WE ALL know runners who never miss a day out on the roads. Torrential downpour, three feet of snow, fever, cold—it doesn’t matter. The Run or The Race rules. You may even be this runner. As much as you don’t want to hear it, there are days when you would be better off skipping your run—or cross-training or exercising indoors. A brutally hot and humid day may be one of those days. The effect of heat stress on marathon and road race injury and performance is predictable, and this article aims to make you a more informed competitor so you know your personal limits regarding heat stress and can answer the question, “When is it too hot for me to run safely?”

SOME BACKGROUND INFORMATION

Body heat is distributed in the core and shell of the body. The body shell is the surface for heat exchange and varies in “thickness” based on the need to lose or conserve heat. Cellular heat is accumulated during running along with a corresponding rise in body temperature. The rise in cell temperature in the essential organs causes the syndrome of exertional heat stroke (EHS).

Core temperature is a sum of exercise-produced metabolic heat plus environmental heat added to the body minus heat lost to the environment. Metabolic heat is a function of intensity and duration of exercise. Environmental heat is gained when the ambient air temperature is greater than skin temperature or from the sun’s radiant heat. Heat is lost from the body by evaporation, conduction, convection, and radiation.

Evaporation is the most powerful means of heat loss to the surrounding environment. High humidity limits evaporation heat loss, and high ambient temperature limits conduction and convection heat loss. Dehydration limits heat transport to the body surface and also limits sweating.

Hot Terms to Know

• ambient temperature—The temperature of the air on a common thermometer (also called the dry bulb temperature).
• black globe temperature—The temperature inside a flat black, four-inch diameter metal sphere hung at chest height in the sun. A measure of radiant and conductive heat gain.
• exercise-associated collapse (EAC)—An athlete requiring assistance during or after endurance activity.
• exertional heat stroke (EHS)—Hot core temperature with central nervous or other organ system dysfunction. Failure to remove heat from the core during activity.
• exertional hyperthermia—Hot core due to muscle work with no systemic dysfunction. Will resolve spontaneously if no more heat calories are added to the system. There is potential for exertional heat stroke if continued heat calories are added to the core.
• green flag conditions—Low heat stress conditions. Relatively safe for road racing. The ambient temperature is less than 65 degrees F, and the relative humidity is less than 100 percent with no sun.
• wet bulb globe temperature—A summation of 0.1 dry bulb temperature plus 0.2 black globe temperature plus 0.7 wet bulb temperature. Developed to estimate the heat stress for people at work in the heat wearing light pants and a light shirt.
• wet bulb temperature—The temperature measured with water evaporating from the mercury bulb. The drier the air, the lower the wet bulb temperature. At 100 percent humidity, the wet bulb temperature should be the same as the ambient temperature.

A SHARED RESPONSIBILITY

Heat-related running injury and heat stroke caused by the exertion of running can be prevented with proper planning by the race organizing committee and proper preparation and good judgment by you. Preventing all exertional heat stroke (EHS) is probably not possible because we cannot control the individual risk variables during running. Primary prevention of EHS is the responsibility of the race administration, but each of us also has the responsibility of knowing how well we cope with hot running conditions.

The race administration is also responsible in high-risk conditions for the secondary prevention of excess morbidity and mortality associated with exertional heat stroke in the circumstances where it is likely to occur. Ironically, it has been shown that participants in shorter, faster road races are at greater risk for EHS than participants in longer, slower races like the marathon, although EHS can—and sometimes does—occur in marathon and longer distance races. The rapid generation of excess metabolic heat in a runner combined with delayed or impaired heat removal puts the faster-paced runners at greater risk.

EHS AND EXERTIONAL HYPERTHERMIA

Definitions of exertional heat stroke and exertional hyperthermia are not chiseled in stone and include a “floating” temperature based on the presence of heat stroke symptoms. These symptoms are normally associated with abnormal brain function. In general, a rectal temperature greater than 104 degrees F is considered exertional hyperthermia.

The clinical picture of EHS is best described as elevated body temperature and altered central nervous system function. EHS victims will most often be sweating and will not always feel hot to the touch. The only adequate assessment of body temperature in the suspected EHS casualty is to take a rectal temperature. The aural canal or tympanic membrane temperature analog thermometers measure a shell temperature in sweating athletes and can easily miss the heat stroke victim.

The symptoms and signs to watch out for with EHS include these: ashen skin color, vacant stare, dizziness, fatigue, weakness, impaired judgment, hyperventilation, flushing, chills, and intense thirst. None of these symptoms or signs is specific for heat stroke, but they should raise the suspicion that heat stroke may be present.

