Environmental monitoring

For the IFs or IOC to accept a bid for an international event, and for the athletes and their entourage to prepare appropriately for a given sport event, they should have access to the historical weather information for the location of the event. The environmental parameters should also be recorded throughout the event to allow the IFs to adapt the implementation of their heat policy accordingly (see further). In addition, the different competition surfaces and the environment surrounding the fields of play (FoP) induce diverse microclimatic environments16 17 that may differ from established weather stations.18 Thus, the measures should be representative of the FoP,19 at approximately 1.2–1.5 m above the surface (or at a depth of 40 cm for water temperature) to represent the conditions experienced by the athlete (figure 1),20 and should include all the parameters required by the IF to implement their heat policy. The instruments should be in a fixed position such as on a tripod (and not handheld) and use well-established sensor technology21 that provides accurate and stable measurements with sufficient resolution to meet decision-making thresholds. Measurement variability due to independent factors, such a fluctuation in an air temperature measurement due to intermittent wind, should be avoided. Data should be sampled in real-time and time-averaged appropriately for each individual variable (eg, longer for black globe temperature than ambient air temperature). Sensors should also be maintained and calibrated according to manufacturer specifications and regularly inspected. In the case of the wet bulb globe temperature (WBGT), there are many commercially available options, but most do not respect the original standards. For example, WBGT requires a true measure of natural wet bulb temperature, a dry-bulb thermometer shaded by a structure eliminating effects of thermal radiation and with mechanically aspirated ambient air and a black globe with a diameter of 15 cm. Although most portable devices do not match those specifications (eg, no aspirated ambient air temperature, smaller black globe), it is recommended to use a device classified under category 1 (have black globe and natural wet bulb temperature sensors) or category 2 (have a black globe and humidity sensor) by the Japanese National Institute of Occupational Safety and Health, with an accuracy within of±2°C (online supplemental appendix 1).22 23 The provision how the environmental measurements are taken (including sampling rate and average span) and who records the measurements should be clear, as well as the mode of communication and the recipient of this information. An example of the standard operating procedures drafted for the 2020 Summer Olympic Games is included in online supplemental appendix 1. Of note, while IFs are encouraged to develop their own risk analyses due to the WBGT limitations (see below), WBGT is currently used by several IFs to guide their heat safety guidelines and recommendations7 and was measured by several IFs during the 2020 Summer Olympic Games (including tennis, triathlon and athletics). Among IFs, there is no common WBGT-based limit/regulation. Human heat balance studies show that even under the same environmental conditions (eg, identical WBGT or temperature/relative humidity), factors like metabolic heat production (eg, marathon vs archery), clothing/equipment worn and air movement (natural or self-generated wind speed; eg, running vs cycling) that vary among sports can affect heat stress.24–28 Studies have shown the effectiveness of using WBGT-based event modification decisions to reduce the risk of exertional heat illness29 and to identify resources (eg, volunteers, equipment and consumables) required at athlete medical stations.30

• Download figure
• Open in new tab
• Download powerpoint

Figure 1

Environmental condition monitoring and reporting. Panels A and B: devices specifically developed for World Athletics and for the Australian Open Tennis tournament, respectively. These are the recommended solutions. Panels C and D: commercially available units used on the field of play. It is not recommended to use a hand-held device (panel C) when the device was stored in the shade between measurements as it led to a 2°C error compared with the stationary device. The device should be fixed on a tripod prior to the event to allow for some time to equilibrate to the environment for accurate measures (panel D). Panel E: the display of wet bulb globe temperature (WBGT) monitor and risk category prepared by the Tokyo Organising Committee of the Olympic and Paralympic Games. This display may confuse the athletes as most do not know the WBGT and may misinterpret it as air temperature. Furthermore, the colour coding was different from what has been implemented by various international federations. Panel F: colour-coded 1–5 Heat stress scale developed for the Australian Open Tennis tournament by the University of Sydney. It provides the actual index (value) and the corresponding recommendation. This feedback method provides a clear message for athletes and is recommended.

Ice supply

Ice is used at different time points (eg, pre-event, during and postevent), in a different format (eg, cold water immersion (CWI), ice-towels, ice-socks, crushed ice and ice cubes) and by different stakeholders (eg, sports teams, athletes, officials and medical personnel). It is therefore important to plan for the provision of a sufficient quantity of medical ice. For example, a single medical ice bath requires 30–35 kg of ice at first plus continuous renewal to maintain the ice bath temperature between 5°C and 15°C.

During the 2020 Summer Olympic Games, the quantity of ice was determined based on: (i) venue setting, (2) competition schedule, (3) the number of athletes, (4) the number of medical ice baths and recovery ice baths, (5) the number of FoP medical tents and (6) empirical analysis of the amount of ice used in pre-Olympic events. Figure 2 summarises the ice provision by sport during the 2020 Summer Olympic Games. These numbers would need to be adapted to the event configuration and environmental conditions. It is the Local Organising Committee’s (LOC) responsibility to have a contingency plan to supply additional ice in times of urgent need, as was the case for the race-walk competitions at the 2020 Summer Olympic Games. In addition to the total of >22 tons of ice used at the competition venues during the 2020 Summer Olympic Games, ice consumption reached as much as 2200 kg per day for the Athlete Village Polyclinic. Moreover, there were three ice machines located in the polyclinic in the Olympic Village, and the athlete residences were also provided with at least one ice machine; each machine producing 1200 kg of cubed ice per 24 hours. Moreover, at the opening of the Olympic Village, 42 tons of cubed ice were delivered and stored in the large ice containers in the residences of each building. Some teams stationed outside the Olympic Village also secured their own ice-making machines in their camps.

