Background
Description of the condition
High ambient temperatures and relative humidities increase metabolic heat gain accompanying exercise and are associated with increased physiological strain and reduced physical work capacity (Wendt 2007). Indeed, wherever excessive imbalances occur between the thermal energy stored and that dissipated to the environment, heat-related illness can occur. Often referred to as heat injury, heat-related illnesses describe a range of conditions that include heat rash (miliaria rubra), fluid retention, muscle cramp, fainting, heat exhaustion and heat stroke (Bouchama 2002). In extreme cases, excessive rises in core temperature above 40˚C result in central nervous system dysfunction, cellular death and multiple organ failure (Coris 2004; Glazer 2005; Sharma 2003). The young and elderly may be vulnerable to extreme heat events (Kovats 2008), though heat illnesses associated with physical exertion are also experienced by athletes, manual labourers and military personnel, particularly when not acclimatised (Bouchama 2002).
The health and financial effects of heat-related illness on everyday life as well as occupational and sports settings are rising, as the increased intensity, duration and frequency of heat wave conditions associated with global climate change take effect (Huang 2011; Luber 2008). Chinese data highlight a 4.5% increase in hospital admission rates with every 1˚C increase in mean daily temperature above 29˚C (Chan 2013), with similar trends reported in Australia (Bi 2011) and the United States of America (USA) (Green 2010). Compounding this elevated burden on the healthcare sector, modelling by Dunne 2013 indicates heat stress to have impaired global labour capacity by up to 10% in recent decades, with this likely to double in peak summer by 2020. The economic consequences of this reduced work are marked, with estimated net costs of USD 2.4 trillion by 2030 attributed to heat-related reductions in work productivity alone (DARA 2012).
Although the risk of exertional heat illness may be reduced by using air conditioning, scheduling physical activity in the coolest time of the day and maintaining adequate hydration (Gupta 2012; Michelozzi 2014), accustomed exposure to hot weather is not always possible. For example, variability in extreme weather patterns (i.e. El Niño and La Niña) (McGeehin 2001), inter-seasonal travel (Hanna 2011), geographic location (Grundstein 2015), and the need for protective equipment in occupational, military, and sporting contexts (Cheung 2000; Holmér 2006; Montain 1994) each unavoidably increase thermal strain.
Heat acclimation is regarded as the most effective means of protecting health against thermal strain. It involves a series of (natural or artificial) exposures to hot conditions in order to invoke physiological adaptations that optimise heat loss mechanisms (Taylor 2006). Importantly, enhanced thermoregulatory efficiency achieved through acclimation may maintain work rate in hot conditions (Chalmers 2014; Lorenzo 2010) and is included in health and safety recommendations for various sports, occupational and military populations (CA DoOSH 2015; Racinais 2015; US Army 2003).
Description of the intervention
Heat acclimation involves a series of adaptations that reduce physiological strain in hot conditions by optimising avenues for heat loss. The treatment requires repeated exposures to an elevated body temperature that can be achieved using passive (i.e. non-active heat absorption from the surrounding environment), active (i.e. heat production caused by greater energy metabolism during exercise), or combinations of both methods. Passive heat acclimation methods include using climate chambers, saunas, water baths and vapour barrier suits (e.g. Fox 1963; Scoon 2007; Stanley 2015; Zurawlew 2015). Exercise-induced heat acclimation may be achieved using constant work-rate (fixed duration or controlled hyperthermia) or self-paced exercise protocols that are usually undertaken in environmental conditions that are hot or humid or both (e.g. Armstrong 1986; Garrett 2009; Gibson 2015; Houmard 1990).Typically, these exposures are administered for 30 to 120 minutes and are repeated across multiple days. Accepted definitions of heat acclimation processes are: up to seven exposures (short-term heat acclimation), eight to 14 exposures (medium-term heat acclimation), and 15 or more exposures (long-term heat acclimation) (Chalmers 2014; Garrett 2011). Adaptations to heat exposure is never permanent. According to Givoni 1973, heat adaptation is lost every day spent without heat exposure at a rate that is twice as fast as the rate with which the heat adaptation was initially gained.
Recommendations for athletes to follow heat acclimation protocols have become increasingly common as a means to protect both health and physical performance during major competitions in hot environments (Chalmers 2014; Guy 2015; Racinais 2015). Occupational safety and health concerns regarding heat-related illness in industry (e.g. military, agriculture, construction, landscaping, oil and gas extraction, and transport) have also led to state-legislated standards that emphasise acclimation (or acclimatization) awareness in the USA (CA DoOSH 2015; Washington State Legislature 2012). However, even in the South African mining industry, where evidence for the use of heat acclimation interventions is long standing (Wyndham 1969), consensus on evidence-based best practice remains elusive. Regardless, to optimise the heat acclimation response, all variables (e.g. environmental temperature and humidity, exercise mode, exercise intensity, exposure duration and number of exposures) should be considered within the logistical and economic constraints of the setting.
