Heat Strain Is Exacerbated on the Second of Consecutive Days of Fire Suppression

We tested the hypothesis that physiological and perceptual heat strain is exacerbated on the second of back-to-back days of fire suppression work despite evidence of full recovery.

Twenty-six career and volunteer firefighters (age = 31 ± 8 yr) completed 20 min of near maximal fire suppression work on consecutive days. Dependent variables were core temperature, heart rate, perceived exertion, and thermal sensation, which were measured before and after fire suppression. Urine specific gravity and body mass were also measured upon arrival at the fire academy as an index of hydration and recovery between days.

Urine specific gravity (1.007 ± 0.006 vs 1.005 ± 0.006), body mass (87.7 ± 16.1 vs 87.8 ± 16.0 kg), heart rate (77 ± 14 vs 76 ± 14 bpm), and core temperature (37.2°C ± 0.4°C vs 37.1°C ± 0.7°C) were not different upon arrival on day 1 compared with day 2 (P ≥ 0.26). The increase in core temperature during fire suppression was higher on day 2 (0.7°C ± 0.3°C vs 1.1°C ± 0.5°C, P < 0.01). Heart rate did not differ (after fire suppression: day 1 = 174 ± 19, day 2 = 169 ± 30 bpm, P = 0.60). The magnitude of increase in perceived exertion during fire suppression was greater on day 2 (6.7 ± 1.6 vs 7.4 ± 1.6 a.u., P < 0.01). Absolute thermal sensation at the end of fire suppression was greater on day 2 (3.8 ± 0.8 vs 4.3 ± 0.6 a.u., P < 0.01).

Physiological and perceptual heat strain is higher on the second of back-to-back days of fire suppression work.