An index of the environmental thermal load imposed on exercising horses and riders by hot weather conditions.

There is a need to determine objectively the environmental heat load imposed on horses competing to an international standard in 3-day-events in environments where there is likely to be a high level of radiation added to high ambient temperatures and relative humidity; the presently used FEI 'Comfort Index' is severely limited in its applicability. It is proposed that the Wet Bulb Globe Temperature (WBGT) Index be used. This index was originally established for man exercising strenuously under harsh climatic conditions with high solar radiation levels. It may be defined as: WBGT = 0.7Twb + 0.3Tg where Twb is the wet bulb temperature of the ambient air, measured in the shade and Tg is the black globe temperature. Climatic conditions should be monitored continuously at a site representative of the detailed topography and nature of the terrain of the competition course. The upper limit for the index for competitions involving fit, fully acclimatised horses, running on ground of optimal going, is recommended to be 32.5 degrees C. This limit should be reduced, by an as yet undetermined amount, for unacclimatised and unfit horses or when the going is less good.