Adsorption Properties of Hair 336 Gaseous Moisture Adsorption 1) The adsorption isotherm: A “map” of the equilibrium amount of gaseous species that adsorbs on to a substrate as a function of progressive increases in the relative vapor pressure (in our case the relative humidity) is known as an adsorption isotherm. Figure 1 contains two adsorption isotherms for hair and water. The solid data points were generated by Chamberlain and Speakman2 in 1931 and show the earliest reported representation of this relationship. Meanwhile, the hollow data points were generated more recently by the author using today’s modern electronic equipment–and thus emphasize the reproducibility of this well-established relationship. From these isotherms, it can be concluded that the moisture content of hair changes considerably as a function of environmental conditions. At reduced humidity, hair contains relatively low levels of moisture but, content increases greatly as humidity rises. Therefore, it is evident that the frequently heard consumer-related question, “How much moisture does hair contain?” does not have Figure 1. Adsorption isotherm for hair and water