The most ominous sign of heat stroke is the presence of abnormal brain function involving bizarre behavior, memory loss (especially event details and name), loss of hind limb function, inability to walk alone, collapse, delirium, stupor, or coma. The presence of the symptoms and signs should alert runners and medical staff of a potential medical emergency. Immediate evaluation and cooling measures should be started.

If the rectal temperature is greater than 104 degrees F and central nervous system changes or other heat stroke symptoms are present, EHS is probably the cause. Immediate cooling treatment can be life-saving in this situation, and delays in applying a radical cooling treatment can result in increasingly severe complications.

There is probably a continuum of changes and symptoms that occur in a runner during the transition from overheated (external hyperthermia) to a full collapse from heat stroke. Exertional heat stroke can ultimately lead to a serious medical injury or death, so preventing EHS and knowing the signs of heat stroke are critical to your safety as a runner.

KNOW THE RISKS

Take it upon yourself to know the risk of variables for EHS and understand the environmental parameters that increase the risk of heat stroke. Be prepared to modify your physical activity to decrease your individual risk of EHS.

High humidity associated with high temperature poses the greatest risk to the body’s ability to dissipate heat and cope with elevated body temperature, especially if it is associated with intense activity or fast-paced races. The risk of heat injury begins to rise rapidly above 65 degrees F and 50 percent relative humidity. With intense work, you can generate 1,000 Kcal of heat per hour of activity, which can raise the body temperature of an averaged-sized runner into the hyperthermic range in 20 to 30 minutes. A fast-paced road race run in hot and humid conditions puts you at risk for hyperthermic injury in 30 to 45 minutes. In longer races, like the marathon, the prolonged, continuous activity can result in hyperthermia despite the slower pace.

It is important to underscore the role of fluid in countering this process. How much fluid you take in while you run or race affects your body’s ability to transport heat through the circulatory system and to lose heat through sweating. There are numerous scientific studies graphically illustrating the effect of dehydration on heat storage and performance. A dehydrated runner will have a higher core temperature, an increased heart rate, a lower cardiac output, and an increased perceived exertion at the same workload compared to a well-hydrated runner.

Drinking fluids during races will not only improve your performance, it will also protect the cooling system during activity. Taking in fluids during your training is essential to being able to tolerate taking in fluids during a race. You have to “practice” this fluid replacement strategy on a regular basis on training runs.

GETTING COMFORTABLE WITH HEAT

Acclimatizing, or physiologic adapting, is critical to performing successfully in the heat and takes at least 10 days of exposure before it kicks in. Gradual and progressive exposure to increasing heat loads during training is the safest way to acclimatize your body to hot conditions. You can induce acclimatization more rapidly with harder training, and athletes who have a higher cardiorespiratory fitness level at the beginning of heat training and runners with a greater V̇O₂max can acclimate to heat faster.

The physiologic effects of acclimatization include decreased heart rate, increased plasma volume, earlier and increased sweating, decreased skin blood flow, and decreased Na+ (sodium) losses with activity in the heat. The end result of acclimatization is decreased core body temperature, increased exercise tolerance time, and decreased perceived exertion in the heat. The greatest improvement in acclimatization occurs after 8 to 12 weeks of exposure, and nearly daily exposure to heat is required to maintain this adaptation. You can

remain acclimatized with as little as 30 minutes in the heat per day. You can lose your adaptation to heat in a few days to a few weeks.

With the ease of modern travel, it is possible for you to enter “hot” races elsewhere in the world during a cold season in your home state. You can induce partial acclimatization before you leave by wearing extra clothing while training in the cooler conditions. (An unexpectedly hot day in the spring or fall can also catch you unprepared for the heat, and in such unseasonably hot conditions, knowing that exertional heat stroke has occurred in relatively cool conditions with temperatures less than 60 degrees F, your decision to race is a personal one.)

When Does the Risk of Heat Injury Increase?

Several other factors increase the risk of heat injury in road racing. Younger adolescents and children as well as older adults are at greater risk in the heat—the younger groups because of their larger mass-to-surface area ratios, the older age group because of decreased efficiency of the heart pump (the circulatory system transports heat around the body as well as oxygen). Health deficits and recent illness will also decrease your resistance to heat and exaggerate the decrease in performance associated with heat. Finally, some medications and drugs will affect your ability to withstand heat during exercise. Always check with your doctor regarding the risk of activity in the heat while on prescription drugs, especially diuretics and tricyclic antidepressants. Remember also that over-the-counter drugs containing sympathomimetic amines (e.g., psuedofed, most cold preparations, ephedrine, antihistamines combined with decongestants) can interfere with your body’s ability to dissipate heat. Creatine and other supplements may also reduce heat tolerance. As always, alcohol and other recreational drugs can increase the risk of heat stroke and hyperthermia, and they will do little to improve performance.

