Only electrolysis, thermolysis, and lasers have shown to be capable of effectively destroying the deep follicular areas. If these areas are not damaged in any way, as they are with wax, the anagen hairs will continue to grow at a speed of about 0.2mm per day and will appear on the skin after a period of time that will depend on the depth of the follicle and on how large a portion of the hair shaft has been removed. If, however, the percentage of the hairs in the telogen phase is high (let’s say 50%) only those in the anagen phase will grow back, giving the impression that a significant removal has been obtained. This is due to the fact that the removal of hairs is certainly greater than 50% also considering those in the catagen phase. Moreover, average follicular damage can induce an anagen hair to pass prematurely into the telogen phase, before starting to grow again after a certain period of time. Significant perifollicular damage is obtained when at least one or all the generative structures of the hair are partially or completely destroyed.

Perifollicular fibrosis, induced through treatment, can also delay the re-growth of the piliferous structures in the case of partial damage; in this case the hair grows back smaller and lighter. There can also be a relative effect using thermolysis or laser treatment, the hair can be white, deep or the skin not sufficiently transparent to the wavelength used, with the result that not all the anagen hairs are destroyed. Besides this, it seems that the telogen hairs are affected to an irrelevant degree with respect to the anagen hairs. In the following paragraphs we will analyze more deeply these details that will indicate the most suitable laser for the hair removal. In this way we will discover that Nd:YAG long-pulsed lasers provide advantages that other systems do not have. This is particularly true for darker skins which make up the absolute majority in countries of Latin origin. We will also see how the right energy and flow are capable of creating an ideal combination in order to obtain an effective and safe hair removal treatment.During the last few decades numerous methods for hair removal, even temporary, have been developed. At different times, more or less valid methods have been introduced, such as shaving, waxing and the use of tweezers, which turn out simply to be temporary solutions. The only non-laser system capable of damaging the hair in a definitive way is through electrical coagulation ( Electrolysis,Thermolysis and the Blend ) , in both versions: continuous and alternating current. Numerous factors are involved in obtaining adequate results from hair removal: this depends on the physiology of the hair cycle, as yet not completely understood, the physical qualities of the tissue, the type of equipment used, the experience of the CME's and particularly the expectations of the patient. Laser hair removal has made great steps forward but, to make a conscious choice, it is necessary to know all the differences that the various types of laser can have. In the figure above we can see the anatomic structure of the hair. In order to obtain its definitive destruction it has been shown that it is necessary to destroy the third inferior part of the follicle, the area around the bulb and the external part of the root. By definitive destruction we mean, in the case where a laser is used, bringing these areas to such a temperature that their biological functions are irremediably jeopardized (photo-coagulation). In order to reach the quantity of energy required to photo-coagulate the deepest follicles (up to 7 mm) or those with the lowest melanin content, it is necessary to provide a greater quantity of energy directly from the laser. If less penetrating wavelengths are used, such as the ruby laser, a lot of energy must be used in order to have enough of it in depth, with a serious risk of damaging the epidermis. These wavelength, despite having a high affinity with melanin, become dangerous as the skin darkens due to the competition between the melanin in the skin and that in the hair. The considerations regarding the use of large diameter spots in order to increase the depth of these lasers are interesting from a theoretical point of view but clash with experimental measures. Z. Zhao and P. Fairchild have directly measured the depth of penetration into the skin and have shown, in "Dependence of Light Transmission Through Human Skin on Incident Beam Diameter at Different Wavelengths", SPIE Proceeding, Vol. 3254, 01.98, two important aspects concerning the increase in the size of the diameter of the laser beam. The first is that even increasing the diameter over a certain size the laser beam does not reach the deepest areas (for example, for the ruby laser this saturation is obtained at 10 mm with white skin). Moreover, the depth of these wavelengths, at 10 mm, is the equivalent of that of the ND:YAG laser at 5 mm.The fundamental characteristic of photo-depilation can be found therefore in the numerical relation between the melanin concentration in the skin and in the hair. This relation is very important in evaluating the effectiveness and the safety of the laser treatment but above all in evaluating which the best sources to use are. The ratio between the temperature of the piliferous bulb and the epidermis can be calculated, in the case of shorter pulses, by the thermo relaxation time, that is the time in which the thermo diffusion from the hair becomes significant.

In this case, for hair colors 10 times darker than the epidermis (very dark skin and black hair), the ratio of the temperatures is shown in the following figure on the left according to different depths. The same calculation is carried out in the case where the relation of color is only equal to 5 (hairs only slightly darker than the epidermis) shown in the following figure on the right.