Ever experienced “dry” hair? Dryness is a common concern in the haircare world, leading many to avoid products and ingredients that are thought to cause dryness or reach for those that claim to moisturize or hydrate hair. 

But what does it really mean to “hydrate” hair? Let’s explore the science behind what hair hydration is, how to maintain the optimal amount of moisture for a healthy look and feel, and how damage affects hair’s moisture balance.

what hair hydration really means

Hydration is a popular topic in beauty. When it comes to skincare, hydration is all about maximizing and retaining a high water content. That’s why many of us use products like moisturizers and humectants designed to draw water into the skin and hold it there.

However, the opposite is true in haircare, where high water content is actually a sign of damage. 

To explain why, we need to first revisit the structure of hair.

The hair is a multilayered structure composed of predominantly keratin proteins. These layers include the outer cuticle, the inner cortex, and, in thicker hair, the innermost medulla. Cuticle cells look a lot like the tiles on a roof and lay flat when hair is healthy. The cells of the cortex (cortical cells) are made up of keratin protein chains that weave together to form larger and larger structures.

When completely dry, hair is up to 95 percent protein, with the remaining weight being internal and external lipids, pigments, and some other minor components. A complex system of chemical bonds stabilizes the keratin protein chains in hair. 

Individual links in hair’s keratin chains are joined by peptide bonds and cross-linked by disulfide bonds. Hydrogen bonds are a type of reversible chemical bond that occurs between keratin chains to maintain and hold a temporary shape. These hydrogen bonds are largely responsible for hair’s elasticity.

how water interacts with hair

In the hair science world, we think about the water content of hair being determined by the relative humidity of your environment, which is the amount of water vapor in the air. The water content of hair is dynamic and naturally fluctuates as the humidity of your environment changes—in scientific terms we call materials with this property “hygroscopic materials.”

As the water content of hair changes, so do its characteristics and properties.

When water enters hair, it makes hair’s structure more flexible and mobile, increasing the space between keratin proteins and causing both the cortex and cuticle to swell.

At low environmental humidity, the water content inside of hair is low, and hair has a smoother cuticle and higher elasticity but can also be less flexible making it more prone to breakage and split ends.

At high environmental humidity, hair loses elasticity and can become overly flexible and limp causing it to tangle and stretch when being combed or brushed which causes fatigue and damage. When water fills hair, it breaks the hydrogen bonds between keratin proteins, causing hair to appear frizzy, and lose its style.

The key to combating these changing conditions is maintaining balance. Healthy hair hydration is about maintaining an equilibrium between the inside and outside of hair, rather than simply adding water. 

how porosity determines water content

A key factor that helps determine how hair’s moisture content changes with humidity is hair’s porosity. Porosity is used to describe the ability of hair to absorb and retain moisture.

The overall damage level of the cortex and the condition of cuticle cells on the surface of hair determine porosity. Hair with an undamaged cortex and tightly packed cuticle cells cannot easily absorb or lose water. Hair with more damage and a compromised cuticle is more porous, meaning water can more easily enter and exit the hair strand.