How Hot Is Hot?  How Safe Is Safe?

"The aim of science is not to open a door to endless wisdom,  but to put a limit to endless error."    --Bertolt Brecht in "The Life of Galileo".

The charts, maps, diagrams, graphs, text and photos in this web site are based on the implicit assumption that something definite can be said about the interaction of weather and sports. There is a great deal of subjectivity and controversy surrounding this idea.  Let's examine what is known, what is suggested, and what is decreed.

How Hot is Hot?

What is Temperature?

Temperature is the degree of heat of any substance. Temperature is measured by linking a calibration standard to a heat-related property of a measuring instrument. As the heat of a target substance changes, the molecules comprising that substance will behave differently. With increasing heat, molecular velocities increase. If the substance is a liquid or gas at constant pressure, the substance will expand as heat increases. In the case of a mercury thermometer, as temperature increases the column of heated mercury expands up the calibrated capillary tube leading out of the bulb reservoir. When the thermometer reaches steady-state, it is possible to observe the height of the mercury column and thereby determine the temperature of the substance being measured.

The ability of a thermometer to measure the heat of a substance is based on the "zeroeth law of thermodynamics", namely that if three or more systems are in thermal contact with each other, and all in equilibrium together, then any two taken separately  are in equilibrium with one another. In other words, the mercury in the thermometer will be in thermal equilibrium with the molecules which comprise the metal reservoir bulb of the thermometer, which will in turn be in thermal equilibrium with the substance whose temperature is being measured.

What is Heat?

Heat is a form of energy. Like all forms of energy, it follows the first law of thermodynamics: When heat is transformed into any other form of energy, or when other forms of energy are transformed into heat, the total amount of energy (heat plus other forms) in the system is constant.

Let's apply this arcane rule to a real-life event: The exercising athlete takes a step. Approximately 15 billion years ago, the Big Bang released an immense amount of energy which eventually led to the condensation of stars, planets and other bodies. The sun, which is an ongoing nuclear fusion reactor, produces radiant energy at a variety of wave lengths, including photons in the range of visible light. These radiant energy photons reach Earth and strike the green leaf of a spinach plant. In the leaf, photosynthesis occurs, transforming the radiant energy of the photon into the chemical energy of a  chemical bond in a carbohydrate molecule. A farmer harvests the spinach leaf and sells it to the athlete. The athlete eats the leaf, and the molecule containing the high-energy chemical bond is absorbed, makes its way into the blood and is carried to a muscle cell.

In order for the athlete to take a step, his leg must move, an event which requires mechanical energy. This is supplied by the shortening of a muscle, which is the result of the interaction of actin and myosin filaments in the muscle cell. Like trillions of tiny oarsmen in a galley ship, the actin and myosin filaments utilize energy released by oxidative reactions to change the spinach chemical bond energy into mechanical energy. The filaments slide over each other and the muscle shortens. The athlete takes a step.

But wait. Although the human machine is semi-miraculous in its ability to transform chemical into mechanical energy, this transformation is inefficient. Only 20-30% of the energy released during muscle metabolism is transformed into mechanical energy. 70-80% of the energy is released as heat energy. Since the warming muscle cell is in thermal equilibrium with the blood which bathes it, the blood becomes warmer. As the blood warms, a region of the athlete's brain, the hypothalamic temperature control center, signals the system to increase  cardiac output and direct more blood flow to the skin. The athlete's face turns red and his veins bulge.

If the athlete produces more heat energy than he can dissipate, his core temperature will progressively rise. At a core temperature between 101.5F and 105F, his system will begin to  malfunction to the point that he must seek relief through marked slowing in his metabolic rate. He must slow his pace, or even  stop. The alternative is collapse and possibly death. To avoid this disastrous rise in core temperature, the body utilizes several heat transfer mechanisms:

Radiation: Any object whose temperature is greater than absolute zero will emit  radiant energy. For humans, this can be shown by infra-red photography.

Conduction: Conduction is defined as heat transfer by molecular contact.   Thermal energy is manifested by the motion of the molecules of an object. Through molecular collisions, the heat of the now-warmer and more rapidly-moving molecules of the first object is transferred to molecules of a second object when that second object is put in contact with the first object. Heat is transferred from hot pavement to a metal can placed on it. Remember: The conductivity of water is about 25 times greater than the conductivity of air. After the sinking of the Titanic, those in the very cold water died of heat loss, while those in the even colder air (in lifeboats) survived.

