Physiological Adaptation to Working in Heat

For the first time in years, outdoor workers performing heavy work in Southwestern Ontario became heat-acclimatized, due to a stretch of high temperatures that met the necessary criteria for physiological adaptation. This was confirmed through field data collected in the Hamilton/Niagara region, where sustained daily heat exposures aligned with Threshold Limit Values (TLV®) recommendations.

According to the TLV® for heat stress, heavy work includes intense arm and trunk activity such as carrying, shoveling, manual sawing, pushing and pulling heavy loads, and walking at a fast pace. These tasks fall within a metabolic workload range of approximately 360 to 470 watts.

Acclimatization for heavy work was reached on July 29th, marking a rare event for the region. However, since then, seven consecutive days have passed without any daily exposures above the heavy work acclimatization threshold (WBGT > 27.5°C). As a result, workers have begun to lose their acclimatization.

Acclimatization Decreases

For those who were fully acclimatized, the process of de-acclimatization is underway. While it is not yet completely lost, under the TLV® framework these workers would now be treated as unacclimatized and would need to re-acclimatize. Fortunately, regaining acclimatization typically takes less time than the initial process, often within two to four days, depending on physical fitness and the length of time away from the hot environment.

Acclimatization was not reached at any point for moderate or light work* categories, and those workers remain at higher risk for heat stress.

We will continue monitoring environmental conditions in the coming weeks to assess whether workers are able to regain acclimatization. This update emphasizes the importance of tracking local heat exposure trends when applying heat stress prevention strategies.
*Please see https://www.ohcow.on.ca/wp-content/uploads/2024/02/heat-stress-risk-factors.pdf for more information on metabolic rates associated with work activities, and what the different types of work are classified as.