Chapter 10 351 humidity step (determined from the adsorption isotherm), together with calculations that provide information on the changing absolute water content of the atmosphere, allow for reasonably good prediction of water adsorption rates. Consequently, it becomes clear that while temperature does not have a sizable influence of the amount of moisture adsorbed by hair, it does have a dramatic effect on adsorption rates. Therefore, it would appear that “bad hair days” during oppressive summer conditions relate in large part to the increased rate by which style-destroying moisture is adsorbed. By means of illustration, 90% relative humidity winter days are by no means uncommon for example, after a good snowfall. Yet, complaints about style retention are not forthcoming. As outlined above, hair will eventually attain slightly higher moisture content on a 90% RH winter day than a comparable summer day, but it is apparent that adsorption rates will be considerably slower. From an experimental standpoint, it is also worth highlighting the influence of sample preparation on adsorption rates obtained by this methodology. The use of larger volumes of hair in these DVS experiments can induce diffusion gradients within the sample, where the inner portions are partially “insulated” by the outer area. Therefore, larger samples are found to yield slower adsorption times under otherwise identical experimental conditions. Moreover, the aspect ratio of hair samples used in this testing is clearly not representative of real-life conditions. The diffusion pathway of water into hair may be debated, but “end-effects” are clearly considerably more pronounced in the short test fibers employed with this approach. Further to this point, it is often claimed that coating the hair surface with hydrophobic materials will slow down rates of water desorption. From the above discussions, it becomes possible to see how difficult it is to test this presumption nonetheless, it does appear spurious. First, healthy hair already possesses a hydrophobic surface coating, the f-layer (see Chapter 1), and yet water readily adsorbs and desorbs. Water is obviously an extremely small molecule that is difficult to deter, and therefore it would appear that the hair surface essentially requires “seal-coating”