## 4 Conclusions  
This study investigated the physicochemical effects of PAW on human hair fibers with different morphologies, focusing on the comparative impact of two formulations: nitrate-rich PAW-N, generated Via a coaxial dielectric barrier discharge system, and peroxide-rich PAW-P, produced using a pin-to-water reactor. The results demonstrated that PAW-N, characterized by high nitrate concentration, low pH, and elevated ionic strength, induced significant oxidative damage throughout the hair structure, including disulfide bond cleavage, lipid peroxidation, protein denaturation, and pronounced cuticle fragmentation--effects particularly severe in porous hair types such as wavy (Type 2) and coily/kinky (Type 4). In contrast, PAW-P, defined by high hydrogen peroxide concentration and lower ionic content, resulted in milder oxidative interactions that preserved cuticle architecture, maintained keratin backbone integrity, and minimized moisture and lipid loss. Straight hair (Type 1), with its compact cuticle alignment and lower porosity, exhibited greater structural resilience under both treatments, especially when exposed to PAW-P. FTIR spectroscopy corroborated these findings through changes in cysteine oxidation (1034 cm\({}^{-1}\)), hydrogen bonding (3400-3200 cm\({}^{-1}\)), and lipid-associated vibrations (2925-2850 cm\({}^{-1}\)), while SEM imaging confirmed surface-level degradation patterns. UV-Vis spectroscopy further revealed differential RONS depletion and byproduct formation, reinforcing the role of hair morphology in modulating chemical reactivity with PAW. Collectively, these findings underscore the importance of selecting PAW formulations according to hair type and treatment goals. While PAW-N may be suitable for applications requiring stronger oxidative action, its high reactivity poses a greater risk to fiber integrity. PAW-P, by contrast, offers a safer and more controlled alternative for cosmetic or routine use, particularly for porous and sensitive hair types. This work provides a mechanistic foundation for the rational development of plasma-based hair care strategies. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app15158573/s1](https://www.mdpi.com/article/10.3390/app15158573/s1)]([https://www.mdpi.com/article/10.3390/app15158573/s1](https://www.mdpi.com/article/10.3390/app15158573/s1)), Figure S1. Image of the natural hair samples used in the study: (a) Sample 1--Wavy hair (Type 2); (b) Sample 2--Straight hair (Type 1); (c)Sample 3--Straight hair (Type 1) with high alignment; (d) Sample 4--Coily/kinky hair (Type 4), Figure S2. SEM and EDS analysis of untreated wavy hair (Sample 1--Control): (a) SEM image; (b) SEM-EDS layered image; (c) EDS spectrum; (d-i) Elemental mapping for C, O, S, Ca, Mg, and Au, Figure S3. SEM and EDS analysis of wavy hair treated with nitrate-rich PAW (Sample 1--PAW-N): (a) SEM image; (b) SEM-EDS overlay; (c) EDS spectrum; (d-i) Elemental mapping for C, O, S, Ca, Mg, and Au, Figure S4. SEM and EDS analysis of wavy hair treated with peroxide-rich PAW (Sample 1--PAW-P): (a) SEM image; (b) SEM-EDS overlay; (c) EDS spectrum; (d-i) Elemental mapping for C, O, S, Ca, Mg, and Au, Figure S5. SEM and EDS analysis of untreated straight hair (Sample 2--Type 1): (a) SEM image; (b) EDS layered image; (c) EDS spectrum; (d-i) Elemental maps for C, O, S, Ca, Mg, and Au, Figure S6. SEM and EDS analysis of Sample 2 after PAW-N treatment: (a) SEM image; (b) EDS overlay; (c) EDS spectrum; (d-i) Elemental maps for C, O, S, Ca, Mg, and Au, Figure S7. SEM and EDS analysis of Sample 2 after PAW-P treatment: (a) SEM image; (b) EDS overlay; (c) EDS spectrum; (d-i) Elemental maps for C, O, S, Ca, Mg, and Au, Figure S8. SEM and EDS analysis of untreated straight hair (Sample 3--highly aligned strands): (a) SEM image; (b) EDS layered image; (c) EDS spectrum; (d-i) Elemental maps for C, O, S, Al, Mg, and Au, Figure S9.