Convection: Convection is the transfer of heat from one place to another by the circulation of heated particles of a gas or liquid. A cold wind blowing over the athlete's skin results in cooling by convection.

Evaporation: Evaporation is the change of a substance from a liquid to a vapor. This phase change requires heat, which is termed the latent heat of the event, and thus results in cooling. It is critical to remember that moisture will always evaporate from any wet surface in contact with air that is not fully saturated with water vapor. Evaporation is the Power House of heat-dissipation mechanisms. For every liter of sweat which evaporates, the athlete will lose 580Kcal of heat.

A quick survey of these four mechanisms leads to the inevitable conclusion that without sweat evaporation, vigorous exercise in a warm environment is not possible: The exercising athlete will quickly overheat and slow down or die.( It must be remembered that in order for sweat to provide any cooling benefit, evaporation of the sweat must occur.) Humans live in a thin cocoon of air which has a high vapor pressure. If wind blows across this cocoon, it will almost always lead to some degree of sweat evaporation. The drier this wind is, the greater will be the rate of evaporation. With 100% relative humidity, no evaporation can occur and sweating is futile.

Systems For Measuring Temperature: Real And Apparent

Working thermometers date to at least the 17th century. The first intelligible meteorological records were made in the early 18th century, using an alcohol-filled thermometer devised by Robert Hook. Fahrenheit introduced the mercury-filled thermometer in 1724, and Celsius worked at about the same time. The early thermometer systems were based on the freezing and boiling points of water. Current systems of measure use a variety of physical standards, including the vapor pressure of helium, the freezing point of silver and the Planck radiation law.

We can measure air temperature with extreme accuracy, but what does it mean? There can be no doubt that the most ancient of papyrus manuscripts referring to weather must contain the phrase, "It's not the heat, it's the humidity." An ordinary dry bulb thermometer is a very rough physical analog to the human body. It measures the air temperature in which both are immersed but is not itself heat-producing, does not have to lose heat to maintain its temperature, and thus is unaffected by air movement. It is virtually insensitive to radiation exchange, and even in bright sunlight registers only a minimal change over ambient temperature. Furthermore, the thermometer does not sweat and thus cannot  simulate this important aspect of human heat exchange with the environment.

Attempts to overcome the limitations of simple air temperature measurement have had mixed results. A large number of biometeorological indices have been proposed, each trying to define how a given set of conditions "feels" to a human in them, or how those conditions "act" by influencing human physiology. These include the Effective Temperature, the Temperature-Humidity Index, Humiture, Summer Severity Index, Relative Strain, Summer Simmer Index, Predicted 4-hourly Sweat Rate Index and Belding Hatch Heat Stress Index, which compares the amount of sweat necessary to maintain thermal equilibrium and the maximum amount of sweat that can be evaporated in the given environmental conditions. As usual, this profusion of approaches reflects the fact that no one of them is entirely satisfactory.

And The Winners Are...

Through a process which is somewhat mysterious, two heat stress measurement techniques have gained sufficient support to be considered the current  standards in the field.

Wet Bulb Globe Temperature. The original work which served as the basis of this standard has been lost. A prominent exercise physiologist of our acquaintance has been looking for the seminal "Technical Paper" by Yaglou   for more than one year, but it has disappeared. Suffice it to say that in the early 1950's the US Marine Corps suffered significant casualties due to heat stroke during training activities at Parris Island, South Carolina. Prompted by this experience, the Department of the Navy commissioned studies on the effects of heat on exercise performance. These studies resulted in a heat index called the Wet Bulb Globe Temperature. In 1989, WBGT was suggested as an international standard (ISO 7243).

The Wet Bulb Globe Temperature is measured using three different thermometers: (1) A standard dry bulb thermometer (Dry Bulb Temperature; DB); (2) a standard dry bulb thermometer whose bulb is wrapped  in a cotton sleeve, the bottom of the sleeve lying in a pool of distilled water, so that the cotton sleeve will always be wet, allowing continuous evaporative cooling of the thermometer's bulb, simulating the evaporation of sweat (Wet Bulb Temperature; WB); and (3) a standard dry bulb thermometer whose bulb is inserted into a large (6 inch) black ball, to allow measurement of the effects of sunshine and other radiant heat (Black Globe Temperature; GT). These three temperatures are integrated as follows:
                                             WBGT = 0.7 WB + 0.2 GT + 0.1 DB

Steadman Heat Index. The WBGT is used by the military, industrial engineers and exercise physiologists to gauge heat stress. In the US, the National Weather Service employs a different standard to provide the general public with advisories regarding the risks of hot weather. This is simply called the Heat Index, and it is based on work carried out by R.G. Steadman and published in 1979 under the title "The Assessment of Sultriness, Parts 1 and 2."  In this work, Steadman calls on a vast number of research papers over a 54-year span to construct a table which uses relative humidity and dry bulb temperature to produce the "apparent" temperature, which is to say the way that a given set of air conditions "feels" to a person who is in them.

