The structure and main function of hair, in addition to decoration, is protection against UV rays and heat loss, through the formation of an invisible insulating layer, which traps air in the area next to the skin. A normally developed hair has an outer part, called the stem, which represents the dead part of the hair. Cutting a hair horizontally shows a structure that can be divided into three parts: the cuticle, which is the first outer part, formed by a single row of transparent, thin cells in the form of scales; the cortex, which is the intermediate part, composed mainly (80%) of keratin, which in turn is composed of amino acids, cystine, serine, glutamic acid, etc.; and the third innermost, central part which is composed of the medullar. Often the focus is exclusively on hair and attention is only paid to aesthetic considerations or the final problem of hair loss.