Heat tolerance of Fire Service Instructors

Fire Service Instructors (FSI) experience repeated fire exposures a median of 13 ± 8 times a month; consequently they may develop an acclimatised state. However, the chronic immunological implications of heat acclimation are yet to be understood. This study aimed to establish whether FSI exhibit an increased heat tolerance and altered immunological response to heat exposures, compared to non-exposed individuals. The study also aimed to identify if heat tolerance is related to symptoms of ill health.

Twenty-two participants were recruited: 11 FSI (age: 41 ± 7 yrs, body mass: 77.4 ± 12.2 kg, height: 174.1 ± 8.2 cm) and 11 non-exposed controls (CON) (age: 41 ± 7 yrs, body mass: 75.9 ± 12.2 kg, height: 177.0 ± 8.1 cm). Participants completed a 40 min heat occupational tolerance test (HOTT) exercising at 6 W kg^-1 (50.0 ± 1.0 °C, 12.3 ± 3.3% relative humidity) on two occasions, separated by 2 months. Physiological and perceptual measures were collected throughout and venous blood samples were collected prior to and post exposure.

FSI displayed significantly reduced peak rectal temperature (T_re) (-0.42 °C), change in T_re (-0.33 °C), and thermal sensation (-1.0) and increased sweat rate (+0.25 L h^-1) at the end of the HOTT compared to CON (p < 0.05). FSI exhibited similar responses to the HOTT as CON for all haematological variables. However, resting interleukin-6, interleukin-1β, and immunoglobulin G were significantly greater in FSI than CON. There was no difference in responses following the 2 month working period. FSI peak T_re was negatively correlated with symptoms of ill health (r_pb = -0.473, p = 0.026) and the number of fire exposures in the previous 2 weeks (r_s = -0.589, p = 0.004).

Despite increased heat tolerance compared to non-exposed individuals, FSI may develop a maladaptation to repeated fire exposures, with elevated resting cytokine levels and an increased prevalence of ill health symptoms.