It is conceived that the activity of water vapor in air space ad jacent to skin is the major factor which influence the sensation of discomfort associated with "hot" feeling, because the high activity resists the heat dissipation process by sweating. If one can maintain the activity of water vapor in the space low by selection of fabric systems, it is likely to result in higher level of comfort (lower discomfort). On the other hand, it is also important to recognize that the comfort factor (or discomfort factor) of a fabric system is governed by the dynamic response of the system to the changes in environmental conditions such as the rate of evaporation of water and the excessive sweating from the skin, rather than the equilibrium characteristics of the system.
　While there are abundant data dealing with equilibrium characteristics such as moisture regain, steady state permeation rate, etc., there seems to be relatively few data available in literature which deal with dynamic changes of those characteristics in relevant environmental changes, For this reason, dynamic water vapor and heat transports in transient state were investigated for fabrics made of polyester, acrylic, cotton, and wool fibers by MU Simulator developed at the Center for Surface Science and Plasma Technology, University of Missouri-Columbia. The simulator is designed to investigate dynamic water vapor and heat transport across layered fabrics under two major cases of pure water vapor and sweating.
　The overall dissipation rate of water vapor is dependent on both the water vapor transport rate and the water vapor absorption rate by fibers which are mutually interrelated. It was found that water vapor transport is governed by the vapor pressure gradient which develops across a fabric layer. When fabrics having the same water vapor permeability are subjected to a given environmental conditions, the actual water vapor transport rate greatly differs depending on the nature of fibers.
　The characteristic differential vapor pressure which develops across the layer is dependent on the water vapor absorption characteristics of the fibers. The differential vapor pressure depends on the balance between the rate of water vapor supply and the rate of water vapor absorption by the fabric material. If these rates are maintained at a comparable level, the higher is the water vapor absorption rate, the lower is the differential vapor pressure for water vapor transport and thus the lower is the overall water vapor transport rate.
　It is important to point out that polyester fabric which has low water vapor absorption showed the highest overall water vapor transport rate. However, this high water vapor transport rate is largely due to high differential water vapor pressure across the fabric (due to low water vapor absorption rate by the polymer).
　Thus, the highest water vapor pressure near the skin is observed with fabric of highest water vapor transport rate. This observation agrees with a general consensus feeling that polyester is relatively less comfortable despite its relatively high water vapor permeability.
　In the case of sweating, the wicking characteristic of fibers plays a very important role. By spreading out liquid phase water in a wider area and depth quickly, dissipation of water is accelerated.
　It was also found that the temperature of air space in between two layers of fabric rises when water vapor transport occurs. The temperature rise is nearly proportional to the water vapor absorption rate of a fabric which is dependent on the chemical nature of the constituent fibers. The temperature rise in the air space by heat of absorption of water by fibers is an important factor when survival factor in severely cold environment (rather than comfort factor in warm weather) is considered.
　The importance of water vapor absorbing capability of fibers is two fold. One is the buffering effect in water vapor transport which reduces the differential pressure gradient and the water vapor pressure in air space adjacent to skin. Another is the release of heat of absorption which raise the temperature of air space adjacent to skin. How these factors influence the wearability of fabrics would depend whether comfort factor or survival factor is the major issue. The wearability of fabrics should be evaluated by considering the distinction between the comfort factor in warm and humid environment and the survival factors in extremely cold environment. A careless application of conclusions obtained in the former conditions to the latter cases would lead to a disastrous consequence. The simulator used in this study enable us to investigate this issue in a well defined manner.
　This study has clearly demonstrated the importance of the dynamic response rather than steady state or equilibrium characteristics of fabrics. Further studies are needed to elucidate effects of combination of fabric layers which have different characteristics, particularly for the purpose of developing improved out-door sport wear for winter sports.

DESCENTE SPORTS SCIENCE Vol.13/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE