Work in the heat and thermal comfort

While working in the heat has decreased in the last thirty years, several recent surveys have shown that heat stress remains a significant complain for almost one every five workers. The most hazardous situations can be characterised by their short exposure time with high physical workload. At the opposite, the huge increase of work situations where complaints focus on thermal comfort explains that prevention has to tackle the most frequently with comfort conditions which satisfy the largest number rather than with hazardous heat exposures.
This booklets reports the physical and physiological aspects of thermal strain and thermal comfort conditions. Its objective is to give the most practical tools to answer the questions of thermal comfort and strain. The second part of this work shows that the recording of heart rate (HR) allows a simple determination of safe exposure limits. In this view, the increase of rest HR due to the thermal strain (EPCT) were recorded on 98 workers during 131 actual work periods representing 18 different work situations. The EPCT are the difference in beats per minute (bpm) between sitting at rest HR before the exposure and the mean HR of the 3rd , 4th and 5th minutes of rest after the end of the exposure. Oral temperature was considered as a good evaluation of core temperature. Variation in oral temperature (dtbu) from the beginning to the end of the heat exposure (dtbu) is linked to EPCT through the relationship: dtbu = 0,04 + 0,029 EPCT. This relationship is significant (p<0,001) and allows to define an EPCT limit of 20 bpm which implies that the increase in the oral temperature is lower than 1 C rendering the thermal strain acceptable.
This result has been obtained from recordings out of almost one hundred workers in varied working and exposure conditions. The EPCT limit of 20 bpm can be used safely to define the physiological acceptability of the heat exposure at a workplace and, if necessary, to determine TLV. The ease of use of the heart rate recorders makes heart rate a reference parameter for the thermal strain quantification as well as for the evaluation of the physical workload at which heart rate can become hazardous.