Introducing the Heat Hangover

Figure 1. Symptoms reported by Queensland based heat-exposed workers on a daily or weekly basis from October to April

Google ‘heat-related illness’ and you’re likely to see an identical spectrum of illness cited on each website: heat stroke, heat exhaustion, heat syncope, heat cramp and heat rash (listed in order of seriousness). Interactions with heat-exposed workers over the past 12 years have identified a disconnect between this spectrum and self-reported symptoms as a result of prolonged occupational heat exposure, a view supported by heat stress survey data. For example, from 918 outdoor workers based in Queensland, Australia, fatigue, headache and irritability were frequently reported on a daily or weekly basis during October to April period (Figure 1; Rogerson et al., 2020). Similar findings were reported for outdoor workers across Northern Australia during October to December (harshest three months for thermoregulation) (Carter et al., 2020).

The similarity of symptoms and delayed onset (latency period between heat exposure and fatigue, headache and irritability symptoms) to an alcohol hangover, has led to this condition being termed a ‘heat hangover’ (Figure 2; Brearley, 2016).

Figure 2. What a heat hangover may look like

Given that heat exhaustion is defined by the inability to effectively exercise in the heat (Casa et al., 2015), it is differentiated from a heat hangover as it occurs at a given point in time. Furthermore, heat exhaustion symptoms typically resolve promptly with proper hydration and cooling (Glazer, 2005). Conversely, heat hangover symptoms persist beyond the work shift.​​

The workplace consequences of heat hangovers are yet to be fully understood. Areas of interest include vigilance, concentration, decision making, execution of physical skills and susceptibility to additional heat hangovers during subsequent work shifts. Negative impact on these and other factors may explain the prevalence of workplace accidents during the hottest months of the year (Xiang et al., 2015) and during severe and extreme heatwaves (Varghese et al., 2019). The impact of heat hangovers is not limited to work, with extended recovery periods for workers in their home environment following 7-28 day work swings where heat hangovers were prevalent.  ​

Figure 3. Core temperature comparison of two underground workers

We are working on identifying the contributing factors. At this stage, it appears that heat hangovers are associated with the area under a worker’s core temperature curve. For example, the blue and green curves of Figure 2 depict the core temperature curves for two workers across a 12-hour shift. Note that the curve of Worker B (Blue) reaching 39.1ºC at ~300 minutes resulting in his immediate cooling and redeployment to a less heat exposed role for the remainder of the shift. This strategy assisted in keeping his core temperature relatively low for the second half of shift whereas the core temperature of Worker A (Green) remained elevated during this period. Overall, the area under the core temperature curve was ~37% higher for Worker A, likely contributing to his heat hangover symptoms at the conclusion of the shift. 

References

Brearley M (2016). Preliminary evidence of a heat hangover, a new heat illness classification for occupational settings? Proceedings of Science of Sport, Exercise and Physical Activity in the Tropics, Townsville.

Brearley M, Harrington P, Field E, Oppermann E, Lee D (submitted). Impact of hot and humid work conditions on perceived heat stress symptoms and management strategies.

Carter S, Field E, Oppermann E, Brearley M (2020). The impact of perceived heat stress symptoms on work-related tasks and social factors: A Cross-Sectional Survey of Australia’s Monsoonal North. Applied Ergonomics. 82:102918

Glazer JL (2005). Management of heatstroke and heat exhaustion. American Family Physician. 71(11): 2133-40 Xiang J, Hansen A, Pisaniello D, Bi P (2015). Extreme heat and occupational heat illnesses in South Australia, 2001-2010. Occupational and Environmental Medicine 72(8): 580-86