• Download figure
• Open in new tab
• Download powerpoint

Figure 2

Ice provision per sport during the 2020 Tokyo Summer Olympic Games. Panel A: values in kilogram per venue. It does not account for the ice provided at the polyclinic and the Olympic Village. Panel B: example of ice storage facilities during the event.

Removing regulatory and technical limitations for in-competition hydration and cooling

In-competition hydration is limited by various constraints, including access to fluids. Therefore, several IFs have implemented changes to their rules, often known as a heat policy, which allows the official in charge to alter the competition format to facilitate athlete hydration and cooling. For example, FIFA rules allow a hydration and cooling break after 30 min of play in each half of a football (soccer) match; the International Tennis Federation rules allow an additional 30 s for each change over in a tennis match as well as a 10 min break after the second set; the Union Cycliste Internationale rules allow one car-feeding during a cycling time trial; the World Athletics rules allow for the installation of a refreshment table on the track for 5000 m and 10 000 m races; and World Triathlon improved the number of aid/drink stations numbers during the run course with a maximum distance of 1.25 km between aid stations. Some adaptations, however, require advanced planning by the LOC, such as enlarging a road section along a cycling road race course to accommodate a feeding station.

Of note, while one could argue that there is no need for an athlete to have access to more fluids than 1 L/hour due to the limited gastric emptying rate (see below ‘Fluid intake in competition’),31 the provision of in-competition feeding/refreshment stations also supports the midevent athlete cooling strategies. Indeed, a study conducted during the 2019 Athletics World Championships showed that most athletes (93%) had a midcooling strategy, consisting mainly of head/face water dousing (65%) and cold-water ingestion (52%).32 The LOC is also an important stakeholder in assisting athletes in their planned precooling methods. For example, athletes often use ice vests32 for precooling, a technology requiring a phase change material to be maintained in a frozen state before use. However, security measures may impede a team from installing a freezer in the secure competition zone after the security sweep. It is therefore paramount for national teams and IFs to agree ahead of time with the LOC if any structural changes are required (eg, providing power for the cooling devices) and how to accommodate the security provisions. However, personal preferences (eg, specific hydration and cooling products) are not an LOC responsibility.

Adapting competition distances and durations

A common belief is that shortening an event may minimise the risk associated with heat stress in athletes. Though it is true that short-duration events such as sprinting are not negatively impacted by heat stress,33 this does not apply within prolonged events of different durations. For example, the organiser of the Tel Aviv marathon cancelled the full marathon in 2013 due to a heat wave but authorised the half-marathon resulting in one death and 20 hospitalisations.34 Indeed, the main determinant of heat strain level in athletes is the exercise intensity, and it has been shown that elite cyclists actually reached higher core temperature during a 40–45 min time trial than during a road race of several hours.9 As such, the Falmouth Road Race, a running event of only 7 miles (11 km), is notorious for its very high rate of EHS.35 Moreover, while shortening an event is technically possible in some sports (eg, the running of the women’s triathlon Olympic test event was shortened from 10 km to 5 km in 2019), this may not be feasible in other sports (eg, a cycling finish line requires time to be designed, secured and installed).

In sports structured to permit extended or additional pauses (eg, tennis, soccer and sailing), introducing more structured breaks and/or longer breaks is a relatively common feature of extreme heat policies in these sports. However, relatively few physiological studies have assessed optimal break frequency and duration across different sports. Nevertheless, it has been shown that while short (90–180 s) breaks alone provide a limited cooling effect if athletes cannot be relocated to a cooler environment,36 such breaks enable the application of active cooling strategies such as ice towels and additional opportunities for rehydration with cool water.37 38 In the context of football (3 min breaks after 30 min play in each half) and tennis (standard 90 s breaks after every odd numbered game, and a 120 s break after each set), cooling strategies during these breaks have successfully attenuated the rise in core temperature.37–39 Extending half-time breaks in soccer by 5 min38 and in rugby by 8 min36 have also been shown to permit greater reductions in core temperature relative to regular half-time durations.

Mist

Spray mist cooling using misting fans is a cooling strategy that involves vaporising droplets of water into the air. These droplets evaporate and cool the air by removing heat via the phase change of liquid water to water vapour.1 Studies have shown that spray mist cooling can reduce ambient air temperatures from 1°C to 12°C, depending on the mister design, environmental conditions such as humidity and air movement and proximity to the mister.40 Strong air movement can reduce the effectiveness of spray mist cooling by dispersing the droplets and cooling tends to be larger in drier environments, although it has also been effectively used in humid climates.40 Thus, optimal cooling seems to be obtained when operating misting fans at an ambient air temperature >30°C, relative humidity <70% and wind speed <3 m/s.40 The use of misting in poorly ventilated enclosed spaces may increase ambient humidity, which in turn could attenuate evaporative cooling from the skin and paradoxically worsen heat strain. While most of these studies have been conducted in urban environment designs, misting fans have also been reported to be a very efficient half-time cooling strategy in rugby league.36

