WATER HARDNESS METALS AND HUMAN HAIR 387 upper end. This observation was independent of the water treatment since 3:1 Ca/Mg was maintained for all water hardness levels. However, the selectivity for calcium over mag- nesium cannot be ascribed solely to the predominance of calcium vs magnesium in the water. This effect has been previously observed in laboratory experiments where bleached hair contained four times more calcium than magnesium following multiple treatments with water containing 1:1 Ca/Mg. In addition to the predominance of calcium vs magne- sium in the water, the sizes of the cations likely infl uenced their keratin-binding affi nities. The site-binding interactions that occur between cations in solution and the anionic groups of a polyelectrolyte are governed in part by the hydration characteristics of the cations (18). Smaller cations possess large, tightly held hydration spheres due to high charge density, while the opposite is true of larger cations (19). Since calcium has a larger ionic radius (1.18 Å), and thus a smaller hydrated radius than magnesium (0.82 Å), the former has been reported to adsorb more easily to polyeletrolytes than the latter (19–21). Baseline calcium and magnesium levels appeared to decrease as oxidative damage in- creased. However, we believe this trend was due to the diffusion of calcium and magne- sium ions out of the swollen hair and into the deionized water used for rinsing the oxidant crème from the hair. This hypothesis has been confi rmed by rinsing the oxidant crème from hair using local tap water (8 gpg 1:1 Ca/Mg ratio). When freshly bleached hair was rinsed with the tap water, the fi nal calcium and magnesium levels were equivalent to the levels in virgin/untreated hair. The effect of water hardness levels was secondary to the condition of the hair differences in metal uptake between the test water hardness levels were quite small in virgin hair, but they increased with oxidative hair damage (Figure 2). Interestingly, calcium uptake from hard Figure 1. Effect of hair condition on the calcium and magnesium content of virgin and bleached (damaged) hair treated with water of different hardness levels. Calcium content was highly dependent on the condition (binding capacity) of the hair substrate. Tukey-Kramer HSD analysis yielded p 0.001 for all hair condition comparisons within the tested water hardness levels.

JOURNAL OF COSMETIC SCIENCE 388 (9 gpg) and very hard (16 gpg) water was not signifi cantly different for virgin and highly damaged hair, and it was different by only 16% for slightly damaged hair (p = 0.005). This clearly indicates that the hair has a fi nite binding capacity, which is determined by the number of available anionic sites. Regardless of the water hardness level, the hair will reach saturation and not associate with any remaining hardness ions in the water. Even upon repeated exposure to soft water (2 gpg), both levels of damaged hair extracted large amounts of water hardness metals, and thus approached their saturation point. Based on a multiple linear regression analysis, it was found that calcium and magnesium uptake was very strongly correlated with hair condition (r2 = 0.98 and 0.99, respectively, p 0.001) and less correlated with water hardness (r2 = 0.59 and 0.72, respectively, p 0.001). This fi nding is signifi cant because it establishes the fact that water hardness metal uptake is not confi ned to individuals who live in hard water regions. The popular- ity of hair treatments that alter the chemical nature of hair, e.g., coloring, bleaching, and relaxing, suggests that even more consumers are susceptible to this uptake if they are exposed to water with any degree of hardness ions. A positive relationship between water pH and metal uptake was observed (Figure 3). This can be related to the binding capacity of the hair because as pH increases, more groups become available for metal interaction due to the progressive ionization of carboxyl groups Figure 2. Effect of water hardness levels on calcium and magnesium uptake by virgin and bleached (dam- aged) hair. n=3. Asterisks (*, **, ***) represent p 0.05, p 0.01, and p 0.001, respectively, obtained by Tukey-Kramer HSD analysis. Uptake was calculated by subtracting the average baseline calcium and mag- nesium content of hair from that of hair treated with water of different hardness levels. Water hardness metal uptake was more driven by the condition or binding capacity of the hair substrate (r2 = 0.98 and 0.99 for calcium and magnesium, respectively, p 0.001) than by the hardness of the water (r2 = 0.59 and 0.72 for calcium and magnesium, respectively, p 0.001). It should be noted that the negative values of magnesium uptake by virgin hair indicate that the hair lost magnesium in an attempt to establish equilibrium between the few binding sites and the low amount of magnesium in the water samples. As the magnesium content in water increased, the hair absorbed more of the ion and approached the baseline levels.