It is not always possible in hot environments, to determine safe work-rest regimens based upon heat stress criteria. Is it then a good health policy to rely upon the self-pacing of the workers? To address this question, we observed the spontaneous work-rest cycles of seven masonry workers allocated to the maintenance of a float-glass furnace and the workers' heart rate and rectal temperature were continuously monitored. Results showed that the regimens adopted by the workers were poorly related to physiological parameters. The duration of each heat exposure was significantly related to the worker aerobic capacity, but not to the heart rate level reached at the end of the working period.
Due to self-pacing of the task, heart rates and rectal temperatures remained within acceptable limits for all workers but one, even though these workers were rather old and had a rather low physical capacity. These favourable results may be ascribed to a spontaneous limitation of the effective working time to about 25% of the shift duration and to the fact that subjects worked, in pairs alternately, to perform the task. It is concluded that self-regulation of the work-rest cycles can be an effective means to protect the workers exposed to hot conditions from an excessive physiological strain, providing that the task has no urgent character and does not involve productivity incentives, and that the workers are well trained to their job.