How Does Hair Grow?
Hair grows from a pouch-like structure below the skin called a hair follicle. The total number of hair follicles is fixed at birth. Humans have more hair follicles per square inch than most higher primates, and the head has more hair follicles than any other part of the body.
Two types of hair can be found on our bodies. Vellus hairs are fine, unpigmented or pale hairs that cover most of our bodies. The thicker, course, pigmented hairs are terminal hairs. Vellus hair can convert to terminal hair with numerous health influences.
Melanin is a dark pigment found in our hair and skin. Melanin, along with other pigments, gives hair its different colors. Melanin granules are deposited in the matrix cells of the hair bulb. The laser energy is absorbed by the melanin.
Everyone's hair grows differently, depending on gender, age, genetics, hormones, medication, and ethnicity, to name a few. All hair goes through three distinct growth cycles.
The anagen phase is the active growth phase for the hair. Its duration is variable and can last up to three years. During anagen, the hair has an abundance of melanin. The hair progresses into catagen or transition. This phase usually lasts from one to three weeks. The third phase is the telogen or resting phase. This dormant phase lasts approximately five weeks to three months, and varies with location. When growth resumes, the new hair pushes the old hair out.
The follicle needs to be treated while in the anagen phase. Percentage and duration of anagen cycles vary with body location. According to the Richards-Meharg table, the scalp has an average of 85% anagen or active growth at any time. In comparison:
- the upper lip has 65% anagen
- the chin has 70% anagen
- the underarms and the pubic area have 30%, and
- the legs have 20-30% anagen hairs.