As its name suggests, laser hair removal is a process where a laser is used on the area to be treated. Laser hair removal destroys the hair follicle, and hence the ability for hair growth by photothermolysis, ie. heat. The laser light is absorbed by melanin, which heats rapidly as it absorbs the laser energy, if enough heat is generated then the hair cannot re grow.
This reliance on absorbtion of laser energy by melanin causes a few problems. Firstly, melanin is present in the skin, so dark skinned people, whether genetic, or by the result of tanning, will need more treatments, each of which is less effect. In some cases they may not be eligible for treatment at all.
A second problem is hair colour. The lighter the hair colour, the less melanin each hair contains. This means that laser treatment is not very effective on the lighter coloured hairs such as red, blond, grey, and doesn't work at all on white hairs. These two factors combined, show that the best subjects for laser hair removal are dark haired, and light skinned people.
This is how the process works. The light from the laser passes through the skin, but cannot penetrate indefinitely. This means that the closer to the surface the hair follicle is the better the process works. Any melanin the laser light encounters absorbs the laser creating heat. As the concentration of melanin is greater in the hair follicle, than the skin, the hair follicle heats up more rapidly. Also, the structure of the skin and the hair follicle being different, the skin cools more rapidly. This feature is exploited more in the newer lasers, as they en corporate a pulse beam. Instead of just one pulse being delivered, the laser is turned on and of many times in a very short space of time. This causes even more heat build up in the melanin of the hair follicle, but lets the skin remain relatively cool. Secondary sources of cooling, such as a jet of cold air, are also used to keep the skin cooler. As the treatment only works on hairs in the anagen phase of the hair growth cycle, several treatments are necessary.
My own experiences with laser hair removal are very recent. I finished a course of 6 treatments only last week, so I can't tell what the long term results are yet. The first treatment is the worst, as there are more hairs to absorb the heat, and it can be very painful. With subsequent treatments the levels of pain experienced drop. After treatment, some hairs fall out immediately. These are hairs in the telogen phase of the hair growth cycle. The treatment has caused them to die and fall straight out, but the follicle itself, not being attached to the hair, has not been damaged, and so these will grow back. It takes two weeks or so, for the hairs that have been affected to fall away and the affects of the treatment to be really known.
The area treated also needs to be looked after properly between treatments. The application of aloe vera gel to keep it moisturised, and as much as possible, kept out of the sun. If the area has to be exposed to sunlight, then a sun block of 30+ has to be applied. This is for two reasons, the most important is that the sun can cause pigmentation problems with the treated area, so that the skin looses all colour. This will prevent further treatments until the skin has recovered. In some cases the skin never recovers, and the colour loss is permanent. A second reason is to prevent the treated area from getting a tan. A tan would cause the next treatment to be less effective, and if the laser operator is not informed of the tan, they would use the previous settings for the next treatment, possibly causing damage to the skin.
Immediately after the treatment it is advisable to apply an ice pack to cool the skin further. I was not advised to do this, and after one treatment, my top lip swelled to three times its normal size, and I developed a big scab. This delayed treatment while it healed, and has left me with a scar. So be careful when you undergo treatment not to miss out this important step.