AT WHAT TEMPERATURES DOES THE BODY PERFORM BEST?

Athletic performance is best when your core body temperature stays in the normothermic body temperature range of 99 to 101 degrees F. Athletes who run in hot conditions almost all drop out of the activity once the rectal temperature is above the 101 to 104 degree F range.

When the winning times of various races are plotted against heat stress measures like ambient temperature, wet bulb temperature, and wet bulb globe temperature, there appears to be an optimum heat load above and below which times are slower. For ambient temperature this is usually around 50 degrees F for the marathon.

Higher heat loads can also adversely affect other performance indicators like exercise time to exhaustion, perceived exertion, and personal motivation. At 50 degrees F, the time to exhaustion with continuous activity is 94 minutes, compared to 80 to 85 minutes at 40 degrees F and 70 degrees F. This finding is also seen at the finish line and in the medical care areas of distance running events where the incidence of race injury rises in the heat.

WHAT REALLY HAPPENS DURING ROAD RACES?

Data accumulated at several running events illustrate the increased risk that hot conditions create for the runners in competition. These data are especially powerful when you compare the same event under different conditions.

The 7.1-mile Falmouth Road Race on Cape Cod in Massachusetts is run on the third Sunday in August, approximately 60 days after the summer solstice. Each year, 7,000 to 8,000 entrants line up to begin the coastline race, which starts in mid-morning and is usually run under conditions of increased heat injury risk. The temperatures are often greater than 65 degrees F with relative humidities in the 50 to 90 percent range. It is not unusual for the race start temperature to be in the high 70s or low 80s.

Each year several participants finish the race with body temperatures greater than 106 degrees F, and 10 to 15 of these runners suffer EHS. The rate of EHS during the Falmouth Road Race is one or two per 1,000 entrants. The vast majority of the treated runners walk away from the finish area in good condition because the race medical team is able to identify and treat the runners with EHS immediately and effectively. Without the continued diligence of the medical team, the race could not be run in August.

In comparison, another race in Falmouth is run on the same course in November, and there are no problems with heat stroke in the cool fall conditions. The contrast on the same course in “hot” and “cool” conditions is striking from a medical perspective. It would be interesting to see the performance data for individual runners on the same course in the different conditions.

Now let’s examine the Twin Cities Marathon of Minneapolis-St. Paul, Minnesota. With approximately 100,000 entrants in 16 years, Twin Cities has had fewer than 25 runners with rectal temperatures greater than 106 degrees F, and only five finishers with EHS.

The race starts at sunrise and is scheduled for a cool time of the year in early October, approximately 100 days after summer solstice. Four of the five runners with EHS participated in the three races that had a start temperature greater than 55 degrees F. The rates of heat stroke for the cool and warm conditions are 1.5 runners per 100,000 entrants and 8 per 100,000 entrants, respectively.

Even in the “hottest” conditions for Twin Cities, the races were in or very near the “green flag conditions,” with a wet bulb globe temperature of less than 65 degrees F. With most of the races run in cool conditions, the average all-injury rate is 18 per 1,000 entrants with a range of 13 per 1,000 entrants at 40 degrees F to 35 per 1,000 entrants at 65 degrees F. The “severe” medical injury rate is less than 2 per 1,000 entrants.

The Boston Marathon, run on the third Monday in April, is scheduled roughly 100 days before the summer solstice. The National Weather Service records for average high temperature, average low temperature, and the average humidity on raceday are nearly identical to the average conditions for the Twin Cities Marathon in Minneapolis in the autumn.

The major difference, from the perspective of heat stress, is the noon start at Boston. The race has experienced a wide range of heat stress conditions, and the noon start requires the elite runners to race in the hottest part of the day while the slower runners finish in “cooling” conditions. Bruce Jones, MD, has shown that the incidence of “all injury” and the incidence of race dropouts increases with increasing wet bulb temperature. The average all-injury rate is 72 per 1,000 entrants with a range of 40 per 1,000 entrants at 45 degrees F wet bulb globe temperature to 125 per 1,000 entrants at 65 degrees F wet bulb globe temperature.