Spray misting can be used to reduce heat stress in mass gatherings.41 42 For someone exercising, misting fans could also enhance evaporative heat loss from the skin without additional perspiration1 and have been reported to improve thermal comfort in sport and leisure settings.36 40 As such, misting was implemented in some warm-up areas during the 2020 Summer Olympic Games (eg, rowing; figure 3). It has also been reported that some race-walkers may choose to use a misting shower.43 However, misting showers are generally not implemented in-competition as: (i) their cooling effect may be limited in athletes who have already reached full skin sweat coverage; (2) the exposure time would be very limited; and (3) they pose a variety of concerns including water-soaked shoes and socks, an increased risk of accidents during sports such as in cycling due to a slippery road surface and the interference of vision for athletes wearing glasses.

• Download figure
• Open in new tab
• Download powerpoint

Figure 3

Example of heat mitigation strategies by the Tokyo Organising Committee of the Olympic and Paralympic Games. Panel A: misting of the boat preparation area. Panel B: shading of the warm-up area. Panel C: shading of the field of play. Panel D: shading of the mixed-zone. Panel E–F: cold bath for recovery at a sport venue and at the polyclinic, respectively.

Surface colour and material

Mega-events are organised in large cities where ambient air and surface temperatures are several degrees higher than in surrounding rural areas; a phenomenon called an 'urban heat island'.44 While the built environment (eg, tall building) can shade the direct short-wave radiation from the sun, open spaces are also required to dissipate the heat trapped between buildings.45 Similarly, while the use of high albedo/reflection materials has been promoted to decrease the surface temperature, the reduction in air temperature may not offset the increased radiant load, manifested as a greater mean radiant temperature, that reflective surfaces induce.46 47 For exercising athletes, it is important also to consider the potential effects of lower wind velocity due to the surrounding buildings48 and spectator stands on their potential for heat dissipation.

Shading

Direct solar radiation has a profound influence on the level of prevailing heat stress in an outdoor environment. Black globe temperature, which is used to help determine the mean radiant temperature—one of the four fundamental environmental parameters that define a thermal environment—is often 10 to 15°C higher than the ambient (shaded) temperature in the middle of a clear summer day. The subsequent radiant heat load can exacerbate physiological heat strain and be detrimental to exercise performance.49 50 Moreover, as for heat stress,51–54 high levels of solar radiation, especially when directed towards the head, may also affect complex motor–cognitive tasks.55 56

Shading, provided by lightweight utility canopies/awning or customised sails, is a simple and effective intervention that mitigates the negative effects of the additional heat load from thermal radiation.57 While for most sports shading cannot be applied on the FoP, it serves as a key heat stress mitigation measure for athletes during mandated rest breaks in play and/or for athlete warm-up and cool-down areas (figure 3). Shading coverage should be sufficient to accommodate the number of players on the FoP for the given sport, permit natural ventilation from prevailing winds from any direction to maximise convection and preferably constructed of material with reflective properties.1 Shading is also important for officials, staff and volunteers exposed to the sun on the FoP. When shading is limited, a rotation in work shifts should be planned to allow adequate relief time from the heat for these personnel.

Recovery and postcooling

Athletes commonly use CWI for recovery, and their use postcompetition is common, especially following heat stress.58 However, their benefits during training have been challenged due to a potential delay in muscle recovery and training adaptation.59 60 Recent observation from the 2019 Athletics World Championships showed that more than half of the endurance athletes planned CWI postrace (unpublished). CWI for recovery is an essential complement to the medical CWI for EHS but has a different rationale. Appropriate triage will direct all athletes with minor heat exhaustion to the recovery ice bath, allowing to spare the medical ice bath within the heat deck for the EHS.61

For example, while the recovery area is under the Sport Department, which is independent from the Medical Department within the Olympic Games operational structure, it appears important for the efforts to be coordinated between the two departments to avoid ice shortage (see previous) and assure that the recovery area may support the clinical needs. It is necessary to supervise (and reassess) the athletes using CWI themselves to recover from heat exhaustion as their status may change to EHS. Thus, a clinician (eg, physiotherapist and athletic trainer) should be supervising the recovery CWI area and have a radio to call the medical station in case of an athlete who was initially triaged as minor heat exhaustion deteriorates and requires transfer to the medical station. In the event of an athlete suffering EHS in a sporting venue without a heat deck, the medical team may also choose to annex the recovery area for emergency cooling (figure 3).

