Each individual hair is formed inside a hair bulb deep in a hair follicle. The follicle is a tiny but powerful factory, which throughout many people's lifetime hardly ever stops working. From a baby's birth for many decades, as much as a century in some people, the follicle continues to produce hairs.
None of these treatments affects the growth of the hair in the hair bulb, even though some may seriously damage the hair shaft.
Finally the hair spontaneously falls out. The follicle rests for a little while, and then starts to produce yet another new hair. This is the hair cycle.
Stages of the hair cycle
Between starting to grow and falling out years later, each hair passes through three distinct stages. These are so important that they have been given special names: anagen (the growing phase), catagen (the intermediate phase) and telogen (the shedding phase).
When you understand these stages you understand how laser hair removal works and why you need multiple treatments.
So, if the hair is up and about and has visible pigment it is in its anagen phase. Its pigments can absorb the laser and the follicle can be destroyed.
When the hair is its resting phase or catagen/telogen phase it is not visible or capable of absorbing the laser light and no destruction of the follicle will occur. So, it makes sense to have multiple treatments in order to obtain the 90% , +, or – 10% permanent reduction standard. The hair needs to be up and about in order to be destroyed.