During 1990 in New York and again in Barcelona in the following year, the International Marathon Medical Directors Association (IMMDA) initiated studies to monitor climatological conditions accompanying several endurance outdoor sports events in different locations. The aim was to identify the changing thermal stress on both a competitor and exposed spectator along the course of any major endurance racing event around the world where there were climatological conditions similar to Barcelona during the 1992 Olympic Games. In this way any combination of type of surroundings, time of day climatological conditions, etc., along an event's course found to present a thermal stress to either an athlete or spectator might be avoided by better scheduling, better routing, increasing the number of aid stations along a route, and other precautions. To evaluate a single measure of the climatological conditions surrounding an event, a monitor with three thermometers was used to obtain the wet bulb global temperature value, which is an index of the risk of heat injury. The present article is an historical review of how this work was started and expanded. Data obtained in New York and Barcelona enabled the President of IMMDA to present, in October 1991, a report to the International Olympic Committee Medical Commission on the possibility of determining daily throughout the period of the Olympic Games the level of thermal stress on both competitors and spectators during long‐lasting events. The events contributing these data were the New York City Triathlon and the Hispanic Half Marathon held in New York City in 1990 and the Race Walking Spanish Championships (20 km), the IAAF Mobil Grand Prix Finals, and the XIV Marathon Catalunya, all held in Barcelona, Spain, in 1991.