Why Does Football Have Two Sets Of Heat Stress Rules?

Football is a very strange game. Playing football requires a protective uniform that covers half of the body surface with waterproof materials. Evaporation of sweat from skin covered by these materials is impossible.

With half of the body's "air conditioner", the skin, rendered incapable of providing any cooling, the burden for sweat evaporation is concentrated on a relatively small amount of skin. Anything that prevents evaporation from this small skin surface, such as a high relative humidity, will make cooling of the football player much more difficult. So, the first heat stress hazard for football players is relative humidity. That hazard is addressed by the guideline set called the Wet Bulb Rule. Wet Bulb Temperature reflects the ability of water to evaporate. The higher the Wet Bulb Temperature, the harder it is for sweat to evaporate, and the more difficult it is to cool the uniformed football player.

The Wet Bulb Rule is of particular importance in regions of high heat and high humidity. This is particularly important in southern states such as Louisiana, Mississippi, Alabama and Florida, but similar conditions can sometimes be found throughout most of the US.

The second heat stress hazard for football players is radiant heat. This is the blazing sun that bathes the desert areas of the US, and can be found over much of the country in August. This heat load is not reflected efficiently in Wet Bulb Temperature. Instead, Wet Bulb Globe Temperature, the heat stress standard used by the US Marine Corps, nicely incorporates radiant heat load into its measurement system.

Wet Bulb Globe Temperature (WBGT) is a very satisfactory heat stress measurement system for the great majority of exercise situations. However, the peculiar heat stress imposed by the football uniform, which can be strongly influenced by relative humidity, can sometimes be underestimated by Wet Bulb Globe Temperature.

To meet this unusual challenge, two sets of heat stress standards are needed. The Wet Bulb Rule covers high relative humidity situations (e.g, Shreveport) and the WBGT Rule covers intense sunlight situations (e.g., Las Vegas). Heat Stress Adviser automatically provides both sets of recommendations to the user. Every decision regarding use of these recommendations must be made based on local conditions, but the most cautious approach is to employ the Rule that is the  COOLEST. So, if one recommendation is "Shirt and shorts only" and the other is "Remove uniform if possible", the "Shirt and shorts only" approach is coolest and therefore safest.

Edward Fox and Donald Mathews (of Ohio State University) published papers on the physiology of exercise in football uniforms in 1966 and 1969. (See references for exact citation.)

These works have continued to provide us with much of what we know about exercise in football uniforms. The 1966 study, carried out in 5 high school football players, showed that 20 minutes of running in a football uniform at Dry Bulb Temp = 74-77°F produced 0.5-0.7 °F more rise in rectal temperature than similar running in a scrub suit. After stopping exercise, core temperature continued to rise for 4 minutes in the football uniform, then very gradually fell from 102°F to 101.2°F over 30 minutes after stopping exercise. Sweat loss was 78% greater in the football uniform.