2007, Piran, Slovenia

Occupational Thermal Problems
highly significant (p< 0.01). The same correlation was not seen between ∆ SBP and the end
heart rate of the test. The scatterplots of the correlations are presented in Figure 2.
r = 0.250
r =-0.736
Figure 2. The relationships of change in SBP vs. heart rate (left) and SBP vs. HF-component
of heart rate variability (right ) are presented in scatterplot figures.
DISCUSSION
This study confirms the finding of our earlier studies (Ilmarinen and Lindholm 2000) that
elevation of body core temperature is not usually the limiting factor in heat stress exercise.
The most typical reason for termination of HST was excessive drop of SBP. In HST this
phenomenon is contributed to elevated heart rate and decreased plasma volume. The
consequence is rapid hemodynamic change, which can cause, for example, heat illness and
heat syncope. One of the most important findings of this study was that change in SPB
correlated more significantly with the HF-component of heart rate variability than with heart
rate alone. The HF-component of HRV may prove a useful tool in the assessment of
hemodynamic balance in a laboratory environment and perhaps also in field studies.
This study also showed that the intended duration of HST should be 60 min. This is enough
time for cardiovascular changes to occur and for factors possibly limiting heat tolerance and
physical performance in heat to appear. In this study, as well as in some earlier reports, it
seems that in healthy individuals the limitations of heat tolerance do not usually appear in the
first 30 min of the heat stress test.
ACKNOWLEDGEMENTS
The authors appreciate the financial support of the Finnish Work Environmental Fund and the
assistance of Finnish Roofing Association and Finnish Asphalt Association. Warm thanks are
also due to Finnish Road Administration and the roofing enterprises ICOPAL Ltd, AL-Katot
Ltd, and Lemminkäinen Corp.
REFERENCES
Ilmarinen, R. and Lindholm, H. 2000. The influence of age and gender on thermoregulatory
control during heat stress. Environmental Ergonomics IX, p 19-22, Shaker Verlag,
Aachen, Germany.
Olesen, B. 1984. How many sites are necessary to estimate a mean skin temperature? Thermal
Physiology (ed. J. Hales), p 33-38, Raven Press, New York.
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