Keeping Sports Participants Safe in Hot Weather
Phillip B. Sparling, EdD; Mindy Millard-Stafford, PhD
THE PHYSICIAN AND SPORTSMEDICINE - VOL 27 - NO. 7 - JULY 1999
In Brief: Keeping in mind the key concepts of heat dissipation and using sound strategies for heat acclimatization and fluid replacement can help keep participants and spectators safe during hot-weather sports activities. Acclimatization to heat requires 10 to 14 days of training. Prudent hydration involves drinking plenty of fluid 2 hours before exercise, 5 to 10 oz of fluid every 15 minutes during exercise, and fluids with increased sodium content after exercise. A sidebar on environmental conditions and heat-related medical encounters during the 1996 Summer Olympics in Atlanta illustrates the importance of prevention strategies at the individual and event level.
From late springtime to early autumn, warm to hot weather is the norm in many regions across the United States. The medical community is called on to help develop heat-illness prevention plans for people in many situations, from marathoners asking if it's safe to train during a July heat wave to a youth soccer team seeking a good fluid replacement strategy for an upcoming tournament.
A working knowledge of how the body dissipates heat will help professionals tailor their recommendations to patients. Changing weather conditions can have serious safety implications, so knowing how to evaluate environmental conditions is immensely useful. Hydration and acclimatization strategies are important for athletes and spectators. (See "Hot-Weather Event Planning: The Atlanta Olympic Experience," below.) Acclimatization strategies can help athletes compete safely in hot environments. Because hydration is important before, during, and after an event, recommending the right amount and type of fluid replacement is crucial.
Dissipating Metabolic Heat
During the transition from rest to heavy exercise, the heat generated from energy metabolism can easily increase 10-fold in active healthy persons, and up to 20-fold in well-trained athletes. About 80% of this energy is released as heat; only a small proportion is converted to muscular work. To counter heat storage and rising core temperature, metabolic heat is transferred from the core to the skin, then dissipated to the environment (figure 1: not shown).
At rest, overheating is prevented primarily through radiation (emission of electromagnetic heat waves through the air to cooler objects), convection (air movement over the body), and conduction (from the deep tissues to the cooler surface). To a lesser degree, sweat evaporation contributes to thermoregulation at rest. The reverse is true during exercise, when evaporation of sweat becomes the major means of heat transfer, particularly when the ambient temperature is above 68°F. The evaporation of 1 L of sweat releases 580 kcal of heat to the environment.
During exercise in a hot environment, the cardiovascular system adjusts by increasing cutaneous blood flow. Shunting blood to the skin improves heat loss but diminishes the blood supply that provides oxygen to working muscles. Consequently, in hot weather the cardiovascular system must meet the competing demands of thermoregulation and muscle metabolism. This is why record-breaking endurance performances are almost always set under cool conditions.
In mild air temperatures, core temperature during exercise is regulated within a narrow range; the heat load from the higher metabolic rate is dissipated primarily by sweat evaporation. However, the combination of a high rate of internal heat production from exercise and a hot environment doubles thermal stress. The internal-to-external thermal gradient no longer favors the transfer of heat from the body to the environment. When the air is dry, sweat evaporation removes heat from the body, even if the air temperature is higher than core temperature. However, when the air is moist, vaporization of sweat is reduced and cooling is greatly retarded (figure 2: not shown).
Under conditions of heavy work, high air temperature, and high humidity, evaporative cooling through sweating (1 to 2 L per hour) may become insufficient, particularly if the exercise is prolonged or hydration is inadequate. This scenario can lead to a spectrum of heat-related problems due to dehydration, hyperthermia, or, most often, the combined effects of the two. For this reason, environmental heat stress should be measured, quantified, and assigned a safety rating.
Measuring Heat Stress
To guard the health of athletes, the medical team at any sports event must measure environmental heat stress and, if necessary, postpone or cancel the event. The wet bulb globe temperature (WBGT) index (table 1) is a practical method for assessing environmental heat stress and the risk of heat illness (1).
The WBGT index takes into account air temperature, relative humidity, and solar radiation by measuring three temperatures. Air temperature is measured using a standard dry bulb (DB) thermometer. Relative humidity is assessed with a wet bulb (WB) thermometer. Air is blown over a wetted wick. Water evaporation, which lowers the temperature, will depend on the amount of moisture in the air. Temperature and relative humidity can be measured on the field using a psychrometer, an inexpensive instrument available at scientific supply houses. Solar radiation is indicated by a globe (G) temperature, from a thermometer with a bulb positioned inside a black metal sphere. (Improvising such a device is simple.) Sophisticated portable instruments that integrate all three temperatures are available but cost several thousand dollars.
