Thermal strain remains difficult to quantify in actual work conditions desp
ite the standardization of several strain models. Indeed, occupational phys
icians must record numerous parameters in order to use these models, which
makes them difficult to apply. In addition, these models are appropriate on
ly in cases of long lasting exposure. In exposures shorter than one hour, w
hich are the most frequent at the workplace, the time limit values (TLV) co
mputed by these models appear unsafe. The purpose of this study was to show
that the recording of heart rate (HR) allows a simple determination of saf
e TLV. In this study, rest HR increases due to thermal strain (EPCT) were r
ecorded on 98 workers during 131 actual work periods representing 18 differ
ent work situations. The EPCT is the difference in beats per minute (bmp) b
etween the mean HR of the 3rd, 4th and 5th minutes of rest after the end of
the exposure and sitting at rest HR before the exposure.
In this filed study, oral temperature was considered as a good index of cor
e temperature. variation in oral temperature (dtbu) from the beginning to t
he end of the heat exposure is related to EPCT : dtbu = 0,032 EPCT. This re
lationship is significant (p < .0001) and the standard error of the estimat
ion (see) is 0,18<degrees>C. Because this study was conducted on a large sa
mple of workers, the results allow to compute a safety limit of EPCT which
protect 95% of the population against an increase of core temperature great
er than 1 degreesC. Thus, maximal mean dtbu increase is given by : mean + 1
,65 see equals 1 degreesC. The mean dtbu increase must then be lower than 0
,7 degreesC (1 - (1,65.0,18)) which corresponds to a maximal EPCT increase
(0,7/0,032) of 21 beats per minute (bpm). From our results, it is concluded
that as long as the EPCT value remains equal or below 20 bpm the increase
in oral temperature is lower than 1 degreesC for 95% of the exposed populat
ion, rendering the thermal strain acceptable.
This result has been obtained from recordings out of almost one hundred wor
ks in various working and exposure conditions. The EPCT limit of 20 bpm can
be used safely to define the physiological acceptability of the heat expos
ure at a workplace and, if necessary, to determine TLV. Easy use of heart r
ate recorders means than heart rate can be a reference parameter for the th
ermal strain quantification as well as for the evaluation of physical workl
oad. Indeed, in the conditions of the present field study, the physical wor
k load reach frequently levels which induce high and even hazardous heart r
ate.