The science of your hair from Clairol print

The Science of Your Hair


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To understand the science of hair color, first it is important to understand WHAT we are coloring. Let’s start with a review of hair structure.


Hair fibers consist of: the cuticle, the cortex and the medulla.

The cuticle is approximately 10 layers of flat, overlapping, scale-like structures that are responsible for the condition of your hair, as it helps protect the inner portion of your hair structure—the cortex.

The cortex is the group of spindle-shaped cells aligned along the fiber, which account for about 70 - 90% of the hair bulk. The cortex contains hair’s natural pigment, called melanin, and is also where the coloring process takes place. The size, shape, and distribution of the melanins determine the overall color of hair.

The medulla is the center of the hair fiber. This is the core of elongated cells running down the middle of the hair fiber.


Since the natural color pigment (melanin) is in the cortex, deep inside the hair, and the cuticle is designed to protect the cortex, dyes or bleaches must penetrate the cuticle, travel through the cortex and remain active long enough to color hair.

There are also a variety of hair types—straight, wavy, curly or anything in between. Minor differences in our genes can make large differences in hair, so it is important to understand your hair type before coloring. For example, since Asian hair is thicker and coarser than Caucasian hair, it can be more difficult for hair color to penetrate and lock in. Therefore, a longer coloring time may be necessary.


  • Hair is found on about 95% of the total skin area
  • The average head contains over 100,000 hair follicles
  • On a baby’s head there are 1,100 follicles per square cm; by age 25 this falls to 600, and by age 50, you’re down to about 250 - 300, but the number depends on the physical type of the individual
  • Hair is very elastic and difficult to destroy, especially by pulling (so strong, in fact, a strand of hair is stronger than a copper wire of the same thickness)
  • Hair grows at a rate of ~1 cm per month
  • Each scalp hair will grow for about 1000 days and then fall out, it than begins the process to fall out while the subsequent phases may last some months.
  • Each follicle grows about 20 new hairs in a lifetime, which grow for several years until they fall out
  • We naturally lose 50 - 100 scalp hairs every day
  • No new follicles form after birth