Scientists assess hair damage in straightening and colouring chemical treatments

The study, which was published in the Journal of Cosmetic Dermatology​​, looked at the protein loss in human hair caused by combined chemical treatments (dye and relaxer) using the validated bicinchoninic acid (BCA) method.

Hair chemical treatments, such as dyeing and straightening products, are known to cause damage that can be assessed by protein loss, so the team from Sao Paolo tested three kinds of straighteners, based on ammonium thioglycolate, guanidine hydroxide and sodium hydroxide, and evaluated them to indicate the least harmful combination.

“Among the straightening products tested, sodium hydroxide promoted highest protein loss,”​ say the researchers.

“These results indicate that when both types of treatments are desired, straightening products based on ammonium thioglycolate or guanidine hydroxide are the best alternatives.”​

Damage levels​

In the study, the team treated caucasian virgin dark brown hair tresses with developed natural brown colour oxidative hair dyeing and/or straightening commercial products based on ammonium thioglycolate, sodium hydroxide, or guanidine hydroxide.

Protein loss quantification was assessed by the validated BCA method which the researchers say has several advantages for quantifying protein loss in chemically treated hair.

When both the straightening and dyeing treatments were combined, the team found a higher negative effect occurred, particularly for dyed hair treated with sodium hydroxide.

In this case, a 356% increase in protein loss relative to virgin hair was observed and 208% in relation to only dyed hair.

The combination of dyeing and relaxers based on ammonium thioglycolate or guanidine hydroxide caused a small increase in protein loss, suggesting that these straightening products could be the best alternatives for individuals wishing to combine both treatments.