It's very important as physical work rate determines body heat production. In turn, this internal heat production combined with the external heat load of the environment determines worker heat stress. Quantifying the conditions workers are exposed to is relatively simple for a fixed worksite, and achievable through portable monitors/weather stations for mobile sites. Many organisations implement such measures to inform their heat stress management and that's a positive. But what about work rate? In our opinion, the role of work rate in occupational heat stress deserves more attention, so we make this point through the research of Tatterson et al. (2000).
Which of the following indoor work environments is more likely to result in worker heat stress?
Hotter conditions = more heat stress, right?
The 32ºC environment has substantially less potential for body heat dissipation, so similar work rates between conditions will result in greater body heat storage, higher core temperature and heat stress compared to the 23ºC environment. But what if workers adjusted their work rate to compensate for the hotter conditions - could they produce similar heat stress between conditions?
Tatterson et. (2000) studied 11 elite Australian cyclists completing 30-min time trials in the conditions previously described, summarised as 23ºC (green dots) and 32ºC (red dots) on the graphs of Figure 1. Despite the different conditions, core temperatures were similar throughout the time trial (Figure 1A), with slightly higher skin temperature and sweat rate in the 32ºC conditions. The similar overall heat stress was due to the lower work rate (reported here as 6.5% lower cycling power output in Figure 1B) and therefore body heat production, in the 32ºC conditions.
Figure 1. Core temperature and power output of elite road cyclists during a 30-min time trial in 23ºC and 32ºC.
Sure, work-as-normal for the vast majority of workers does not require periods of maximal workload as per this elite cycling research example. Yet, similar to the cyclists, workers that are free to self-pace their effort are constantly adjusting work rate based upon such factors as experience, health and internal feedback. We wrote this about pacing in a paper on electrical utility worker heat stress (Brearley et al., 2015):
"By understanding the tasks required, number of staff available, time frame for completion, anticipated climatic conditions and factoring in their personal experience, physical fitness, and acute physiological status, workers can initiate behavioural and work load adjustments, thereby selecting an appropriate pace to complete the task and prevent excessive body heat storage. It is reasonable to expect that regular exposure of the utility workers to hot conditions has enabled pacing strategies to be routinely applied and modified."
The Tatterson et al. (2000) example highlights the role of work rate in the development of heat stress. While knowledge of expected and actual environmental conditions are important, also understanding the work rate required to complete a task, and that harsher environmental conditions are likely to lower the sustainable work rate and therefore, productivity of individual workers, allows alterations of schedule, redeployment of resources and tailoring of controls to mitigate heat stress.
Brearley M, Harrington P, Lee D, Taylor R (2015). Working in hot conditions--a study of electrical utility workers in the northern territory of Australia. J Occup Environ Hyg. 12(3):156-62.
Tatterson AJ, Hahn AG, Martin DT, Febbraio MA (2000). Effects of heat stress on physiological responses and exercise performance in elite cyclists. J Sci Med Sport. 3(2):186-93.