The charts are laid out in a straightforward way. At the bottom of each chart is a listing of dates throughout the year. The first day of the month and the 15th day of the month are used for each month and displayed in standard form: January 1st is 1/1, January 15th is 1/15, etc. Along the left side of the chart are markings to indicate Heat Index in degrees F. Lines are drawn across the chart to indicate the American College of Sports Medicine levels of risk for heat-related injury. These levels are meant to take into account not only heat and humidity, but also the effects of wind and sunshine.

We have spent a great deal of effort in establishing the proper positions for these risk zones. In this work, a substantial amount of data from the Atlanta Olympics micro-climate profiling studies has been used.  A model was constructed linking the American College of Sports Medicine 1996 guidelines for prevention of heat illness during distance running (as expressed in Wet Bulb Globe Temperature), and National Weather Service Heat Index values. The results are as follows:

ACSM Risk Category Wet Bulb Globe Temp NWS Heat Index Temp
Dangerous Zone             >90F              >115F
Very High Risk Zone          82 - 90F         98 - 115F
High Risk Zone          73 - 82F         80 - 98F
Moderate Risk Zone          65 - 73F         65 - 80F
Low Risk Zone          < 65F          < 65F

Even so, these guidelines, which are approximations,  may eventually require minor adjustment as more data becomes available.

These risk zones ("low risk zone," "moderate risk zone," "high risk zone," "very high risk zone" = "event delay threshold zone",   and "dangerous zone") are to be considered advisory. Untrained and unacclimatized athletes are much more likely to experience heat injury at any risk level than are those who are better prepared. Even so, extremely fit and acclimatized athletes can occasionally experience serious exercise-related heat injury, even in the "low risk zone." The chief benefit of these zone markings is to give the athlete, coach or event organizer an overview of relative heat stress likely to be encountered during an event held at a specific time and place.


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