On the Effect of Sex on Heat Strain During Prolonged Work in the Heat

Abstract

Workers in many industries commonly experience excessive elevations in core temperature and heart rate (heat strain), which can compromise productivity and safety. These elevations in heat strain could be exacerbated in women, due to impairments in evaporative heat loss (sweat evaporation) relative to men, which exacerbate body heat storage during semi‐nude, short duration (30 min) exercise at a moderate metabolic rate (~200 W/m2). However, in an occupational setting, work is often performed for prolonged periods (>180 min) at a similar intensity (on average) while wearing protective clothing that can impede sweat evaporation. As such, the extent to which such sex‐related impairments in heat dissipation may be exacerbated during prolonged, moderate‐intensity work while wearing protective clothing remains unclear. This is a critical knowledge gap, as such information is necessary to inform sex‐specific occupational heat stress guidelines, which are currently unavailable. We therefore sought to evaluate whether core (rectal) temperature and heart rate (% heart rate reserve, HRR) would be exacerbated in women relative to men during prolonged, moderate‐intensity work in non‐heat stress and high‐heat stress conditions while wearing standard work clothing. To evaluate this hypothesis, 15 healthy women (18–42 years) and 19 men (18–45 years) completed two trials (separated by >48 hours) involving 180‐min of treadmill walking at a moderate metabolic rate (200 W/m2) in a non‐heat stress (21.9°C, 35% relative humidity; wet‐bulb globe temperature [WBGT] 16°C) and high‐heat stress (41.4°C, 35% relative humidity; WBGT 32°C) environment, while wearing single‐layer cotton coveralls (~1 Clo). Trials were terminated upon completion of 180‐min work, volitional fatigue, or a core temperature ≥39.5°C. Mann‐Whitney U‐tests were used to identify between‐group differences in work duration, while unpaired t‐tests (2‐tailed) were used to assess between‐group differences in peak core temperature and HRR (average of the final 5‐min of work). All participants completed 180 min in the WBGT 16°C however, in the WBGT 32°C, 7 men and 7 women terminated prematurely. Nonetheless, median (range) work duration was similar between groups (women: 158 min [75 – 180] vs. men: 145 min [78–180]; p=.79). Peak core temperature (mean [SD]) was similar between women and men in the WBGT 16°C (37.8°C [0.3] and 37.6°C [0.3], respectively; p=.07) however, contrary to our hypothesis, this response was also similar between men and women in the WBGT 32°C (38.8°C [0.4] and 39.0°C [0.4], respectively; p=.23). Peak HRR was also similar between women and men in the WBGT 16°C (47% [11] and 43% [16]; p=.44) and WBGT 32°C (81% [13] and 86% [16]; p=.41), respectively. Our preliminary findings indicate that sex‐related impairments in heat loss (previously observed in dry heat) do not appear to exacerbate heat strain in women relative to men during prolonged, moderate‐intensity work while wearing standard protective clothing. However, whether a given level of heat strain is equally detrimental to the health of women and men has yet to be determined, and this should be considered prior to implementing or dismissing sex‐specific guidelines for industrial workers.Support or Funding InformationThe Government of Ontario.