Heat Strain in Middle‐aged and Young Men During Prolonged Work in the Heat
Authors: Friesen Brian J. , Notley Sean R. , McCourt Emma , McCormick James J. , Larocque-Graham André , Boulay Pierre , Kenny Glen P. 
Published: April 2020

Workers employed in laborious occupations often experience excessive rises in core temperature and heart rate (heat strain), particularly in hot environments, which can elevate heat‐injury risk. That increase in heat strain may be even greater among middle‐aged workers, who display impaired evaporative heat loss relative to young adults during short bouts (≤30 min) of exercise at a moderate metabolic rate (~200 W/m2) consistent with the average metabolic demands of arduous work, while wearing minimal clothing in dry heat. However, the extent to which those impairments may worsen heat strain during the more protracted work duration (≥180 min between main meal breaks) associated with arduous occupations and whilst wearing protective clothing, which restricts heat loss, remains unclear. Therefore, we sought to assess core temperature and heart rate in young and middle‐aged men wearing single‐layer cotton coveralls during prolonged work in both non‐heat stress and high‐heat stress conditions, hypothesizing that those responses would be exacerbated in middle‐aged relative to young men in high‐heat stress conditions only. To evaluate this hypothesis, we assessed core temperature (rectal) and heart rate (expressed as a percentage of heart rate reserve) in physically active, but not endurance‐trained, middle‐aged (n=10; 58 [SD 4] years) and young (n=10; 22 [SD 3] years) men. On nonconsecutive days, participants performed 180‐min of treadmill walking at a moderate metabolic rate (200 W/m2) in either non‐heat stress (21.9°C, 35% relative humidity; wet‐bulb globe temperature [WBGT] ~16°C) or high‐heat stress (41.4°C, 35% relative humidity; WBGT ~32°C) conditions with minimal air flow (<0.5 m/sec), while wearing single‐layer cotton coveralls (~1 Clo). Trials were terminated upon completion of 180‐min, volitional fatigue or core temperature ≥39.5°C. Data were averaged over the final 5 min of work and compared between‐groups within each condition using unpaired t‐tests (core temperature, heart rate) or Mann‐Whitney U‐tests (work duration). All participants completed 180‐min in the 16°C WBGT condition. However, while several participants terminated prematurely in the 32°C WBGT condition, median (range) work duration (middle‐aged: 180 min [63 to 180 min]; young: 175 min [78 to 180 min]) did not differ between groups (p=.513). Core temperature did not differ significantly between‐groups during either the 16°C WBGT (middle‐aged: 37.5°C [SD 0.3]; young: 37.6°C [SD 0.3]; p=.573) or 32°C WBGT (middle‐aged: 39°C [SD 0.3]; young: 39°C [SD 0.4]; p=.795) conditions. Similarly, heart rate reserve was similar between‐groups during the 16°C WBGT (middle‐aged: 33.8% [SD 10.1]; young: 36.9% [SD 12.6]; p=.546) or 32°C WBGT (middle‐aged: 76.6% [SD 10.8]; young: 77.6% [SD 11.6]; p=.835) conditions. These preliminary findings indicate that age‐related differences in heat loss (previously observed in dry heat) do not appear to exacerbate heat strain in physically active, healthy middle‐aged relative to young men during prolonged, moderate‐intensity work in hot environments while wearing single‐layer cotton coveralls.Support or Funding InformationFunding: The Government of Ontario