because half of what this address was ever about is hair. The Brazilian blog that published here from 2008 to 2016 wrote mostly about hair colour, and the addresses that still carry links today are, overwhelmingly, its posts on de-yellowing bleached blonde and on copper and red shades. Those links now arrive at articles that answer the same underlying question with a document attached rather than an account of somebody's bathroom. About explains the change of hands, and the corrections log records which addresses were forwarded where.

The other five articles are at the article index: three on reading an ingredient label, two on how the Brazilian, American and European regimes work.