Understanding the hair growth life-cycle
To fully understand how hair removal works it is useful to have an idea about the three different stages of hair growth. Each hair follicle continues cycles through these phases; telogen, anagen and catagen.
Telogen Phase
The telogen phase is the resting phase of a hair follicle. 10%-15% of all hair follicles are in the telogen phase at any one given point in time. This phase lasts for about 100 days for hairs on the scalp and much longer for hairs on the eyebrow, eyelash, arm and leg. During this phase the hair follicle is completely at rest and the club hair (white material formed by divided cells adhered to the root of the hair) is already formed. If you pull out a hair in the telogen phase you will see the root covered with this solid, hard, dry, white material. About 25-100 telogen hairs are shed normally each day.
Anagen Phase
The anagen phase is when a hair follicle is actively growing hair. The cells in the root of the hair divide rapidly and add to the hair shaft causing hair growth. Typically, hair grows about 1cm every 28 days. Scalp hair stays in this active phase of growth for 2-6 years. The amount of time each hair follicle remains in the anagen phase is genetically determined and finishes with the beginning of the catagen phase.
Catagen Phase
The catagen phase happens at the end of the anagen phase and is a brief transitional stage signalling the end of the active growth of a hair. It lasts 2-6 weeks while a club hair is formed. This process cuts the hair off from its blood supply and from the cells that produce new hair, forming a club hair when the part of the hair follicle in contact with the lower portion of the hair becomes attached to the hair shaft. When a club hair is completely formed, about a 2 week process, and the lower part of the follicles squeezes to push the hair up and returns to its normal position, the hair follicle enters the telogen phase.
When a hair follicle is in the anagen phase, it is when Intense Pulsed Light is effective in halting the process and effectively removing its growth capability. This is also the reason for which a series of sessions is necessary. As you can see, there will always be some hair follicles which are not active, so follow-up treatment can be necessary to catch unwanted hairs which have restarted their growth cycle.