Chapter 10 353 14% when exposed to 100% relative humidity, while the length will increase by around 2.5%23 (although the absolute magnitude of this effect can be altered by factors such as hair health and the presence of external strain, among others). Therefore, from a hair care perspective, it is not a stretch to imagine how this effect, compounded over the 100,000–150,000 fibers on a typical head, could alter aesthetic considerations such as feel and manageability (more on this to come as this chapter progresses). Nonetheless, this swelling can be beneficial in terms of enabling materials to penetrate into hair. Many chemical treatments (e.g. perms, colorants, bleaches) are formulated at elevated pH, which is recognized to increase swelling and thus facilitate diffusion of actives into hair. Consequently, it is frequently heard that low pH conditions cause de-swelling of fibers–with this viewpoint possibly being reinforced by old wives’ tales involving the supposed benefits of applying vinegar or lemon juice to the hair. In actuality, this is incorrect little effect on swelling is seen over a wide range in pH and increases only towards both the very upper and lower extremes of the scale.24 According to Valko & Barnett, swelling of hair fibers is constant between pH 2 and pH 9 with progressive increases occurring above and below this range. Upon approaching the high and low ends of the pH scale, the ability for either acid- or base-induced protein hydrolysis becomes more likely. Therefore, these conditions can permanently alter the hair structure, which represents the control factor behind swelling. In evidence for this supposition, it is noted that cleavage of cystine disulfide bonds during the perming process is known to result in considerable swelling,25 while their reformation during the oxidation/ neutralization step produces partial reversal.26,27 Nonetheless, complete reformation of all bonds is not possible, leaving fibers with depleted tensile properties and a heightened tendency for swelling. Indeed, the extent of swelling is sometimes used as one quantifier of hair damage. This heightened degree of swelling in damaged fibers further facilitates diffusion, and in consumer language, the hair is often