Different chemistries have been designed to turn curly hair straight, but in all cases questions remain about their safety, finds Rachel Brazil
Consumer groups are concerned about recent epidemiological studies showing correlations between use of hair relaxers and a higher risk of both breast and uterine cancer. The cosmetics industry insists their products are safe, and all chemical ingredients undergo rigorous testing, but in the US litigation is now under way to try and prove otherwise.
The chemical key to the traditional relaxing processes is breaking disulfide bonds in hair. Hair is made from layers of the protein keratin, with an outer protective fish scale-like cuticle, surrounding an inner cortex of keratin filaments, which are held together by disulfide bridges between cysteine amino acids.
Breaking those bonds to permanently straighten hair needs a high pH, achieved using sodium hydroxide, which in the first relaxers was carried in an emulsion of water, petroleum jelly, mineral oil and emulsifiers. Straightener kits commonly containing up to 2.5% sodium hydroxide – with a pH of about 13. The hydroxide ions swell the cuticle to reach the cortex, where they break disulfide bonds, leading to a new bond between a cysteine residue and a dehydrated serine residue known as a lanthionate – and the process itself known as lanthionisation. After drying, heating and neutralising, the hair will be straight.