No matter which laser or light-based device is used, the concept of how laser hair removal works is the same - selective photothermolysis. The “selective” in the case of this treatment means that the lasers are more highly absorbed by some things than others. The lasers that are used for laser hair removal are specifically chosen because their wavelength is more highly absorbed by darker colors than lighter ones. This concept is exploited in laser hair removal because there is more melanin in hair (the dark pigment that gives our skin and hair color) than there is in the surrounding skin.
The “photothermolysis” basically means using light (photo) to create heat (thermo) to destroy cells. As the laser (or light in the case of Intense Pulsed Light or IPL) passes over the skin it is absorbed by all the pigments in the skin and hair. As the pigment absorbs the light, it changes that light energy into heat energy. The more pigment there is, the more heat is produced. Because dark hair has much more pigment than the surrounding skin, it absorbs more light and creates more heat than the surrounding skin. As that heat is generated very quickly, it radiates out from the hair shaft almost like a shockwave, and in so doing it super-heats the hair follicle, where the papilla that feeds and grows the hair lies. If enough heat is generated, it can kill (lysis) the cells in the hair follicle and prevent it from ever growing a hair again.
For this concept to work, there are a few conditions that have to be met:
1. Quite obvious from the description, the hair must be dark. If there is no melanin in the hair, such that it is white, gray, or blonde, laser hair removal cannot work. Even red hair does not absorb the light well because it has a different kind of melanin than brown or black hair. This also means that thicker hairs (simply because they are bigger and hold more melanin) tend to be better treated than finer hairs.
2. The hair must be in the hair follicle. If it is pulled out by waxing, tweezing, threading, etc. there is no “heating coil” to absorb the light energy and create heat to kill the follicle. Since the hair follicle has no melanin itself, there is currently no way to target the follicle directly, only indirectly through the hair.
3. Only a certain percentage of hair follicles can be destroyed at any one time, because hairs must be targeted when they are in the “anagen” or growth stage, one of three phases of the natural hair production cycle. Different body parts have differing numbers of hairs in the “anagen” phase at any one time, the scalp being the highest with about 70% of hairs actively growing at once. As a result, multiple treatments are necessary.
4. While all skin types can be treated (given the appropriate laser), the greater the difference in pigmentation between the skin and the hair, generally the better the result. The darker an individual’s skin becomes, the more melanin they have, and the skin begins to heat more with the application of laser or light. Therefore darker skinned individuals must be treated at lower energy levels and often require more treatments to attain good hair reduction.