Abstract: Sudden cardiac death accounts for approximately 45% of duty-related fatalities among United States firefighters. Strenuous physical exertion is recognized as a trigger of sudden cardiac events. This study describes the duration of strenuous physical exertion on-scene preceding a fatal cardiac event by situation encountered during firefighting duties. Data provided by the National Institute for Occupational Safety and Health Fire Fighter Fatality Investigation and Prevention Program for duty-related firefighter fatalities over a 9-year period were analyzed. Among the 235 fatalities investigated, 45% had a cause of death that was cardiac in origin. Time (mean ± SD) spent on-scene in strenuous work was 30 ± 26 min, 14 ± 15 min, and 47 ± 52 min for fire, non-fire emergency, and training situations, respectively. Across all emergency operations and training, 31% of fatalities occurred among firefighters who performed ≤10 min of strenuous work, whereas 13% of fatalities occurred among those who performed >60 min. Study findings indicate that there is considerable variability in the duration of strenuous work preceding fatal cardiac events during firefighting duties. Notably, a high percentage of fatal cardiac events occurred after a relatively brief period of strenuous work, suggesting that the performance of any strenuous work, even that of short duration, may be sufficient to provoke a cardiac event in a firefighter with underlying cardiac disease.