The Science of Hair Structure: Why It Matters for Your Hair Health

Have you ever wondered why your hair behaves the way it does—or why certain products and treatments work (or don’t)? The answer lies deep within the structure of your hair.

Understanding the science of your hair can empower you to make better choices for your hair care routine, prevent damage, and finally get the results you’re looking for—whether that’s more definition, less frizz, or longer-lasting color. So let’s dive into the 3 layers of the hair and the bonds that hold it all together.

The 3 Layers of Your Hair Strand

1. The Cuticle Layer: Your Hair's Protective Shield The outermost layer of your hair is called the cuticle, and it's made up of clear, overlapping cells that resemble shingles on a roof. This layer acts like a doorway between the outside world and the inner part of your hair.

The quantity of cuticle layers you have determines how easily moisture, products, and treatments enter (or exit) your hair. Some people have fewer cuticle layers—this is often called fine, high porosity hair, which absorbs water quickly but can lose it just as fast. Others have more densely packed cuticle layers—coarse, low porosity hair—which resists water and product absorption, often making moisture retention a challenge (but not impossible).

2. The Cortex: Where Color, Strength, and Moisture Live Beneath the cuticle is the cortex, the thickest part of the hair strand. This is where your melanin lives—the pigment that gives your hair its natural color. When you color your hair, this is also where artificial pigment is stored.

The cortex also stores water and provides strength and elasticity. Think of it as the powerhouse of your hair—it does the heavy lifting.

3. The Medulla: The Core (That Not Everyone Has) At the very center of the hair strand is the medulla. Not everyone has this layer—people with very fine and/or very light blonde hair often don’t. The medulla houses atomic-level bonds that give hair its internal structure and shape. While the medulla isn’t essential for the hair to exist, it plays an important role in hair integrity and how your hair responds to certain treatments. Regardless on if you have a medulla layer or not, you still have all of the following bonds in your hair.

The Bonds That Hold Your Hair Together

Inside the cortex and medulla are bonds—tiny chemical structures that give your hair its strength, shape, and ability to be styled. Let’s break them down:

1. Hydrogen Bonds Hydrogen bonds are the most flexible and forgiving. They break and reform easily.

• Broken by: Water or heat

• Reformed by: Drying or cooling to room temperature

• Examples: Braiding wet hair for waves, roller sets, flat ironing, blowouts

These bonds allow for temporary styling changes. As long as you’re gentle, hydrogen bonds can be broken and reconnected again and again without causing long-term damage.

2. Salt Bonds Salt bonds are pH-sensitive and help maintain your hair's internal stability.

• Broken by: Shifts in pH (like hard water or harsh shampoos)

• Reformed by: Rebalancing pH (with pH-appropriate products)

• Issues with imbalance: Dullness, frizz, color fading, dry or oily scalp

Since your hair thrives at a pH of around 5.5, using products outside this range can lead to problems you may not realize are chemistry-based.

3. Disulfide Bonds These are the big ones—the bonds that define your hair’s natural shape. They are strong, but if broken, the damage is permanent.

• Broken by: Chemical processes (perms, relaxers), bleach, extreme heat

• Cannot be naturally reformed

Disulfide bonds are what hold the structure of your curl pattern, which is determined by the shape of your follicle. When you chemically relax or perm your hair, you’re softening and reshaping these bonds. If done incorrectly or carelessly, it can result in severe damage or breakage.

Ever seen a viral video where someone’s curl falls off with the curling iron? That’s a disulfide bond failure caused by heat.

Bleach can also melt disulfide bonds. It’s designed to remove melanin from the cortex, but once that job is done, it keeps going—consuming protein and bonds if left too long. That’s how hair can literally melt off during color services.

Can Disulfide Bond Damage Be Reversed?

Nope—but it can be supported.

There are now products like Olaplex and K18 that help reinforce weakened disulfide bonds by temporarily filling in cracks or providing structural support. However, let’s be clear: these do not reverse damage. They help prevent further damage and support hair health when used consistently, but they are not a permanent fix.

Why This All Matters

Knowing how your hair is built—layer by layer and bond by bond—empowers you to:

• Choose better products

• Avoid unnecessary damage

• Get better, longer-lasting results from your routine

• Protect your natural curl pattern

• Communicate clearly with your stylist

Whether you’re considering a color change, a new styling routine, or just want healthier curls, understanding your hair’s science gives you the power to make smart, sustainable choices.