With 5 percent body weight loss due to dehydration, work capacity can be decreased by as much as:

• A. 5 percent
• B. 10 percent
• C. 20 percent
• D. 30 percent

Answer: (D)

Dehydration lowers work capacity because even a moderate loss of body water reduces blood volume, making it harder for the heart to pump blood to both the skin for cooling and the working muscles. The body compensates by raising heart rate and increasing vascular strain, which can speed fatigue. Heat load compounds this: with reduced plasma volume, core temperature rises more quickly, perception of effort increases, and muscle function can decline. In firefighting settings—where heat, protective gear, and demanding physical work converge—about 5% body weight loss can reduce ability to sustain work by as much as about 30%. The combination of faster fatigue, greater thermal strain, and higher effort required explains why the impact can be so large.