Why multiple sessions are required to permanently remove the hair growth?
Any hair on our body or face features three fundamental stages of growth: the active growth phase called Anagen, the regression phase called Catagen and the resting phase called Telogen.
Effective hair removal can happen only during the Anagen phase. During this phase the hair develops from the follicle that is located in the dermis (inner layer of the skin) being attached to the cell that is responsible for hair formation and growth. If laser light is applied during this phase, it is capable of transiting sufficient heat through the hair itself to the connected growth and development cell, thus permanently removing it.
However during other phases the hair follicle is no longer attached to the formation and growth cell or this connection is weak. Hence the application of laser light will destroy the visible hair body and its follicle itself, but may not at all or only partially affect the underlying growth cell. In this case the hair may reappear again; albeit it will be much weaker and will also take it much longer to develop.
Statistically it has been confirmed that on overage during sequential 6 sessions all hair would have passed through its Anagen phase and targeted during one of the treatments. With that being said, individual circumstances may wary significantly, with some people reporting complete hair reduction after 2-3 sessions and some requiring up to 9 treatments.