Acclimation

Hot ambient conditions limit an athlete’s capacity to dissipate the heat produced by the contracting muscles in the environment, therefore imposing thermal and cardiovascular stress limiting exercise capacity.6 66 Fortunately, repeated exposures to heat allow for specific physiological adaptations allowing to minimise this thermal and cardiovascular stress via an enhanced sweat response, a lower decrease in plasma volume, electrolyte conservation and a range of other adaptations described previously.67 68 This process, to train in the heat, is referred to as heat acclimation when using simulated environments such as hot rooms, saunas or baths and heat acclimatisation when exposed to naturally hot areas/environments.69 The term ‘acclimation’ is used throughout this document as a generic nomenclature for both. Generally speaking, a ‘heat acclimation session’ is when an individual exercises in the heat with a core body temperature between 38.5°C and 39.8°C for at least 60 min,70 along with elevated skin temperatures, sweating and skin blood flow.7 Heat acclimation is considered one of the primary countermeasures to protect the athletes’ health and performance in hot environments.7 Research studies have consistently reported that performance in the heat increases after days/weeks of training in the heat,71 or that athletes are less likely to suffer a heat-related medical event when they are acclimated to hot ambient conditions when required to do intense exercise.65 72 The physiological and performance benefits of heat acclimation have been extensively covered in numerous recent narrative reviews,67 73 meta-analyses,68 consensus statements,7 15 Delphi consensus74 and practical recommendations.70

Tapering and maintaining acclimation

While the principles of heat acclimation are relatively well understood, and it has clearly been shown that performance is increased after 1 or 2 weeks of training in the heat,71 several athletes are still reluctant to modify their usual precompetition training plan (eg, altitude camp, taper period and travel arrangements) to accommodate heat acclimation just before a competition. Given that the decay of acclimation appears to be slower than its induction,76 those athletes could acclimate up to 1 month in advance and then briefly reacclimate for a few days before competing.77 78 Indeed, a subsequent acclimation stimulus within a month of a first stimulus induces faster and greater adaptations.76 79 It is also possible to minimise the decay in the acclimation phenotype between the heat-acclimation period and the competition by maintaining regular active (or even passive) heat exposures.75 78

Hydration principles

Healthy humans are generally well hydrated80 with a day-to-day variation in total body water of 0.2%–0.7% of body mass.81 82 However, the rise in sweat rate when exercising in the heat can dramatically change this fluid balance. A reduction in total body water is associated with a decrease in plasma volume and an increase in plasma osmolality83 that will increase the core temperature threshold for vasodilation and sweating,84 85 reduces the sweat rate for any given core temperature86 and decreases cardiac filling and challenges blood pressure regulation.87–89 Dehydration will therefore exacerbate the rate of heat storage and cardiovascular strain imposed by exercising in the heat, thus reducing exercise heat tolerance.90–92 Given the potential limit in accessing and ingesting fluid during the competition (see further), the general principle of hydration should be to avoid hypohydration (less than 2% body mass) during the periods of training in the heat, especially immediately before and during intense training and competition exercise in the heat. Any increase in fluid consumption should be progressive and not limited to competition day as an acute increase in fluid absorption may first increase the urine production.93 The fluid needs for an athlete training in the heat are highly individual but consistent within the individual when influencing variables are consistent. Fluid needs should be adapted based on changes in body mass and/or plasma osmolality and/or urine specific gravity. Hydration measures should be obtained in the morning to avoid confounding effects of recent diet and exercise.94 The following examples can also be used as a general guide at first before individualised data are ascertained: 6 mL of fluid per kg of body mass every 2–3 hour,7 or 2 L per day plus 1–2 L per hour of exercise plus 1 L for each 5°C increase in ambient temperature above 21.5°C.95

Fluid intake in competition

Sweat rate and body mass changes are highly variable between athletes participating in a given sport32 96 97 making any recommendation on an absolute fluid quantity complex.7 98 Sweat rates will also change for a given individual depending on the environmental conditions (ie, temperature, humidity, wind and sun exposure), metabolic rate, clothing worn and heat-acclimation status.63 99 While some athletes may sweat ~1 L/hour, others may sweat >3 L/hour,96 97 100–103 an amount likely higher than what athletes can absorb during exercise. Indeed, fluid consumption has been estimated at ~0.5–0.7 L/hour for the elite marathoner and ~1.1 L/h for the elite race walk (20 km).32 104 Even if athletes, such as tennis players (ie, frequent breaks) or cyclists (ie, carrying their water bottle), have more opportunity for hydration, there is also a limit in gastric emptying rate.31 While emptying rate may be higher in some large athletes, values of ~1 L/hour have been reported while exercising in the heat.105 Therefore, hydration should not aim to fully compensate for the sweat loss during competition but rather to limit the level of dehydration.7 98 To do so, while there is a debate regarding the need to follow a hydration plan versus drinking to thirst,106–108 the vast majority of elite athletes have a hydration strategy based on personal experience.32 Body mass changes are however highly variable, even between athletes of the same level of participation in the same competition.32 Lastly, hydration does not have to be linear through time. For example, a cyclist may focus on early hydration to account for the delay in absorption, allowing him or her to limit fluid intake during the final climb for body weight advantage. Personalising in-competition hydration strategies, however, require accounting for the rules of the sport, fluid storage and transportation logistics. Furthermore, beverage temperature becomes an important factor since cool beverages (10°C–15°C) are reported to increase the volume of fluid consumed over a given period of time and improve performance.109

Rehydration for recovery

Rehydration is an important component of recovery, especially after exercising in the heat. When a rapid replenishment is wanted, athletes have been suggested to consume 150% of body mass losses in the hour following exercise cessation.110 111 However, such rapid rehydration is infrequently necessary and is not always feasible considering gastrointestinal limitations. In most cases, athletes may replace 100%–120% of body mass losses.7 Postexercise rehydration is achieved through both fluids and foods and should also account for the other losses during exercise such as electrolytes (see next paragraph), carbohydrate112–114 and amino acids.115 For example, for lactose tolerant individuals, chocolate milk is an appropriate recovery drink with a carbohydrate-to-protein ratio of 4:1, contains sodium116 and may also better restore fluid balance after exercise than a standard carbohydrate-electrolyte sport drink.115 117

