Relaxers, perms and smoothing treatments

Key takeaways

• There are three different chemistries. Alkaline relaxers (sodium hydroxide "lye", or guanidine hydroxide "no-lye") straighten permanently. Thioglycolate perms and thio relaxers break the hair's disulfide bonds and re-form them in a new shape. "Keratin", "Brazilian" or "progressive" smoothers cross-link the hair with formaldehyde-type agents or glyoxylic acid and fade over months. See sodium hydroxide and guanidine relaxers, ammonium thioglycolate, formaldehyde and releasers and glyoxylic acid.
• All of them damage hair in the lab. In one swatch study, alkaline relaxers and thio each raised protein loss to about 2.5 ug/g versus 1.12 in untreated hair, and glyoxylic acid reached 3.5, with similar break force across products. Velasco 2022
• Scalp harm is documented. A review of 33 studies linked relaxers to scalp eczema, burns, inflammation, hair loss and shaft damage, and noted that long-term effects are unknown. Hatsbach de Paula 2022
• Cancer signals are observational and still debated. In the US Sister Study, using straighteners in the past 12 months was linked to uterine cancer (hazard ratio 1.80, and 2.55 for more than four times a year; 378 cases among 33,947 women). Chang 2022 The Black Women's Health Study (44,798 women) found no overall increase for heavy use (HR 1.18, CI 0.81 to 1.71) but higher risk in postmenopausal women (HR about 1.6-1.7). Bertrand 2023 A 2026 Sister Study analysis reported higher thyroid (HR 1.71) and pancreatic (HR 2.66) cancer with little dose response, and called for confirmation. Bailey 2026 These are association studies and cannot prove cause.
• "Formaldehyde-free" is not a guarantee. In a simulation of four professional smoothers, even products labelled formaldehyde-free had the potential to meet or exceed occupational limits, and bulk formaldehyde ranged from 0% to 11.5% by weight. Pierce 2011
• Glyoxylic acid is under EU review after kidney injury reports. French authorities reported four acute kidney failure cases in October 2024, and a 2026 European safety mandate asks whether up to 16% is safe in hair straighteners. A case report with mouse experiments linked glyoxylic acid straighteners to crystal-forming kidney damage. Robert 2024
• The ingredient mix matters. A 2026 scoping review of 21 studies found keratin treatments more often contain formaldehyde or releasers, while relaxers more often contain endocrine-disrupting chemicals and fragrance ingredients. Weaver 2026

The science

Perms and thio relaxers: thioglycolic acid reduces disulfide bonds and the neutraliser re-forms them. Raman analysis shows that high-temperature stress relaxation after reduction speeds up disconnection of disulfides, and the oxidation step raises cysteic acid in the cuticle and outer cortex. Kuzuhara 2016 Perm damage is partly reversible at the bond level but cumulative at the cuticle.

Alkaline relaxers: a pH above 12 swells the fibre and rearranges its cross-links. What is documented is protein loss and scalp injury (above).

Smoothing treatments: formaldehyde (as methylene glycol in water) cross-links keratin when heated, and the heat step is when gas is released. The US FDA states that heating releases formaldehyde gas, that air levels above 0.1 ppm can irritate eyes and airways, and that IARC classes formaldehyde as a human carcinogen. The FDA has issued warning letters to makers of smoothing products. FDA

What it means for your hair

• Curl 3C-4C: relaxers are most used here. Bleached or colour-treated hair should not be relaxed, and relaxer plus bleach is a common cause of breakage (practice knowledge, not trial-tested). Consider the cancer-association data above, which come from cohorts with many Black women and show the strongest signals in frequent users.
• Fine or low-density hair: perms and relaxers can make hair look thinner through breakage.
• High porosity: already damaged fibres take up chemicals faster, so do a strand test first.
• Pregnant or breastfeeding: avoid smoothing treatments as a precaution (no controlled data).

Myths and facts

• Myth: keratin treatments add keratin protein. The straightening effect comes from the cross-linkers.
• Myth: "no-lye" is gentler. Both are strong alkalis, and the swatch study found similar protein loss and break force for sodium and guanidine hydroxide. Velasco 2022
• Myth: glyoxylic acid is the safe formaldehyde alternative. See the kidney reports above.
• Fact: heat-free alternatives such as stretching, silk-press technique and wash-and-go avoid these chemistries.

Bottom line

If you relax, limit retouch frequency, keep product off your scalp, never relax over bleach and stop at any burning. Avoid heat-activated smoothing treatments, and if you do have one, ask for the full ingredient list and good ventilation. Stylists need ventilation and respiratory protection with smoothing products. Use bond builders as damage limitation only.