A healthy strand of wet hair can stretch up to 30% of its original length without breaking.

Human hair is primarily composed of a tough protein called alpha-keratin, which is organized into a complex structure of cortical cells. These cells contain macrofibrils and microfibrils held together by various chemical bonds, including disulfide bridges and hydrogen bonds. When hair is dry, these bonds are stable and provide the hair with its structural rigidity and shape.When hair becomes wet, water molecules penetrate the hair shaft and disrupt the hydrogen bonds between the keratin chains. This process is known as plasticization, which temporarily increases the fiber's flexibility and allows the protein chains to slide past one another. According to trichology studies, a healthy hair fiber can extend by roughly 30% of its length in this state without sustaining permanent structural damage.The elasticity of hair is a primary metric used by stylists and dermatologists to assess the integrity of the hair cuticle and cortex. If the hair is chemically damaged from excessive bleaching or heat styling, the protein structure becomes compromised. In these cases, the hair will either snap immediately when pulled or remain stretched out like a piece of chewed gum rather than snapping back to its original size.Research published in the Journal of Cosmetic Science highlights that the moisture content of hair can fluctuate based on environmental humidity, but full saturation provides the maximum possible stretch. This unique mechanical property is why experts recommend using a wide-tooth comb on wet hair. Because the hair is in its most vulnerable and elastic state, rough brushing can easily lead to over-stretching and breakage.