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Background: Hair-straightening treatments may involve the use of thermal devices, which potentially cause damages in the cuticle and cortex of the hair fibers. Particularly, the heat causes denaturation of the α-keratin and the degradation of the components of the hair cortex., Objectives: The aim of this study was to evaluate the effect of heating on the cuticle and cortex of the Caucasian and Asian virgin hair by using heat iron flat., Methods: The effects of the heating in the tresses of Caucasian and Asian virgin hair were analyzed by thermogravimetry/differential scanning calorimetry-Fourier transform infrared/mass spectrometry (TG/DSC-MS/FTIR), protein loss analysis (PLA), Fourier transform infrared (FTIR-ATR), and scanning electron microscope (SEM)., Results: TG/DSC-MS/FTIR results were: formation of the H 2 O between 25-170°C and CO 2 , SCO and H 2 S from 200°C. Denaturation temperature = 237°C. Protein loss: Asian hair>Caucasian hair. Data presented statistically significant alterations, α = 5, P ≤ 0.05, n = 3. FTIR-ATR: changes in the secondary structural conformation of the protein of hair cuticle. SEM: heat caused damage to hair cuticle., Conclusions: The results evidenced the importance of the control of temperature in the procedures involving heat. The damage caused by thermal devices showed be dependent of the ethnicity analyzed (Caucasian and Asian hair)., (© 2019 Wiley Periodicals, Inc.)