This is the first thing I teach my clients about their hair. Its importance is immeasurable. With a healthy cuticle layer your hair will have fewer tangles, more shine and be soft to the touch. Your color and restructuring treatments will last longer with out fading away. So here's the test:
If you looked at a hair under a microscope, you would see that it looks like shingles on a roof. When the hair is in good condition, these layers lay down tightly on top of one another. Creating shine is simply the reflection of light. It will also tangle less. Tangles occur because unhealthy (ruffled) cuticle layers catch on each other as the hairs pass by each other. Dryness and fading occur when you wash your hair. The soap and water get into the inner layers and rinse out the color and condition. To combat an already open cuticle layer, an acidification treatment is needed. The microscope never lies, and only professional acidification treats this problem. It also doubles as an extender of the life of your color's tonal effects.
Porosity: The ability of the hair to absorb and retain moisture. Hair is like a sponge. The better condition the sponge is in, the better it can absorb moisture, and the longer it can maintain its moisture level. There are a few things that contribute to an unhealthy cuticle layer.
Mechanical damage, caused by heat/thermal appliances, chemicals, combing and brushing, and cutting with poor quality or unkempt shears.
Environmental damage is caused by the sun, wind, cold or other elements drying out or tangling the hair. Aging hair. As it becomes longer and older it has been washed, dried, and ironed more. How old are your ends? Product build up: Particular ingredients in some products leave a build up on the hair which prevents your hair from being able to absorb the conditioning treatments you do give your hair. Most non professional products have these ingredients in them to prolong the shelf life of the product. They are not water soluble and remain on the hair shaft, and cannot be removed. When a heated appliance is applied over top of these ingredients the plastics are permanently bonded to the hair as if the hair has been laminated. Now through repetitive applications your hair has become more and more coated causing gravity to pull your hair down, and that usually finds a person using even more product to counter act these unnecessary issues. Prevent these events by using ONLY professional products. Beware however some professional products do also build up on the hair. Your hair dresser should be able to analyze your hair's needs and recommend something that is appropriate. Remember also that too much build up on the hair shaft will cause color failure. The color molecules need to penetrate the cuticle layer and enter the cortex layer to be deposited or to complete the lifting/toning process successfully. This is why understanding the science of the hair structure is key to knowing how to achieve the desired results.