Effect of heat exposure and simulated physical firefighting work on acute inflammatory and cortisol responses

Objective: To examine firefighters' hormonal and immune markers during consecutive days of physical firefighting work performed in hot compared to mild ambient temperatures. Methods: Firefighters completed 3 days of simulated physical firefighting work in either hot (HOT condition; n = 19; 33°C) or mild temperature conditions (CON condition; n = 18; 19°C). Participants provided regular daily samples for the determination of salivary cortisol and plasma cytokine levels (IL-6, IL-8, IL-1β, TNF-α, IL-4, and IL-10). Results: The HOT condition elicited higher IL-4 and a trend towards elevated afternoon and evening cortisol when compared to the CON trial. The HOT condition also produced lower levels of IL-1β compared to the CON across time points and a decrease in IL-1β between days of work. IL-6 increased across time points and between work days, but this finding was not different between mild and hot conditions. Immune-endocrine interactions revealed a rise in morning IL-6 that was related to elevated daily cortisol levels, independent of condition. Conclusion: Findings demonstrate the possibility that firefighters are able to regulate normal acute immune and hormonal responses to multiple days of simulated physical work in hot and mild ambient temperatures. Further research is necessary to determine if the responses continue under non-simulated conditions and in response to more extreme temperatures possible on the fire-ground.