On April 11, 2001, the National Institute for Occupational Safety and Health (NIOSH) received a health hazard evaluation request from International Brotherhood of Electrical Workers (IBEW) Local 2331 Union officials at Thomson Multimedia, Inc., in Circleville, Ohio. The request described heat strain, heat exhaustion, and worker fatigue among the workers in the facility's television glass panel and funnel forming department. A NIOSH industrial hygienist and a NIOSH medical officer visited the Thomson facility on July 18 and 19, 2001. Environmental temperatures were recorded at ware handler and press operator areas on three different production lines using wet-bulb globe temperature (WBGT) monitors. To assess workers' physiological effects, personal monitoring was performed, including continuous monitoring of workers' core body temperature (CBT), skin temperature, and heart rate. Workers' pre- and post-shift weights were recorded. During breaks, workers were interviewed about health symptoms they may have experienced as a result of working in a hot environment. Recorded WBGT readings reached highs of 94.2 , 95.4 , and 102.8 F in various areas of the three forming lines, with average readings of 91.2 , 91.5 , and 98.8 F, respectively. Only one worker exceeded a core body temperature (CBT) of 101.3 F, the American Conference of Governmental Industrial Hygienists (ACGIH) guideline for medically selected, acclimatized workers. However, the guidelines were exceeded for a few, non-consecutive minutes during the shift. None of the monitored workers had a sustained heart rate in excess of ACGIH's evaluation criteria (180 beats per minute [bpm] minus the individual's age in years). Two of five workers monitored exceeded 1.5% loss of body weight during their shift, a sign of risk for heat strain. Eighty-two percent of interviewed workers reported experiencing dehydration while at work in the past. Other symptoms reported included fatigue, lightheadedness, nausea, and near-syncope. NIOSH investigators concluded that hazardous heat stress conditions can exist at the facility. However, physiologic data from monitored workers did not show signs of heat strain on the day monitoring occurred. Temperatures in the work environment suggest a need for employees to take full advantage of the workplace controls for prevention of heat strain. The workplace controls include not exceeding the work/rest schedules and using the cooled break room during rest breaks, particularly when environmental temperatures are high during the summer months. Workers' hydration status needs to be maintained. Communication between employees and management needs improvement, particularly in the area of reporting symptoms of heat strain experienced by the employees.