Of particular interest is the correlation coefficient for the data comparing injury and heat stress that shows that 70 percent of the risk of all injury in the Boston Marathon is correlated to the heat stress. Jones has also compared the Boston Marathon incidence of casualties on a warm (68 degrees F) versus a cool (48 degrees F) raceday. The incidence of symptoms associated with heat injury (including nausea, vomiting, and collapse) were all increased on the warm day. The occurrence of muscle cramping was equal in both warm and cool conditions, so we can assume that exercise-associated muscle cramping is probably not related to heat but rather to muscle and neural fatigue. Also of interest is the increased incidence of both ankle sprains and blisters in the warm conditions, possibly due to muscle fatigue and sweaty, wet feet.

Grandma’s Marathon in Duluth, Minnesota, is scheduled for the third Saturday of June, which is very near the summer solstice. Even with its current starting time of 7:30 A.M., the race has experienced a wide range of heat stress conditions. The odds ratio for “medical care” is 1.84 per 1,000 entrants if the average temperature is greater than 60 degrees F, and the odds ratio that runners will need to receive I.V. fluid in the finish area medical tent is 2 if the wet bulb temperature is greater than 55 degrees F, again pointing out the increased risk to the runner in warmer conditions.

PREVENTION STRATEGIES FOR EHS

As mentioned earlier, the primary prevention of EHS is a responsibility each runner shares with the race administration. Passive strategies—that is, ones that don’t require race participants to modify behavior—are the most powerful means of preventing heat injury and are within the complete control of the race administration.

Race scheduling, for example, determines the “inherent” risk of the race. A race scheduled away from the summer solstice will avoid peak radiant sun exposure and the highest ambient temperatures. Be an informed runner and find out what are the average high and low temperatures along with the average relative humidity and the extreme high temperature for raceday of the event you’re considering. You can learn this information via the National Weather Service in the reference section of any library. If the National Weather Service data indicate that a particular day in a particular location is beyond your personal heat limits, it may be prudent to avoid the race. Common sense dictates so.

The next factor in preventing heat injury is the race start and finish time. From the perspective of heat injury, the “safest” races will have early morning start times to avoid the heat of the day. This allows the elite runners to compete in the cooler and faster conditions and the slower runners to finish in warming rather than cooling temperatures.

If a race does not have a plan for extreme heat conditions, develop your own individual cancellation policy. The unfortunate reality of road racing is that runners often surrender their normal behavior controls to the race administration, assuming the race will only be run if the conditions are in a safe range for all competitors.

The American College of Sports Medicine (ACSM) has road race recommendations for hazardous heat conditions, designed to guide race administrators in their effort to prevent heat injury. The ACSM heat stress prevention cascade is based on wet bulb globe temperature, which accounts for humidity, radiant heat, and ambient temperature.

Some simple heat stress guidelines are as follows:

• temperature of 65 degrees F and relative humidity of 100 percent increases the risk of heat injury
• temperature of 73 degrees F and relative humidity of 100 percent dramatically increases the risk of heat injury
• temperature of 82 degrees F and relative humidity of 100 percent is dangerous for vigorous and continuous activity.

The responsibility to “cancel” an event is shared by runners and the event. If an event does not intend to cancel under any circumstances, then the race administration should at least publicize the environmental conditions and risks to the runners before the start of the race so each athlete can make a wise decision regarding the environmental risk of the day.

Several factors can influence the outcome of a race in hot conditions. The heat wave of 1995 in the Midwestern U.S. was studied by the Center for Disease Control (CDC), which found that as little as one half hour per day spent in an air-conditioned environment could make a difference between life and death for the elderly. A prerace air-conditioned environment may help runners withstand the heat stress of a race.

Acclimatization to heat with a gradual increase in heat exposure to race conditions will help you resist heat stress during competition. It takes one to two weeks to improve heat resistance. Be leery of a second and third day in “unexpected” heat. Hydration is critical to heat response, and you need free access to fluids during training and before, during, and after races.

Races scheduled in moderate to high-risk heat conditions should anticipate heat injury and develop triage and treatment protocols for the race. The race medical team should train the volunteers to recognize and treat exertional heat stroke on-site to decrease the risk of death by heat stroke. Low-risk race volunteers should be trained in recognizing and treating EHS.

SUMMARY

Race organizers should strive to develop competition opportunities away from the hot times of the year and move the start times to the early morning hours to improve performance and reduce the risk of heat injury. You need to be aware of race cancellation and modification policies (including a “no cancellation” policy), so you can make a rational decision regarding at the start of each race. Your risk of heat illness increases if you have been ill, are not well hydrated, well nourished, and well rested, and if the conditions are warmer than you can normally tolerate. Runner safety should take priority over tradition, sponsors, coaches, and fans in all events and individual decisions to participate in road races. Understanding heat illness and knowing your personal heat limits will make for safer racing and could save your life. Can you answer the question, “When is it too hot for me to run safely?”