Electrolytes

Sweat includes electrolytes, mainly sodium118 119 and therefore sweat loss cannot be compensated by water alone. ‘Heavy’ and ‘salty’ sweaters have been recommended to include sodium in their diet before, during and after exercising in the heat (eg, 3.0 g of salt added to 0.5 L of a carbohydrate-electrolyte drink).7

The sports drinks provide moderate levels of key electrolytes (eg, sodium) to help replace sweat losses and increase the voluntary intake of fluid. Timing of the use of electrolyte supplements can include pre-exercise hyperhydration, sometimes beneficial before a race in hot conditions.98 Symptomatic exercise-associated hyponatraemia (blood sodium <125 mEq/L) can occur in endurance events. Contributing factors to exercise-associated hyponatraemia include overdrinking of hypotonic fluids and excessive loss of total sodium before, during and sometimes even after the event.120

The Institute of Medicine80 has recognised that public health recommendations to limit sodium ingestion should not be applied to individuals with elevated sweat loss due to exercising in the heat. While exercise-associated muscle cramps are mainly due to premature muscle fatigue121 and not salt depletion,122 exercise in the heat may also promote muscle cramping in some athletes,13 probably when the sodium deficit reach 20%–30% of the exchangeable sodium pool.123 Thus, most athletes should include a solution with 0.5–0.7 g/L of sodium in their hydration plan when exercising >1 hour.111 124 125 Sodium supplementation may be increased to 1.5 g/L for athletes prone to exercise-associated muscle cramping in the heat,126 or rather 1.5 g/hour as organoleptic properties of beverages are affected if the sodium concentration is >1 g/L. As for postexercise recovery, this can be achieved through a combination of fluids and solid foods. Consuming beverages with sodium and/or a small amount of salted snacks or sodium-containing food at meals will help to stimulate thirst and retain the consumed fluids.127

Carbohydrate

Energy availability may not be the limiting factor when exercising in the heat,128 as high ambient temperatures reduce the overall capacity to perform a prolonged exercise.129 Fluid ingestion during exercise in the heat should, therefore, focus on maintaining optimal hydration status rather than on substrate provision. For prolonged exercise >1 hour in hot environmental conditions, consuming fluids containing diluted carbohydrates and electrolytes (<6% carbohydrates and 0.4–0.85 g/L of sodium) may improve overall fluid consumption due to the increased palatability. Carbohydrate supplementation may include 30–60 g/hour of carbohydrates for exercise >1 hour,125 and up to 90 g/hour for events >2.5 hours.130

Warm-up and precooling

Athletes undertake preconditioning exercises to ‘warm-up’ before competing or even before an intense bout of training. While this activity may increase core temperature and therefore exacerbate the thermal and cardiovascular stress during prolonged exercise, an increase in muscle temperature may conversely benefit muscle contractility.66 Importantly, the effect of warming-up are both temperature131 132 and non-temperature133 134 dependent. Indeed, a preconditioning activity will begin the metabolic and circulatory adjustments135–138 and may induce a postactivation potentiation139 along with psychologically preparing for the upcoming task.140 These various effects have been reviewed with recommendations made elsewhere on the warm-up structure.66 134

Clothing

Ultraviolet rays (UVRs) induce a range of skin responses141 and are the main aetiological agent of skin cancers.142 UVR exposure can be minimised with appropriate clothing (including head and neck protection).143 To this end, sun-protective clothing made from lightweight fabrics that absorb or reflect UVR has become a standard in outdoor activities. Garments are designed to cover as much skin as possible (eg, back of the neck) and fit loosely to enable better convective heat exchange with the clothing microenvironment that sits between the skin surface and inner clothing layer. Similar to the sun protection factor (SPF) rating of sunscreen, the ultraviolet protection factor (UPF) indicates how many units of UVR are blocked.144 145 A UPF of 25 means that only 1/25 (4%) of UVR penetrates the fabric, so the higher the UPF, the greater the protection against UVR. Fabrics with UPF <15 are not considered protective while UPF >50 provides minimal additional protection. However, while protecting from external environmental factors such as UVR, clothing can disrupt heat transfer from the skin surface and thus impose a thermoregulatory burden.146–148 Therefore, while staff and officials should use clothing for sun protection, this strategy needs to be adapted depending on the metabolic heat production of the athlete. Importantly, the main avenue for heat dissipation during heat exposure and/or exercise is sweat evaporation. It is therefore recommended that clothing should impair sweat evaporation as little as possible, and the exposed skin area available for evaporative heat loss in athletes is maximised.15 93 For summer sports where the potential for sweat evaporation may be reduced by protective equipment, the heat stress risk of non-heat acclimated athletes can be managed by permitting a progressive period of heat adaptation with minimal equipment coverage over several days before full protective equipment ensembles are worn.74 The common practice of some sports such as using large ‘bibs’ (ie, sleeveless shirts with numbers and logos for identifying and commercial purposes) may also have to be adapted to provide the athletes with garments of appropriate fabric, size and fitting so as not to impair sweat evaporation.

