Abstract: Between July 29 to August 2, 1996, Employee #1 was engaged in roof maintenance work in the high desert area of Palm Springs and Yucca Valley. On July 31, he did moderate work on a roof for 4 to 5 hours, and felt ill afterward. The next day he worked with a reduced workload and out of the direct sunlight. Employee #1 still felt ill while driving to his job the following morning and he returned to his maintenance base. He was driven to the hospital, where he was eventually diagnosed with heat-related illness. WBGT threshold limit values and metabolic heat load were calculated for Employee #1's roof work on July 31 using weather data from August 2, which was similar. The employer's heat stress prevention instructions included a 60/30 ratio of work to rest. The ACGIH recommended ratio is between 50/50 and 25/75 work to rest. Employee #1 had suffered a heart attack the previous year, but had been cleared to return to work.