The heat stress index is calculated as WBGT = (0.1 x DB temp) + (0.7 x WB temp) + (0.2 x G temp) (1). The high weighting (0.7) of the wet bulb temperature reflects the potent effect of humidity on heat stress. When using these guidelines (table 1) to gauge risk of heat stress, the assumption is that athletes are wearing warm-weather clothing such as a loose-fitting T-shirt and shorts. Revise guidelines appropriately if athletes are wearing football uniforms or other occlusive clothing that imposes a barrier to heat dissipation.
Acclimatization to Heat
Physicians can reduce the risk of heat-related injuries by helping coaches and athletes include acclimatization and fluid-replacement strategies in their training programs. Acclimatization is the process of physiologic and psychological adaptation to a new environment, such as when a person moves from a cool climate to a hot climate or simply adjusts from spring to summer.
Many of the physiologic benefits of heat acclimatization are similar to those of physical training: reduced heart rate, core temperature, and utilization of muscle glycogen, as well as increased cutaneous blood flow, plasma volume, red-cell filterability, and work time until exhaustion (2,3). During acclimatization, sweat rate increases, sweating starts earlier, and the electrolyte content of sweat decreases. Well-conditioned athletes have a higher heat tolerance than their sedentary counterparts; in other words, fitness improves heat tolerance. Regular vigorous training induces "internal heat stress," which enhances the physiologic adaptations that are similar to acclimatization.
Acclimatization typically requires 10 to 14 days in the warmer environment, but 75% of the adaptation is believed to occur within 5 days (4). Initial exercise sessions should be shorter and less intense than normal training levels. For example, the first few sessions may last only 15 to 30 minutes at moderate intensity (50% to 70% of V*o2 max), compared with a standard high-intensity 60-minute workout. Workouts should build to normal levels over the next 7 to 10 sessions.
Attention to clothing is also important. Fabrics that minimize heat storage and enhance sweat evaporation should be selected. White or light colors, cotton or other breathable fabrics, and designs that maximize skin exposure are beneficial.
Hydration is important because chronic dehydration retards acclimatization. Hydration level can be tracked by monitoring daily fluid intake and body weight (same time of day and standard conditions). Rehydration recommendations are discussed in the next section.
Though fluid replacement is important in preventing heat illness, excessive hyperthermia may also occur without dehydration when exercise intensity—and thus metabolic rate—is very high, as in a 10-km footrace. Extensive findings from military basic training in the heat show that exertional heatstroke occurs mainly within the first 2 hours of exercise (5). Soldiers who are unacclimatized, unfit, and/or overmotivated are most vulnerable. Investigators report that nearly all serious cases of heat illness can be prevented if proper procedures are followed: avoiding the hottest hours of the day, regimenting rehydration, scheduling rest periods (10 minutes of rest for every 60 minutes of exercise), and matching physical efforts to fitness level.
Maintaining Hydration in the Heat
The combination of exercise and high environmental heat stress can produce dehydration. Fluid ingestion during exercise in the heat reduces dehydration, core temperature, and cardiovascular strain (6) and increases exercise performance (7). Fluid ingestion reduces plasma adrenaline, which may reduce heart rate and maintain skin blood flow (8). However, the optimal dosage has been debated over the years.
The American College of Sports Medicine (9) suggests that the rate of fluid ingestion during prolonged exercise should attempt to match fluid losses from sweating or follow a more generic guideline of 150 to 300 mL (about 5 to 10 oz) every 15 minutes during running or similar vigorous activity. The generic guideline is a good starting point, but it does not allow for individual differences in sweat rates, and so in practice, the generic guideline may result in either too little or too much fluid intake. For example, the elite marathoner who sweats 1.5 L per hour and ingests the bottom of the range incurs more than a 2-L deficit over the course of a marathon. However, the jogger who consumes near the top of the range and sweats 1 L per hour incurs a fluid excess of 800 mL during the marathon.
Sweat-rate determination may be the best approximation of individual fluid requirements in the heat. Sweat loss is easily estimated in the field by measuring the difference between preexercise and postexercise body weight. This measurement appears to be remarkably reproducible in individuals when training and acclimatization are stable. Data obtained in our heat-stress field studies revealed minimal intraindividual but high interindividual variation in sweat rate among homogeneous well-trained, heat-acclimatized male distance runners (10). The mean weight loss in individual runners varied less than 0.2% after two 20-mile runs in humid heat. The mean dehydration level for the group was a 4.0% body weight loss with a range of 2.1% to 5.4%, though the runners consumed about 750 mL of fluid per hour.
