Chapter 10 337 a straightforward answer. Namely, the water level is a dynamically changing variable that continuously alters as we pass from room to room, indoors to outdoors, and encounter different humidity conditions. To emphasize this point, Figure 2 shows the result of using sensors to track the temperature and humidity conditions experienced by an individual during a 24-hour period, and demonstrates how the moisture content of hair will be constantly changing and readjusting as we go about our daily duties. As will be described shortly, a sizable amount of time is required for the moisture content of hair to equilibrate under a given set of conditions, and so it must be concluded that this state is rarely (if ever) attained. An evaluation of the water content during progressive decreases in the relative humidity constitutes a desorption isotherm, which, as shown in Figure 3, takes a somewhat different path to the adsorption behavior. Accordingly a hysteresis exists, and it becomes evident that equilibration of air-dried hair (i.e. high to low water content) results in higher water content at a given condition than heat-dried hair (i.e. low to higher water content). The generally Figure 2. Temperature and humidity conditions encountered during a 24-hour period taken from Ref. 3 with permission of the authors.