Getting Down to the Science of Hair

By Mary E. Leonard

It’s easy to think of the hair on our heads as an accessory, something that communicates identity or even social status. Our hair can powerfully influence how we feel about ourselves, and hairstyles have been used as a form of self-expression throughout history. In biological terms, hair is part of the integumentary system, which also includes our skin, associated glands, and nails.

The Physiology of Hair

A strand of hair has two parts—a shaft you can see above the skin and a follicle beneath the skin’s surface. The shaft is a flexible cylinder made of non-living cells that are filled with protein, primarily keratin. The living hair follicle beneath the skin widens at the base to form a hair bulb, which is the structure that actively produces hair.

Hair follicles are attached to small glands that secrete sebum. Sebum is an oily substance that forms a protective layer on the scalp, lubricating it and sealing in moisture. Sebum also transports fat-soluble vitamins like vitamin E to the skin’s surface and acts as an anti-fungal and anti-bacterial barrier. Sebum is a mixture of fatty molecules, including:

• Fatty acids
• Cholesterol
• Glycerides
• Squalene
• Wax

Lathering Up Puts Surfactants to Work

A little sebum helps keep your scalp and hair healthy, but too much can make hair look dull or stringy. Washing hair with shampoo cleanses the scalp and removes excess sebum. At the molecular level, ingredients in shampoo known as surfactants help loosen sebum and lift it off the scalp.

Surfactant molecules are amphiphilic, which means they contain distinct water-soluble (hydrophilic) and fat-soluble (hydrophobic) components that naturally align at the interface between air and water. In shampoo lather, the water-soluble components orient themselves towards water, while the fat soluble components orient themselves toward the sebum layer on your scalp.

As you scrub your scalp, mechanical action breaks the sebum into small particles. Surfactant molecules surround the particles in a process called solubilization. This makes the particles more water-soluble, so they go down the drain when you rinse.

Condition for the Win

Sebum is slightly acidic due to its fatty acid content. The pH of sebum ranges from ~4.5 to ~6.0 (pH 7 is neutral), so the scalp is a naturally acidic environment. Shampoos are relatively alkaline, or basic, so they can increase the pH of the scalp and leave hair feeling dry. Applying conditioner after shampooing restores acidity. The fatty alcohols and oils in conditioner also smooth the outer cuticles of hair shafts, which helps them retain moisture and makes hair softer, shinier, and easier to detangle.

Now that you understand some of the science behind hair and hair care, you can start decoding those mysterious ingredient lists to find products that keep your hair at its best!