In highly trained, well-acclimatized athletes, some dehydration is probably inevitable. Sweat rates greater than 3 L per hour have been observed in the laboratory (11). Attempting to match sweat loss beyond 1 to 1.5 L per hour may produce gastrointestinal discomfort and be impractical because 0.8 to 1.0 L per hour seems to be the upper limit for fluid absorption.
What Type of Rehydration Drink?
The formulation of an oral rehydration solution to be used before, during, and after exercise in the heat is important. Historically, plain water was thought to be the ideal fluid to drink during exercise. Early research (12) suggested that as the level of carbohydrate (CHO) increases in a solution, gastric emptying slows. However, recent studies (13,14) indicate that 5% to 8% CHO-electrolyte sports drinks are well tolerated during exercise in the heat and improve endurance. However, hypertonic solutions (greater than 12% CHO) such as fruit juices and soft drinks may cause gastrointestinal distress and impair exercise performance in the heat (9).
Recently, it was observed that during 1 hour of cycling in the heat, the benefits of fluid and carbohydrate for high-intensity performance were additive (7). High water intake (1,330 mL) improved performance by 6.5% compared with a lower intake (200 mL); adding 79 g of CHO to the high-volume regimen improved performance another 6.3%.
The old rule that only water or a dilute sports drink should be consumed in the heat is not justified by current research. Ingesting about 1 L per hour of a 6% to 8% CHO-electrolyte drink can provide adequate fluid and CHO to maintain hydration and increase performance during prolonged exercise (about 1 hour or longer). Also, cooling the fluid (to about 50 to 59°F) improves palatability and may aid gastric emptying and heat dissipation.
Hyperhydration Before Exercise
Hyperhydration (or "excess drinking") before hot-weather exercise is another recommended strategy. The current guideline (9) is to ingest 500 mL (17 oz) of fluid 2 hours before competition. The rationale is that gastric emptying is enhanced when the stomach is relatively full (15). Because chronic mild dehydration is not uncommon in some athletes, hyperhydration may also prevent athletes from beginning competition already dehydrated. A pre-event CHO drink may improve performance and be particularly useful for sport activities that last less than 1 hour and in which drinking is minimal (ie, soccer, field hockey). Though precompetition hyperhydration may initially be unacceptable to athletes in running-based sports, it can become more tolerable if practiced in training sessions.
When athletes are recovering from exercise in the heat, sodium replacement maximizes rehydration (16). Sodium is more important for fluid restoration after exercise than during exercise (17). In a recent study (18), male and female subjects who consumed only 350 mL of fluid at the beginning of a 2-hour postexercise recovery period had greater fluid retention and plasma volume restoration with chicken noodle soup (334 mmol/L of sodium) and broth (110 mmol/L of sodium) than with water or a sports drink (16 mmol/L of sodium).
Optimal postexercise rehydration requires both higher fluid volume replacement (more than 150% of weight lost) and higher sodium content (60 vs 20 mmol/L of sodium) compared with rehydration during exercise (19). Plain water ingestion delays rehydration because it decreases plasma osmolality, reduces thirst drive, and increases free-water clearance. Even most commercially available sports drinks may have suboptimal sodium (20 to 30 mmol/L) when used exclusively for postexercise rehydration. Thus, it appears that higher levels of sodium may be necessary to promote body fluid restoration after exercise in the heat.
Hot-Weather Event Planning: The Atlanta Olympic Experience
In the years preceding the 1996 Olympic Games in Atlanta, there was a looming dread that the hot, humid weather typical of the southern United States would spoil the celebration (1). Legitimate concerns were voiced about a potentially high incidence of heat illness among athletes, not to mention lackluster performances. Concerns extended to the officials, volunteers, and spectators who would face long hours in the sun.
Fortunately, the worst fears were not realized. Medical, sports, and mass media groups educated athletes, coaches, volunteers, and spectators on preparing for and coping with the heat. At nearly all venues, organizers ensured the availability of fluids, shaded areas, first aid stations, and air-conditioned recovery rooms. In our experience, these strategies were successful.
The measurement and interpretation of environmental heat stress using the wet bulb globe temperature (WBGT) (table 1) was important to Olympic organizers, coaches, athletes, medical personnel, mass media, and the general public. These data influenced Olympic officials to schedule endurance events when heat stress would be lowest. Environmental data and acclimatization guidelines were provided to coaches and athletes worldwide to assist them with training programs. During the Games, weather data and drinking water were readily available and alerts were released if conditions became high-risk or hazardous.