Despite our better understanding of sweat physiology149 and clothing requirements,150 it remains unclear how different commercially available clothing should be selected depending on the prevailing level of heat stress (eg, tropical vs desert), skin colour, previous exposure/tan and activity.

Sunscreen

Using sunscreen is one of the main preventive strategies to protect the skin from UVR.142 Sunscreen application recommendations are to apply ~2 mg per cm² of protection to exposed skin,151 indicating that adult athletes may need ~15–20 mL of sunscreen per application.152 It has been recommended that athletes use water-based sunscreen (ie, non-greasy) over oil-based sunscreen that may affect sweating.93 However, there are large differences in the sweating response to different water-based sunscreens.153 Sunscreen can work as a chemical sun filter or as a physical sunblock. Chemical sunscreen absorbs into the skin and then absorbs UV rays, converts the rays into heat before releasing from the body. The active ingredients in chemical sunscreens include avobenzone, octinoxate and oxybenzone.145 Physical sunblock sits on top of the skin and reflects the sun’s rays. The minerals titanium dioxide and zinc oxide are the main active ingredients in physical blocks. When comparing an organic chemical sun filter (oxybenzone) and an inorganic physical sunblock (titanium dioxide), both water-based with an SPF 50, Aburto-Corona reported that the latter may impair sweating.153 While the study design was different from real-world utilisation, athletes should be encouraged to use sunscreen when necessary given the importance of sunscreen in preventing skin cancer. Future studies are needed to identify the specific ingredients in sunscreens that interfere with sweating and their interactions with the thermoregulatory response.143

Sunglasses

UVR (<400 nm wavelength) is not necessary for sight but is a risk factor for cataract and macular degeneration and may damage the retina in children.154 UVR can be removed by wearing sunglasses (marked UV400) with a wraparound shape to prevent reflective UVR. Short blue visible light (400–440 nm) is not essential for sight but is a risk factor for the adult human retina and should be removed with specific sunglasses for adults >50 years old.154 Wearing sunglasses when appropriate is recommended on various IOC and IF educational documents.93 Some contact lenses may also absorb those wavelengths.154

FoP supervision

Time to reduce the internal body temperature is the key factor for the success of EHS treatment. Therefore, the FoP medical plan should facilitate the identification of the athletes suffering from heat-related illness and their rapid transfer to a medical station. In the case of endurance sports like race walking, long-distance running, or triathlon, a course that has been planned in laps, may reduce the transfer time to the finish line and/or a medical station. Organisers should however keep in mind that the length of the lap may be influenced by the number of competitors (mass vs elite competitions). For road events spread along a long distance (ie, without lap), positioning medical stations along the route may minimise the transfer time and allow the EHS to be treated immediately (on-site) by trained practitioners as opposed to transferring the patient to the nearest hospital. Thus, the race protocol should supersede the local emergency medical system in case of EHS and should be implemented as a common understanding between the FoP medical and local emergency medical services. Assigning spotters at regular intervals on the competition course to inform the triage medical manager of any athlete in distress (as in the 2019 World Athletics Championships) is helpful. For competitions with many participants or competitions where the visibility is not optimal (eg, buildings, tents, turns, etc) spotter towers may be installed to provide a better view of the finishing area. The spotters should be in radio contact with the triage manager and the sweep team, and there should be a well-practised rescue procedure in place for rapid retrieval of collapsed athletes.

Finish area pretriage

Medical personnel should be deployed along the final few hundred metres prior to the finish line. Ideally, the main medical station should be in the immediate vicinity of the finish line, with direct and unhindered access to the finish area. The majority of the event medical staff should be stationed in the medical station with a FoP team directly observing and with direct access to the finish area. Where transfer time from the finish area to the medical station is short, then athlete transfer can be undertaken using wheelchairs. However, where the medical station is not in the immediate vicinity of the finish area, specifically designated transportation methods (eg, golf carts) should be available to transfer athletes from the course to the medical station using predefined routes and FoP points of entry.

A pretriage to discriminate between exhaustion and EHS should be commenced immediately in the finish area. It should be supervised by a medical professional experienced in the recognition and management of exertional heat illness and a knowledge of the sport.

Although athletes may want to rest (often seating or lying) after crossing the finish line, they should be encouraged and helped to walk immediately to avoid congestion of the area as this could jeopardise subsequent triage. It is important that the finish area is kept clear and the pretriage staff should strive to rapidly triage and move competitors away from the finish area. Following a rapid clinical assessment, athletes showing benign signs of exhaustion should be directed to a recovery area, where they can be further observed, rest, rehydrate and cool down. Athletes with more severe symptoms, such as central nervous system dysfunction (including severe difficulty walking and standing), must be transferred immediately to the medical station and to its heat deck (see further) for a complete assessment/triage.

Finish area care

All competitors should be offered water, ice or ice towels at the finish area. Athletes with central nervous system disturbances should be pretriaged and immediately directed towards the medical station. They should not be encouraged to drink until exercise-associated hyponatraemia has been excluded.61

Although ice packs and ice towels may provide subjective benefits to the users, their effect on both the core temperature reduction and orthostatic intolerance (postfinish line collapse) is limited for the purpose of EHS treatment. The skin surface exposed to cold is too small to trigger a significant core temperature decrease in a short period of time.156 For that reason, ice packs and ice towels should be given to competitors at the finish area for immediate relief from heat stress, but they do not replace more aggressive cooling such as CWI (aiming at cooling internal body temperature and enhance central venous return) for athletes suffering EHS.157

Athletes sent to the recovery area should be accompanied by their entourage or members of the medical team. Walking (or assisted walking) may support the circulation by activating the lower leg muscle pump, recirculating pooled blood in the lower extremities and reducing the incidence of collapse.

