Adsorption Properties of Hair 346 However, in this author’s opinion, the use of lesser equilibration conditions all too often leads to highly questionable results and conclusions. It is again noted that dramatic changes in adsorption behavior are not usually seen, so accurate detection of minor differences involves meticulous experimentation. Recommendations based on extensive experience involve allowing two days for initial degassing of the sample and at least 6–8 hours equilibration at each relative humidity. 3) Rates of water adsorption: In addition to quantifying the amount of water adsorbed at a given humidity, it is also necessary to consider the rate. Figure 11 shows an expanded view of adsorption data for the 0–10% step in RH, and illustrates the duration necessary to reasonably approach equilibration. In actuality, it can be demonstrated that extremely small changes are still occurring even after 24 hours, but the vast majority of this process occurs over the timeline shown. The generally accepted approach for quantifying adsorption rates involves reporting diffusion constants, and a survey of the existing literature5,15 shows typical values for the adsorption of water and hair being in the order of 10-9 cm2/s. However, the physical significance of these numbers is not readily apparent, in that it is not clear whether these values represent fast or slow adsorption. For this reason, we advocate borrowing an idea from the field of heterogeneous kinetics16 that first involves normalizing the y axis between 0 (weight of the sample at the initial equilibration condition) and 1 (weight of the sample at the final equilibration condition), and then evaluating the time required to reach the midpoint of the adsorption process (i.e. the half-time, t 0.5 ). This approach has been demonstrated in Figure 12, where under these conditions it is observed that slightly over 40 minutes was required to adsorb 50% of the total amount possible when undergoing this change in humidity. This same process can be performed for each of the adsorption steps shown in Figure 10, with the results being given in Figure 13. Therefore, it is evident that adsorption rates vary considerably with differing changes in climatic conditions. Similar conclusions can be