The NWS uses this table to provide its Heat Index values. The NWS does not use adjustments for the effects of sunlight and wind (but the assumption of a 5.8 mph wind is "built" into the NWS heat index), although Steadman outlined these adjustments in his papers, presumably because there is too much microclimate variation to make such adjustments very meaningful. Instead, the NWS just states that "Exposure to full sunshine can increase heat index values by up to 15F. Also, strong winds, particularly with very hot, dry air, can be extremely hazardous" and leaves it at that. Steadman's Heat Index Table and multiple regression analysis equations approximating that table can be found in The Library.

Setting Boundary Lines Based on Heat Index

If what has gone before has seemed theoretical, what follows qualifies as either sincere seeking after elusive truth or bone-headed arbitrariness, depending on your point of view. As best we can determine, there were no heat stress rules or guidelines of any significance for a long time. There are still no laws governing heat stress limits for workers in the US workplace, although OSHA does have a set of guidelines. Prior to the installation of air-conditioning, the Federal Government sent its Washington, D.C., office workers home whenever the heat-humidity index topped 90F.

Prior to 1956, the US Marine Corps used a heat stress index system based on dry bulb temperature and relative humidity. Yellow Flag (modified training) went into effect when dry bulb temperature was 90-100F. and humidity was low (exact level?). Red Flag (suspended training) went into effect when dry bulb temperature was 90-100F and humidity was high (exact level?) or when dry bulb temperature exceeded 100F.

Beginning in 1956, a training system based on WBGT was instituted by the Marine Corps. In the WBGT system, Yellow Flag (modified training) went into effect at a WBGT of 85-87.9F., and Red Flag (suspended training) went into effect at WBGT of 88F. and above.  Yellow Flag applied only to recruits in the early weeks of training, however. In a paper published in 1957, it was claimed that these changes had resulted in a significant lowering of the rate of heat-related illness. The following statement was made: "With the exception of one day in 1955, no cases were reported for either year when the mean daily WBGT level was below 81F, which thus represents the threshold level for heat casualties." 40 years later, in 1996, in a review of 217,000 Marine recruits, it  was noted that the real "threshold level" for increased heat injury risk is a WBGT of 65F. That the 1957 paper missed the mark by 16F gives some inkling of the limitations of a  small initial dataset.

By 1998, the Marine system had evolved into a fairly complex structure. WBGT measurements are taken hourly from 7 am to 8 pm daily from the 3rd Monday in April until the 3rd Monday in October, using non-electronic instruments. When WBGT reaches 84F, measurement frequency is increased to every 30 minutes. WBGT measurements are taken from multiple stations around the island. Each station runs its own heat risk flags. The current flag system is as follows: WBGT 90F and above = "Black Flag"; WBGT 88-89.9F = "Red Flag"; WBGT 85-87.9F = "Yellow Flag"; WBGT 75-84.9F = "Green Flag".  In addition, there is a "cool room" where personnel experiencing heat illness are kept. When this "cool room" is filled, there is  "Administrative Black Flag" status, to prevent generation of further need for "cool room" treatment. Activities permitted during each flag condition vary according to station. Almost all outdoor training requiring vigorous effort is stopped during Black Flag conditions. Limitation of activity during other Flag conditions is determined by the type of activity and the acclimatization and conditioning status of the recruits involved.

From 1974 to1993, the NCAA used a heat stress guideline system to try to avoid heat injuries in long-distance running events. These guidelines used WBGT limits which varied by event, with lower thresholds for longer races. The Injury Risk Guidelines, which triggered event postponement, are WBGT's as follows: 1500 m - 92.8F, 3000m - 88.5F, 5000 m - 84.6F, 10,000 m - 81.5F. At the same time, the NCAA published a second set of WBGT guidelines, designed to alert event organizers of temperatures which were felt to lead to decrement in performance. These guidelines are as follows: 1500 m - 84.6F, 3000 m - 80.6F, 5000 m - 77F, 10,000 m - 73.6F.