Lastly, the finish line area should be kept in order, with no athletes lingering in the area for longer than necessary. At this time, medical volunteers should actively look for early recognition of EHS, as these athletes need to be immediately transported to a heat deck for prehospital treatment. Transfer from the finish line to the medical station is generally managed by wheelchair with recent data suggesting that endurance event organisers should plan 1 wheelchair per five competitors for elite international athletic events in hot and humid conditions.13

EHS prehospital management

EHS is characterised by extreme hyperthermia (>40.5°C) that is accompanied by signs of central nervous system dysfunction (eg, irrational behaviour, altered consciousness and loss of consciousness).158 It is a medical emergency that warrants immediate treatment (ie, whole-body CWI) to ensure survival and minimise the chance of long-lasting sequela.14 158 159 Therefore, medical volunteers must be proficient in: (1) the transfer of the collapsed athlete from the FoP to heat deck, (2) rectal temperature assessment, (3) the transfer of the collapsed athlete to whole-body CWI bath and (4) determination of whether the collapsed athlete requires advanced care beyond heat deck.61 The medical team’s training should be hands-on and include venue and event specific considerations, such as how the transfer to the heat deck will vary depending on the location and timing of the collapse. A practical guideline is presented in table 1, and a detailed list of equipment needed along treatment procedure has been published in the IOC EHS guidelines.61

While not exclusive to EHS, mass participation events should have designated hospitals that will admit collapsed athletes who require advanced medical care (ie, acute cooling of those who are not transported to heat deck, hypothermic overshoot after medical ice bath, follow-up for EHS recovery). Such partnerships will allow the medical organiser to capture the full scope of medical services rendered and medical outcomes from participation in the event.

Athlete monitoring

As mentioned previously, several IFs have heat policies allowing them to alter an event in case of heat stress. However, the decision is currently informed by environmental parameters. Obtaining real-time athlete data would better inform the decision makers on how the athletes are currently coping with the environmental conditions. The technology in this area is rapidly developing,160 with several studies having already recorded thermal responses during world championship events.9 32 The first real-time monitoring occurred during the 2020 Summer Olympic Games (figure 4). Knowing an athlete’s physiological status however, raises an ethical dilemma,161 as there is no ethical or legal grounds for making a decision to withdraw an athlete from a race against his or her will. Real-time physiological monitoring may however inform the LOC and/or IF at the organisational level to determine if an event should be altered when participants appear to have difficulties coping with the extreme conditions.

• Download figure
• Open in new tab
• Download powerpoint

Figure 4

Real-time monitoring of athlete’s thermal responses during the 2020 Tokyo Summer Olympic Games. Panel A: dashboard accessible online allowing to follow an athlete during a race walk event (the route consisted of 1 km loops at Odori Park, Sapporo). Panel B: example of data recorded during the male 10 000 m and being visualised in real time (Olympic stadium).

#1: environmental monitoring

Monitoring environmental conditions will allow an IF to estimate the environmental risk for their athletes, similar to the heat stress monitoring programme that has been implemented by the International Volleyball Federation (beach) for all international competitions since 2009.65 166 Monitoring the environmental conditions is also the first step to inform the implementation of a heat stress policy. However, while all components of any indices (eg, natural wet-bulb temperature, black globe temperature and dry-bulb temperature for WBGT) must be physically measured, they are often estimated using only temperature and humidity with assumptions on cloud cover and wind speed.162 All variables required to calculate the chosen heat stress index should be physically measured (using the recommendations provided previously) and not derived or estimated from other measures.

#2: define physiological/physical parameters

Not all sports present the same thermoregulation challenges due to different heat production (ie, intensity and duration) and dissipation (eg, clothing, air velocity). For example, the relatively high velocity in cycling may allow for air movement around the rider and favour the convective heat loss.71 167 With an ambient temperature of 35°C and relative humidity of 50%, the maximal convective and evaporative cooling capacity of a cyclist at 40 km/hour has been estimated to be 43% higher than a runner at 20 km/hour and 60% higher at 50 km/hour.168 However, such convective advantage will disappear during up-hill cycling, where the racing speed is lower, or when the air temperature is higher than the skin temperature. Risk thresholds should also be adjusted according to the required clothing and protective equipment by the rule of the sport (and the possibility to adapt them).