Average air temperatures and relative humidity readings for Atlanta during the 17 days of the 1996 Summer Olympic Games were 74.2°F and 83% at 7 am, 84.9°F and 63% at 1 pm, and 81.9°F and 67% at 7 pm (2). These were similar to 45-year (1945 to 1990) mean values for Atlanta. Based on the WBGT, the mean risk of heat illness over these 17 days was moderate at 7 am and high at 1 pm and 7 pm. The risk was hazardous on two of the 17 days at 1 pm and 7 pm.
Nielsen (3) has theorized that marathoners will be unable to maintain thermal balance when air temperature reaches 95°F and relative humidity is greater than 60%. Fortunately, this extreme level was not reached during the Olympic Games. From our records, the greatest heat stress occurred on the fourth day with an afternoon air temperature of 93.9°F and 51% relative humidity (2). The women's and men's marathons were begun in the early morning under moderate conditions; air temperature and relative humidity were 70.3°F and 93%, and 73.4°F and 92%, respectively (4).
At selected venues, detailed environmental measurements were collected at 10-minute intervals during competition (4). These nearly real-time data were provided to medical personnel, venue managers, and public address announcers so that they could warn athletes, officials, and spectators of worsening conditions and the need for appropriate precautions.
Spectators and volunteers accounted for most of the medical visits associated with heat illness. Factors that can contribute to low heat tolerance in apparently healthy individuals include low fitness, poor acclimatization, dehydration, improper clothing, obesity, physical exertion, and prolonged heat exposure. Persons with previous heat problems are also at increased risk for heat illness. Additionally, middle-aged and older men and women have less tolerance for heat stress than young adults. Assuming that a large proportion of the estimated 2.2 million spectators and 70,000 volunteers had two or more of these characteristics, it is remarkable that fewer than 1,000 were treated for heat illness. Our interpretation is that most people took the warnings seriously, took reasonable precautions, and used common sense.
Among the athletes, only about 5% of those who sought or received treatment from a physician did so because of heat illness. Among those at greatest risk, the long-distance runners, fewer than 11% were treated for heat problems by medical personnel. These low numbers can be attributed in part to the supreme level of fitness among Olympians. Unlike spectators and volunteers, most Olympians have few of the characteristics associated with heat intolerance.
According to a report (5) summarizing all medical encounters during the Olympics, of 10,715 persons treated by a physician, only 9.9% (1,056) were seen for heat-related illness (heat cramps, dehydration, heat syncope, or heat stroke). Heat-related illness was the most common condition among spectators (n=752, 21.6%), but it was the least common condition among athletes (n=95, 5.3%). No cases of heatstroke were reported. Spectators and volunteers accounted for most (88.9%) of the visits for heat-related illness. Of 101 persons admitted to a hospital, only 2 were admitted for heat exhaustion.
In a separate analysis (4) of 3,992 track-and-field athletes, 1 in 34 sought medical care for heat-related illness. No athlete presented with exertional heat stroke. When analyzed broadly by type of event (track vs field), the incidence was 1 in 26 (115/2,948) for track athletes and 1 in 522 (2/1,044) among field athletes. Though field athletes may be exposed to high heat stress for several hours, their sporadic activity and ability to maintain hydration substantially decrease their risk of heat illness.
For endurance athletes (runners and walkers in events longer than 5,000 m), heat illness incidence was higher at 1 in 9 (62/559). This is predictable because a prolonged high metabolic rate is required and the climate was unfavorable. The average WBGT for the long-distance events was 72.9°F, which is at the high end of the moderate-risk range. All distance events were held in the early mornings and mid-evenings.
Endurance performances were remarkably better than anticipated (6). In the women's 10-km walk, a new Olympic record was set (41:49), while in the men's 20-km walk, the winning time was within 10 seconds of the Olympic record. The winner of the women's marathon posted the second-fastest time of the year (2:26:05). In both the men's and women's 10-km races, new Olympic records were established at 27:07.34 and 31:01.63. In nearly all the distance events, the races were closely contested, with silver and bronze medalists finishing within seconds of the winners.
Dr Sparling is a professor and Dr Millard-Stafford is an associate professor in the Department of Health & Performance Sciences at Georgia Institute of Technology in Atlanta. Both served as members of the Heat Stress Task Force and Olympic Medical Support Group for the Atlanta Committee for the Olympic Games and are fellows of the American College of Sports Medicine. Dr Sparling is an editorial board member of The Physician and Sportsmedicine. Address correspondence to Phillip B. Sparling, EdD, Exercise Research Laboratory, Dept of Health & Performance Sciences, Georgia Institute of Technology, Atlanta, GA 30332-0110; e-mail to [email protected].