A person running a marathon in a latitude greater than 40 degrees during the spring or early summer may not be acclimatized to warmer temperatures. This can result in a large number of people collapsing and requiring medical attention. Roberts developed an equation for estimating the number of runners who will not complete a course based on the wet bulb globe temperature (WBGT).


Marathon selection: run in a location at greater than 40 degrees latitude


Runner selection: non-elite regional resident, not yet fully acclimatized to a warmer temperature


Reasons for not completing the race:

(1) exertional heat illness

(2) dehydration

(3) overhydration with hyponatremia

(4) death

Wet Bulb Globe Temperature

Percent Unsuccessful

< 1°C

rises rapidly above 6%

1 to 13°C

around 4 to 6%

> 13°C

rises rapidly above 6%


percent not completing the marathon =

= ((0.514 * ((WBGT)^2)) - 8.097 * (WBGT)) + 78.926) / 10


Decision Level


Percent Unsuccessful

Do Not Start

20.5°C (69°F)



17°C (64°C)




• The impact of altitude was not considered.

• This would not apply to a race run at a lower latitude or under other conditions.


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