2007, Piran, Slovenia

Environmental Ergonomics XII
Igor B. Mekjavic, Stelios N. Kounalakis & Nigel A.S. Taylor (Eds.), © BIOMED, Ljubljana 2007
THE HEAT TOLERANCE TEST – A VALUABLE SCREENING TOOL
FOR SUSCEPTIBILITY TO HEAT
Daniel S. Moran 1 , Yair Shapiro 2 , and Yoram Epstein 1
1 Heller Institute of Medical Research, Sheba Medical Center, Tel Hashomer and Sackler
Faculty of Medicine, Tel Aviv University, 2 The College of Judea and Samaria, ISRAEL
Contact person: dmoran@sheba.health.gov.il
INTRODUCTION
Tolerance to sustain heat stress lies within a wide range of individual variations. Under the
extreme conditions of exercise-heat stress, even healthy, well acclimated, and physically fit
individuals will ultimately store heat at a rate that will cause body temperature to rise. In most
cases it is an uneventful condition, but in the extremes it develops into heat stroke.
Those individuals who are not able to sustain heat, and whose body temperature will start
rising earlier and at a higher rate than others under the same conditions, are defined as "heat
intolerant". The reasons underlying heat intolerance are ample. Some of those reasons are
congenital, which reduces the ability to thermoregulate efficiently (e.g., ectodermal dysplasia,
anhidrosis), and thus will always result in heat susceptibility, while other conditions are
acquired and could be considered as temporary, and not affecting the thermoregulatory center
or the heat dissipation mechanisms (e.g., dehydration). However, there are some acquired
circumstances, such as large scarred burnt skin area, that will affect the heat dissipation
mechanism to a degree that will result with a permanent condition of heat intolerance (HI).
For workers in the industry and for soldiers, who are routinely exposed to unfavorable
environmental conditions, the realization whether a previous condition of exertional heat
stroke (EHS) they encountered was due to a temporary or permanent HI is valuable in
protecting them from successive episodes of heat stroke.
METHODS
To identify an individual's susceptibility to exercise-heat stress, a Heat Tolerance Test (HTT)
was performed according to the following protocol: 120 min exposure to 40 o C and 40%
relative humidity in a climatic chamber while walking on a treadmill, dressed in shorts and a
T-shirt, at a pace of 5 km/h and 2% elevation. According to our experience, it is expected that
under these conditions of exercise-heat stress, at the end of the 120 min exposure rectal
temperature of heat tolerant individuals (HT) will be 38.0 ±0.3 o C, their heart rate will be 120
±15 bpm, and their sweat rate -780 ±160 g/h. Accordingly, heat intolerance is ascertained
when throughout the exposure to heat: rectal temperature exceeds 38.5 o C, or heart rate
exceeds 145 bpm. Obviously, the larger deviation from the expected values in a healthy
individual, the more pronounced is the state of heat intolerance (Figure 1; only rectal
temperatures shown).
Two hundred and fourteen formerly exertional heat stroke soldiers underwent the HTT, about
6 weeks following their release from hospital. Roughly, 10% of these subjects were found to
be heat intolerant following the test. Both sub-groups were comparable in all aspects, but
differed in the duration in service. The HT subjects were longer in service, and thus more
experienced, than the HI soldiers. The general characteristics of the two subgroups are
depicted in Table 1.
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