What is hair hydration: myths and reality How does water interact with hair?

The effect of water on the fiber.

Human hair is made up of 90-95% proteins, mainly keratin, with 3-10% water and 1-9% lipids. The water content significantly affects the hair's mechanical properties, such as elasticity and tensile strength. Water disrupts the hydrogen bonds and salt bridges that give hair its structure, leading to swelling and reduced mechanical properties. 

When hair absorbs water, it causes changes in the keratin structure, affecting the hair's shape. While increased moisture is generally not beneficial for most hair types, it can help highly curly hair by making it more manageable and easier to detangle. However, swelling and cuticle elevation can still occur.

Hair's internal water content influences its visual and mechanical characteristics. Low moisture results in a smoother cuticle and improved elasticity, but can increase breakage risks. Conversely, too much water can lead to excessive flexibility and tangling, causing damage to the hair fibers. Additionally, disruptions in hydrogen bonds can lead to frizz and styling challenges, while low moisture levels can lead to static electricity.

Written by Human hair : A unique physicochemical composite, J Am. Acad. Dermatol, 2003.