The primary principle behind laser hair removal is selective photothermolysis (SPTL), effect on a targeted tissue (melanin) with minimal effect on surrounding tissue or skin.

There are two types of melanin in hair. Eumelanin gives hair brown or black color, while pheomelanin gives hair blonde or red color. Because of the selective absorption of photons of laser light, only hair with color such as black, brown, or reddish-brown hair or dirty blonde can be removed. White hair, light blonde and strawberry blonde hair does not respond well. Laser works best with dark coarse hair.

Hair removal lasers have been in use since 1997 and have been approved for "permanent hair reduction" by FDA. Indeed, many patients experience complete regrowth of hair on their treated areas in the years following their last treatment. This means that although laser treatments with these devices will permanently reduce the total number of body hairs, they will not result in a permanent removal of all hair.

Comparisons with other removal techniques

Comparison with intense pulsed light

IPL, though technically not containing a laser, is sometimes incorrectly referred to as "laser hair removal". IPL-based methods, sometimes called "phototricholysis" or "photoepilation". IPLs offer larger diameter targets.

Comparison with electrolysis

Electrolysis is another hair removal method that has been used but the treatment is slow and tedious compared with typical newer laser hair removal.

Comparison with shaving

Shaving is removes hair only temporary and can lead to irritation of the shaved area.

Comparison with waxing

Waxing is another option for hair removal but not permanent.

Types of Laser -

Pulse width (or duration) is one of the most important considerations. The length of the heating pulse relates directly to the damage achieved in the follicle. When attempting to destroy hair follicles the main target is the germ cells which live on the surface of the hair shaft.

Spot size, or the width of the laser beam, directly affects the depth of penetration of the light energy due to scattering effects in the dermal layer. Larger beam diameters or those devices that has a linear scanning results in deeper deposition of energy and hence can induce higher temperatures in deeper follicles. Hair removal lasers have a spot size about the size of a fingertip (3–18mm).

Fluence or energy density is another important consideration. Fluence is measured in joules per square centimeter (J/cm^²).

• Contact cooling

• Air cooling: forced cold air

When treating hair (and other skin conditions) is power density – this is a combination of energy, spot diameter and pulse duration. These three parameters determine the results.

Number of sessions

Hair grows in several phases (anagen, telogen, catagen) and a laser can only affect the currently active growing hair follicles (early anagen). Hence, several sessions are needed to damage the hair in all phases of growth and force it to revert to a vellus non-colored small hair. Multiple treatments depending on the type of hair and skin color have been shown to provide long-term reduction of hair. Most patients need a minimum of eight treatments. Waiting from three to eight weeks between sessions, depending on the area being treated. The number of sessions depends on various parameters, including the area of the body being treated, skin color, coarseness of hair, reason for hirsutism, and sex. Coarse dark hair on light skin is easiest to treat. Certain areas (notably men's faces) may require considerably more treatments to achieve desired results.

Laser does not work well on light-colored hair, red hair, grey hair, white hair, as well as fine hair of any color, such as vellus. For darker skinned patients with black hair, the long-pulsed Nd:YAG laser with a cooling tip can be safe and effective.

Side effects and risks

Some level of pain, including itching, pink skin, redness, and swelling around the treatment area or swelling of the follicles (follicular edema). These side effects rarely last more than two or three days. The two most common serious side effects are acne and skin discoloration.