This enlightened policy recognized the dual nature of heat stress guidelines: Those designed to protect the athletes, and those designed to allow a good race, about 8F lower. In general, those guidelines were validated: There were no severe heat injuries with those guidelines in effect, and the performance decrements predicted by increasing heat stress were for the most part confirmed.

In 1975, the American College of Sports Medicine published a position statement on prevention of heat injuries during distance running. According to this statement, races greater than 10 miles should not be conducted when the WBGT exceeded 82.4F. In 1984, the ACSM statement was revised to include races of any distance, along with specific recommendations for warning competitors of the relative risks of heat injury at various WBGT indices. This flag system for WBGT was devised: Green Flag = Low Risk - <65F ; Yellow Flag = Moderate Risk, 65F-73F; Red Flag = High Risk - >73-82F; and Black Flag = Extreme Risk, >82F-90F("Event Delay Threshold" = 82F). Above 90F was the Dangerous Zone, a designation in line with the US Marine Corps standard. [Most recently, a WBGT "Event Delay Threshold" temperature for "Elite" events has been set at 87F.]

In 1979, R.G. Steadman published "The Assessment of Sultriness, Parts 1 and 2," as described above. The heat index he proposed was adopted by the US National Weather Service, and in about 1980 the NWS made the following guideline recommendations, based on temperature and relative humidity information, but without reference to the effects of wind and sunshine: 80-90F, Fatigue possible; 90-105F, Heat Injury possible; 105-130F, Heat Injury likely; 130F and above, Heat Injury highly likely. The basis for these designations is not spelled out. We have made inquiry with the NWS, but have never received an answer regarding the support for these guidelines.

The American Academy of Pediatrics has given this series of guidelines for WBGT temperatures and sports: <75F - No limits, but watch for heat problems; 75-78.6F - Longer rest periods in the shade, rest periods every 15 minutes; 79-84F Stop activity of unacclimatized and other persons with high risk, Limit activities of all others(disallow long distance races, cut down further duration of other activities); >84F, cancel all athletic activities.

In December, 1997, the Australian Open Tennis Tournament set a temperature standard for play. At a dry bulb temperature of 104F, the referee can stop matches until the temperature falls. There is no accommodation for radiant heat, wind or relative humidity. This standard appears to ignore the sophisticated physiologic considerations involved in other heat stress guidelines, and as such seems regressive. What sets it apart as a landmark event is that it sets some limit to the heat stress which the competitors must endure. The bar, strange as it may seem, has at least been set.

How Safe Is Safe?

A Few Dry Bulb Temperatures For Your Consideration:

212 Boiling point of water
Temperature in a Finnish Sauna
136 Highest temperature ever recorded at the Earth's surface (Libya)
135 Highest US temperature (Death Valley)
130  Temperature reported during 1-97 Australian Open Tennis Tournament
122 Highest European temperature (Spain)
118 Temperature reported during 7-95 Tennis Tournament, Washington D.C
108 Human core temperature which cannot be sustained during exercise
107 Human core temperature maintained at rest for 1 hour during whole
body heating for treating certain cancers and AIDS 
Core temperature sustainable by top endurance athletes during exercise
Core temperature at which most athletes collapse
104    Temperature at which the Australian Open Tennis Tournament allows
  postponement of matches
100    Temperature at which US Marine Corps recruit physical training was
  previously halted
90     Temperature at which the Women's Tennis Association allows a
10-minute break between 2nd and 3rd sets
88     1996 US Olympic Trials, Women's 10 Km Walk, 6/18 (33%) of athletes
  required medical attention for heat-related problems
88.0-      73.4 1996  Olympics, Men's Marathon, 14/124 (11%) of runners required
  medical attention for heat-related problems     
1996 Olympics, Women's Marathon, 21/86 (24%) of runners required
  medical attention for heat-related problems    

This recital of temperatures, heat standards and heat injuries underscores the complex nature of discussions of temperature and safety. In its simplest terms, the relationship between athletes and heat stress can be broken down into four elements: The degree of environmental heat stress (air temperature, relative humidity, wind, radiant heat), the athlete's metabolic rate (how hard he is working), the athlete's overall fitness (as judged by ability to take up oxygen), and the athlete's ability to endure/dissipate heat (related to acclimatization and genetic factors).