In addition to the characteristics of the sport (eg, swimming vs cycling vs running vs sailing) and the competition (eg, distance, course), the characteristics of the athletes should also be considered (eg, age and acclimation status). If the chosen heat stress index allows, parameters that broadly define the athlete’s thermoregulatory demand should be integrated when interpretating the index. For example, heat acclimation status profoundly alters the boundary between compensable and uncompensable heat stress secondary to modification in maximum skin wettedness.169 Similarly, injuries such as skin burns, or spinal cord damage, lead to significant physiological impairments to sweating.170 171 If physical parameters such as the surface area-to-mass ratio are distinctly different along with competitors morphology (eg, children vs adults), parameters should be adjusted accordingly.172

#3: define the acceptable level of heat stress risk

The acceptable risk is specific to the population that the policy is designed to protect. Elite-level professional competitors may expect to tolerate (and be appropriately conditioned for) higher levels of heat stress than amateur community-based competitors playing the same sport. Seasonal changes (eg, early summer) or unseasonal weather (eg, heat wave) present additional risks. The readiness of the organising committee and medical services in managing potential cases of exertional heat illness should also be considered (eg, a competition without a proper heat deck61 cannot afford the same level of risk as a competition where exertional heat illness can be safely managed).

#4: generate a risk assessment format that is easy to understand and implement

Even the most evidence-based heat stress risk assessment will be limited in its effectiveness if the output is not easily comprehended or actionable by stakeholders such as athletes, physicians, coaches and event organisers. For example, raw WBGT values are reported with °C units and can be misleading because they are often lower than air temperature (figure 1). Therefore, outputs from any heat stress index should be scaled appropriately in a format that is accessible and familiar. A traffic light (red, amber and green) system or a 1–5 scale (figure 1) is easily understandable as it is used in many circumstances of everyday life from environmental hazards (eg, fire) to rating systems (eg, ridesharing, movies, hotels etc).

#5: incorporate evidence-based heat stress mitigation strategies that are feasible and implementable

Associated with each risk threshold defined by the scale selected in #4, recommendations can be triggered for implementing a strategy that serves to reduce physiological heat strain (online supplemental appendix 2). These strategies are informed by the scientific evidence and may range from recommendations to the athletes to providing extra services (eg, hydration station and cooling bath), modifying the rules of the sport (eg, additional/longer break) and even postponing the event. Equally important, these strategies must be compatible with the setting, in terms of available resources and opportunities for their use in the context of the standard format of the specific sport without unduly interfering with play.

Other methods and verification

The previous five steps are based on a thermophysiological analysis of the sport. This approach was implemented to develop an extreme heat policy for the Australian Open Grand Slam Tennis tournament as detailed in online supplemental appendix 2. It differs from the common ‘sequence of prevention’ of sports injuries that aim to establish first the extent of the problem, then the aetiology and mechanism of the injury, then introduce preventive measures before assessing their effectiveness (by repeating the first step).173 Irrespective of the method employed, it is recommended that IFs work towards developing comprehensive heat stress guidelines that reflect the risks specific to their sports and ensure consistency across competitions within one sport. It is also recommended to assess the effectiveness of the heat policy and to modify it accordingly if required. Lastly, it is recommended to use a 1–5 scale for consistency and clarity in reporting the level of risk and the associated countermeasure (figure 5).

• Download figure
• Open in new tab
• Download powerpoint

Figure 5

Example of a heat stress scale that could be adapted and used across sports. The scale uses a common 1–5 colour-coded rating with the exact value indicated. The current preventive measures to action are openly listed to inform athletes, organisers, medical services and other stakeholders. The listed modifications are non-contractual examples; each IF is in charge of their heat policies.

Limitations

Body temperature has been used as a health indicator for millennia and is still one of the first vital signs measured. The research on the effect of heat on healthy active humans is however much more limited and historically driven by the requirements of the mining industry and the military. While there has been extensive research on exercising in the heat in recent years, most knowledge is derived from laboratory studies using recreational athletes. Therefore, there is a need for research in elite athletes, practising their sports outdoors. This need is further exacerbated by the ethical concerns associated with the technological developments that allow the monitoring of athletes in real time (figure 4).

It was decided for these recommendations to focus on the athlete, their entourage and the event organisers. We have not included spectators in these recommendations, but event organisers are encouraged to seek advice from public health experts and coordinate their effort with the local health services. In some instances, the spectator’s protection is also directly related to the event organiser. For example, the IOC working group agreed with the Tokyo Organising Committee of the Olympic and Paralympic Games to allow spectators to bring one plastic bottle filled with water through security (up to 750 mL to accommodate the 21 oz/600 mL sport bottles). Such negotiation involved all concerned stakeholders (eg, security, sponsors) and was carried out 2 years before the event.

Perspectives

The current document covers a range of topics. For some of them (eg, acclimation), there was already extensive referenced literature available. For other topics (eg, mist, shading), more research is required that replicates the conditions athletes experience.

IFs and event organisers are also encouraged to conduct systematic monitoring of the environmental conditions and health hazards during their event to establish an epidemiological database and inform future policies. Along these lines, it is important to understand how widespread heat decks are used, their effectiveness and the problems encountered. With this information, it will be possible to review and modify the heat deck recommendations based on clinical data in the future.

In addition, while research and scientific publications may be considered the two first steps of risk mitigation, it is also necessary to communicate with the different stakeholders using their own language, including video and social media for athletes, policies for IFs and organisers, medical workshops for clinicians, etc. In the case of the Tokyo Summer Olympic Games, the IOC developed educational material based on the previous leaflet from the Union Cycliste Internationale and World Athletics, using athlete role models. For local medical volunteers, the Tokyo Organising Committee of the Olympic and Paralympic Games hosted specific training on exertional heat illness prehospital management.