2007, Piran, Slovenia

Acute and chronic heat exposure
PHYSIOLOGICAL STRAIN DURING ACCLIMATION IS PARTLY
REDUCED BY LOWERING RESTING CORE TEMPERATURE
Bernhard Kampmann 1 , Peter Bröde 2 , Martin Schütte 2 , Barbara Griefahn 2
1 Division of Applied Physiology, Occupational Medicine & Infectiology, Department of Safety
Engineering, Bergische Universität Wuppertal, Germany
2 Leibniz Research Centre for Working Environment and Human Factors at the University of
Dortmund (IfADo), Germany
Contact person: kampmann@uni-wuppertal.de
INTRODUCTION
Repetitive exposure to heat usually results in an adaptation of the body to the conditions of
climate and work, leading to a decrease of physiological strain (acclimation). Depending on
climatic conditions and work intensity, heart rate and body temperature will decrease during
heat exposure whereas sweat rates will increase. Some studies show that during the
acclimation process body core temperature is reduced in total so that it will be found lowered
already during the resting period before heat exposure (e.g. Buono et al., 1998). This paper
studies the extent to which the lowering of the resting core temperature contributes to the
decrease of body core temperature during acclimation by means of a re-analysis of data from
controlled laboratory experiments in a climatic chamber (Griefahn, 1997).
METHODS
The analysis is based on the registration of core temperatures (Tco) measured in the rectum
during four series of acclimations. Three series were carried out at equivalent heat stress in
terms of WBGT = 33.5°C, but different characteristics (warm-humid, hot-dry, radiation), one
series took place in a neutral environment (25°C). In each series eight semi-nude (0.1 clo)
subjects (2 women, 6 men) participated during consecutive days in a resting period followed
by two hours of work on a treadmill (4 km/h on the level). Table 1 presents the climatic
conditions during the series. Differences in the course of resting Tco (at end of first resting
period) and final Tco (end of last work period) were tested by repeated-measures ANOVA.
RESULTS
Figure 1 presents an example of an individual showing a lowering of Tco during acclimation
to dry heat not only at the end of the work period, but also for the resting values before the
start of heat exposure.
The results of ANOVA (Table 2) confirm the lowering of the final Tco during the acclimation
procedure, and the influence of the climate (Griefahn, 1997). Post-hoc tests revealed that the
effects of the different types of heat stress did not differ significantly from one another, but
differed significantly from those of the neutral condition. For the resting values a significant
lowering of Tco during the series showed up without a specific influence of climate.
The attenuation in physiological strain during acclimation, calculated by subtracting the
values at the first day from the actual values of final Tco, is illustrated by Figure 2, together
with the corresponding change in resting Tco. The resting Tco decreased by about 0.2°C in a
similar way for each of the four series. However, for the final Tco, calculation of the
differences reduced inter-individual variability, and the attenuation effect appeared to increase
from the neutral to the warm-humid, hot-dry and radiant heat stress condition. This was
proven using ANOVA showing a significant difference for the radiant heat condition not only
compared to the neutral, but also to the warm-humid condition.
385