The heat stress of sitting in a 175F sauna with a 28% relative humidity for a short time (10 minutes) has been estimated to approximate the effort involved in a brisk walk. It has been claimed that anyone who can walk into a sauna can probably walk out of one. So tremendous heat, encountered briefly and at rest, can be tolerated by almost anyone.

At the other end of the spectrum, Nielsen has pointed out that marathon running is "a fight against physics." By calculating rates of heat generation and the sweat rates which that heat burden engenders, she has shown the maximal sustainable work rates allowed by various climatic conditions. At an air temperature of of 95F and a relative humidity of 60% (heat index = 114F, WBGT ~90F), a high-intensity marathon is no longer possible. For an athlete to proceed at a world-record pace in such conditions would lead to rising core temperature. At that point, the athlete would have to decide to slow down immediately, or to refuse to slow down and in short order to collapse. Of course, the athlete could slow down to a walk, in which case he could finish his marathon even in hellish conditions. But it would be a poor race indeed.

So the question "How Safe Is Safe?" must be understood to contain three separate elements: The severity of the weather conditions, the fitness of the athlete, and the amount of performance stress being applied to the athlete. Each of these elements can be manipulated to increase or decrease the heat safety of an event.

Event organizers have a great deal of control over the severity of weather conditions, a modest amount of control over athlete fitness, and substantial control over performance stress.

Weather conditions: Below 60F, heat-related injury rarely occurs. There is a progressive and essentially linear rise in heat injury as the WBGT rises from 65F to 75F. Between WBGT 75 and 85, injuries continue to rise, but at a much slower rate. This almost certainly reflects the effect of decreasing performance, as athletes slow up in response to rising core temperature. There is very little written about heat injury rates at WBGT >90F, since properly-planned endurance competitions rarely are held in such conditions. The message here is clear: Event organizers should aim for the lowest possible heat index over 50F. Even relatively small decreases in heat stress can yield  benefits, and 8F reductions in WBGT will have a major impact on both injuries and quality of play. As shown in "Weather Roulette in Pittsburgh," moving an event 15 days earlier on the calendar and 2 hours earlier in the day can reduce average heat index by 10F, a major benefit. Event organizers must measure temperature and relative humidity or WBGT at the event site during the event to fully recognize heat stress risk.

Athlete fitness: Event organizers must realize the critical role of heat acclimatization in determining the athlete's response to heat stress. Events played in very hot weather early in the spring-summer period, before athletes have had the 10-14 days of heat exposure necessary to develop heat tolerance, are at risk of producing a large number of heat-related injuries. Furthermore, heat tolerance can be lost relatively quickly. Athletes returning to competition after illness, injury or final exams will also be at increased risk. Obviously, sufficient water must be available at the event site, and adequate shade if the event permits it.

Performance stress being applied to the athlete. There is absolutely no doubt that heat stress is cumulative. An athlete who has been exposed to high heat stress on one day is clearly at increased risk of heat injury the next day, even if temperatures are cooler. In multi-day events, there must be adequate spacing for recovery. Furthermore, prolonged same-day exposure to high heat stress, even at a relatively low effort level, takes a significant toll. The athlete who sees an event as life-or-death will have a significant likelihood of continuing to maintain a high effort level even when his core temperature is rising. It has been shown repeatedly that heat injuries in endurance events are more likely to occur near cut-off times. Event organizers who want to avoid heat injuries will take these factors into account. It must also be remembered that a fairly consistent finding in severe heat injury is athlete confusion. In fact, the degree of confusion appears to increase linearly with increasing core temperature. Relying on the good judgment of the athlete regarding his ability to continue exercise is an unsound policy.

Conclusion: It is our feeling that the great tangle of facts, suggestions and decrees outlined in this article reflect the complexity of the questions, How hot is hot? How Safe is safe? The answers to these questions, it turns out, are situation-dependent and probabilistic. Every day, every competition and every athlete is a casino. We walk into the casino and we make bets. Insightful gamblers, who respect the seriousness of the game and have prepared diligently, will make a vast number of winning bets, and a few losers. Others will flounder. The game cannot be simplified: It is inherently complex. There will always be those who deny this reality, and they will propose simple solutions. Every complex problem will prompt suggestions of simple solutions, and all of those simple solutions will be wrong.

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