367 Chapter 11 Hair Damage Crisan Popescu DWI at the RWTH Aachen University Introduction A discussion of hair damage requires a reference point, a control from which to gauge the various degrees and effects of damage, and herein that point is defined as healthy hair. Although apparently a common sense notion, it is not easy to define a “healthy fiber” in a clear, objective way. For the sake of simplicity, this condition is taken to be the state in which the hair emerges from the follicle. In other words, the fiber grows out of the follicle in its best condition, and is damaged continuously by external factors acting on it. This is, in fact, another way to state that hair plays a primarily protective function for the body.1 The complex structure of hair enables it to fulfill its purposed task by interacting with the various factors that cause its permanent erosion. These degrading processes may be grouped into: chemical, mechanical, thermal, and radiation factors. The Chemistry of Hair Damage Hair is made up of hard alpha-keratin (see Chapter 1), a fibrous protein, and has a large amount of cystine in its composition (the matrix contains around 21 mol% cystine calculated from the sulphur content of gamma-keratose of human hair1). The ability of this particular amino acid to establish disulphide bonds is responsible for the high degree of cross-linking of the fiber and