How the intervention might work
Humans regulate core temperature through changes in autonomic (e.g. sweating and shivering) and behavioural (e.g. work rate) thermo-effector responses (Cabanac 1977; Hartley 2012; Schlader 2009). During physical exertion in hot conditions, heat is gained through both endogenous (i.e. increased metabolism required to complete work) and exogenous sources (i.e. transferred from the surrounding environment) (Wendt 2007). The maintenance of core temperature within a homeostatic range at rest (˜36.8 ± 0.5 ˚C) (Hanna 2015) is mostly achieved via convection of heat to the skin surface and radiation of heat to the surrounding environment (Sawka 1996). However, once core temperature meets or exceeds that of the external environment, sweating provides the main heat loss mechanism through evaporation (Sawka 1996). Relative humidity, air flow and skin exposure to the external environment all influence evaporative heat loss and when compromised (e.g. when wearing personal protective equipment), exacerbates heat strain and leads to people working slower or for shorted periods of time to avoid excessive thermal injury (Marino 2004; Tatterson 2000; Tucker 2006).
Notably, repeated exposures to an increased core temperature achieved via exercise, environmental means, or both, induce physiological adaptations associated with greater thermal tolerance (Armstrong 1991; Garrett 2011; Taylor 2006). Acclimation to a hot environment evokes a complex multi-system response that results in improved rates of heat loss that are associated with cardiovascular, endocrine and nervous system changes (Francesconi 1996). The classic signs of heat acclimation include a lowered heart rate, cooler body temperature (core and skin), and earlier and larger sweat responses to exercise in hot conditions (Sawka 1996). These adaptations may be associated with plasma volume expansion stimulated by repeated heat exposures (Nielsen 1993; Patterson 2004; Senay 1976), which are attained via altered kidney function that maintains body water and electrolyte concentrations (Francesconi 1996). This allows for the maintenance of skeletal muscle blood flow during exercise (Chalmers 2014) and evaporative cooling by sweating (Libert 1983; Nadel 1974), while reducing the risk of dehydration.
The physiological adaptations experienced during heat acclimation markedly enhance thermal comfort (Daanen 2011; Petersen 2010; Sunderland 2008) and lower ratings of perceived exertion in hot conditions (Armstrong 1991; Castle 2011; Pandolf 1977). This is key as the combination of both lowered physiological and perceptual demands allow for longer tolerance times and more work to be completed in the heat (Chalmers 2014; Lorenzo 2010). However, physiological adaptations are variable depending on acclimation mode, duration and frequency, and individual responses will impact subsequent thermal tolerance and physical performance (Lambert 2008; Racinais 2012; Racinais 2014). Moreover, it should be noted that the protective and performance benefits of heat acclimation are limited (Hanna 2015), and in extreme weather, high task motivation or confidence in heat tolerance or both should not come at the expense of appropriate work-rest schedules (Lucas 2014). To guide the scope of the review, we developed a logic model (Figure 1) in accordance with Anderson 2011 that outlines: 1) potential benefits and 2) adverse effects of heat acclimation interventions in alleviating exertional heat stress.
Figure 1.
Logic model describing the potential benefits and adverse effects of heat acclimation
Why it is important to do this review
Exertional heat illness is a major concern amongst applied practitioners (Casa 2015) and its rising incidence rate in physically active populations is likely to be exacerbated as global warming continues (Brocherie 2015; Lucas 2014; Mueller 2012). Considering the health concerns and the associated costs to industry and government, it is important that there are evidence-based guidelines to best inform heat acclimation procedures (Taylor 2006). The effects of heat stress on active paediatric and adolescent populations have been given a lot of attention (Bergeron 2011; Casa 2009; Marshall 2010). However, there are no practical recommendations available that are based on empirical evidence (Armstrong 2007; Bergeron 2005). A recent systematic review and meta-analysis of eight small studies (including six observational and two randomised controlled trials) reported short-term heat acclimation (≤ 7 exposures) to increase aerobic performance outcomes (Chalmers 2014). While Chalmers 2014 has practical implications for athletes and coaches involved in team sports, outcome measures were restricted to athletic performance only. Applications of the heat stress Standard ISO 7243 indicate greater work tolerance of a high wet bulb globe temperature (i.e. a heat stress index that incorporates temperature, humidity and radiation) following acclimation (Parsons 2006), without detailing how this is achieved. Accordingly, questions remain as to the optimal dosage effects of heat acclimation, underlying mechanisms, and the potential for adverse outcomes on health and performance across all populations and contexts (e.g. occupational, military, and sports).