2007, Piran, Slovenia

Environmental Ergonomics XII
Igor B. Mekjavic, Stelios N. Kounalakis & Nigel A.S. Taylor (Eds.), © BIOMED, Ljubljana 2007
Subjective evaluations of thermal sensation, thermal comfort, and skin wetness
sensation were recorded every 10 minutes.
Physiological parameters measured: Metabolic heat production was calculated from
oxygen uptake, analysed by a Vmax29 (SensorMedics Corporation, CA, USA), heart
rate (Polar Sport Tester heart–rate recorder, Polar Electro OY, Kempele, Finland), 13
skin and rectal temperatures (YSI-400 and 700 thermistors, accuracy ±0.15° C,
Yellow Spring Instruments, OH, USA), body weight before and after the experiments
(Mettler ID1 Multirange, Mettler Toledo, OH, USA). Mean skin temperature (MST)
was calculated using the average of all measured skin temperatures. Clothing
physiological parameters: Temperature (YSI-400 thermistors, accuracy ±0.15° C,
Yellow Spring Instruments) and moisture (relative humidity, % RH, moisture sensors
HIH-3605-B-CP, Honeywell, NJ, USA) between clothing layers, sweat accumulated
in the clothing (weight increase during experiment).
RESULTS AND DISCUSSION
Moisture in the clothing, measured as % RH, demonstrated a marked change in the
rate of increase for five of the six test subjects after an initial period of a minor
moisture increase (Figure 1). This change does not happen at the same time for all the
subjects, but varies between 42 and 88 minutes into the test. Interestingly, the time
course of the rise in % RH follows the time course of skin temperatures at the same
location (middle back) for each subject. We also observed an increase in other local
skin temperatures at the same point in time, especially at the shoulder, neck and
posterior thigh where PCM was located. Figure 2 demonstrates the rise in PCM
temperature followed by an increase in skin temperature at the same location after 90
minutes in the climatic chamber for one subject. This is closely linked to the increase
in sweat production, and followed by a 0.3°C increase in rectal temperature for this
subject during the final 30 minutes of the exposure. The mean rectal temperature for
all six subjects demonstrates an increase at the same time as the temperature of the
PCM starts to increase following the characteristic stable temperature of the phase
change period. At this time the PCM has changed from a solid to a liquid state, and
has no more capacity to absorb excess body heat (Figure 3). Furthermore, subjective
perceived wetness of skin was also improved by the PCM (Figure 4). The subjective
evaluation of skin wetness followed the same time course as for the skin temperatures
and clothing relative humidity.
The results suggest that there is a relation between the PCM and thermo-physiological
effector mechanisms. The sudden increase in relative humidity in the clothing
suggests that skin and core temperature have been held at a level below the effectorspecific
threshold temperature for sweating. Once PCM has lost its cooling capacity,
the integrated signals from peripheral and central temperatures receptors crosses this
threshold, and as a result sweat production increases in order to facilitate heat loss.
However, it is evident that the capacity of the quantity of PCM employed in this study
is not sufficient for a warming period lasting for two hours under these environmental
conditions of heat stress.
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