the elasticity of differently treated relaxed hair under humid condi- tions, only high-MW peptide treated hair remained significantly stiffer. Effects of peptide treatments on hair breakage The summary of premature breakage (defined as hair breakage at extension ≤ 20% strain) at 20% and 80% RH is presented in Table 1. At 20% RH, 24% of virgin hair and 30% of relaxed control hair broke prematurely. At 80% RH, the premature breakage in all of the tested hair types was significantly reduced. However, mid- and high-MW peptide contribution to the reduction of pre- mature breakage of relaxed hair at both humidities appears to be stronger than that of other actives. Performed chi-squared test confirmed that observed breakage trends are associated with the applied treatments (the null hypothesis of the chi-squared test that no relationship exists between breakage and treatment was rejected). The break stress data at 20% and 80% RH are compared in Fig. 8. As expected, virgin hair was significantly stronger than relaxed hair at both humidities. The difference in virgin hair break stress at both humidities was insignificant. At 20% RH, the break stresses of mid- and high-MW keratin peptides treated hair were significantly higher than that of the relaxed hair control by ~18.6% and ~16.3%, respectively. The break stress of hair treated with leucine was significantly lower than the relaxed hair control. Whereas we have not found significant difference between virgin hair equilibrated at 20% and 80% RH, the humidity effect on Figure 5 Longitudinal cortex cracks of virgin and relaxed control hair. 6 © 2020 The Authors. International Journal of Cosmetic Science published by John Wiley & Sons Ltd on behalf of Society of Cosmetic Scientists and Societe Francaise de Cosmetologie International Journal of Cosmetic Science, 1–12 Larger hydrolyzed keratins reduce hair breakage E. Malinauskyte et al. (a) Virgin hair control (b) Relaxed hair control