Healthy wet hair can stretch up to 30% of its original length without breaking.

The elasticity of human hair is primarily determined by the cortex, which makes up about 80% of the hair shaft. This layer contains long chains of keratin proteins held together by chemical bonds. When hair is wet, water molecules enter the cortex and temporarily break hydrogen bonds, allowing the keratin fibers to slide past each other and stretch further than they could when dry.Research published in the International Journal of Cosmetic Science highlights that healthy hair can extend 30% beyond its resting length when wet, whereas dry hair typically breaks after stretching only 20%. This physical property is a direct result of the alpha-helix structure of keratin. These coiled proteins act like microscopic springs that can uncoil under tension and recoil once the pressure is released.The balance between moisture and protein is critical for maintaining this structural integrity. Excessive chemical treatments, such as bleaching or high-heat styling above 350°F (177°C), can permanently damage the disulfide bonds that provide hair with its strength. When these bonds are compromised, the hair loses its ability to return to its original shape, leading to brittleness or a gummy texture.Trichologists use the elasticity test to diagnose the internal condition of the hair fiber. A study by the Society of Cosmetic Chemists notes that hair with low elasticity is often porous, meaning the protective outer cuticle is raised or missing. Maintaining a pH balance between 4.5 and 5.5 is essential for keeping the cuticle closed and preserving the natural elasticity of the inner cortex.