Sweat, Page 5

Nielsen concludes this tour de force of calculation by examining how fast a marathoner could run, as limited by weather conditions. For this example, she specifies a 67 kg runner, air temperature of 95°F (thus assuring that all heat loss will be due to the evaporation of sweat), and varies the relative humidity, thus limiting the amount of sweat which can evaporate per hour. Here are the absolute physical limits to marathon times, as constrained by the ability of the environment to accept heat from the evaporation of the runner's sweat:

Running speed (km/h)	Marathon time	Minutes	Sweat Evaporation Rate in liters/hour
19.4	2 h 10 min	130	2.10
18.0	2 h 20 min	140	1.95
16.9	2 h 30 min	150	1.80
15.8	2 h 40 min	160	1.70
14.9	2 h 48 min	170	1.60
14.0	3 h 00 min	180	1.50
12.7	3 h 20 min	200	1.35

The world record for a marathon is 2:06:50. If the environment only permits a sweat evaporation rate of 2.1 liters/hour, this record pace is unattainable. As conditions progressively worsen to an air temperature of 95°F and a relative humidity of 80% (heat index = 136°F, WBGT~96°F), only 1.35 liters of sweat could evaporate per hour.The best marathon time which could be accomplished by any marathon runner in such conditions would be 3 hours and 20 min. This would be the marathon run in hell, and no reasonable event organizer would ever permit it.

But what if a runner decided to defy physics and increase his pace in the face of a core temperature which had already risen to high levels? Hanson and Zimmerman reported on 4 novice long distance runners who experienced heatstroke with loss of consciousness. These runners, who were competing in races of 10, 16, 32 and 42 km, each tried to increase his running pace by approximately 0.5 -1.0 min/km late in the race. Race conditions were variable, with heat index values of 61°F, 79°F, 76°F and 78°F, respectively, indicating low (61), moderate (76,78), and moderately-high (79) risk of heat injury. Sun exposure was 60% to 100%, and windspeed varied from 3 mph to 12 mph. In each of these runners, the onset of clinical heatstroke occurred within 5 to 15 minutes of the attempted increase in work rate. These athletes were literally teetering on the edge of disaster, and a brief increase in work load pushed them into the abyss. By the grace of expert medical attention, all survived. Core temperature is an unforgiving trial judge, and those who ignore its dictums are severely punished.

This lengthy examination of mechanisms by which heat is dissipated in hot conditions drives home the importance of the evaporation of sweat as a prime cooling process. In conditions in which the theoretical maximum evaporation of sweat has not been reached, the ability to sweat more should confer a competitive advantage to an athlete. Since sweat is exclusively produced by skin, having more skin should be useful.