Objectives In order to make recommendations to protect wildland firefighters (WLFFs) from heat-related illness, the role physical exertion plays on core body temperature (Tc) in the work environment needs to be considered. We aimed to estimate the association of time-varying job tasks with differing exertion levels on change in Tc, among WLFFs engaged in fire management activities, while controlling for ambient conditions and individual characteristics. In addition, we examined whether duration of tasks modified the relationship between task and change in Tc.

Methods We used data from The Wildland Firefighter Heat Related Illness study, a repeated measures study with individual-level data on 301 WLFFs from 2013 to 2016. Job tasks over one full shift were recorded and categorised into four levels (sedentary, light, moderate and high) based on estimated exertion levels. Tc was measured every 15 s using an ingestible sensor and personal monitors measured ambient conditions. The analysis used generalised estimating equations with an autoregressive correlation structure.

Results We found statistically significant associations between job tasks with increased physical demand and increases in Tc. When compared with sedentary tasks, the estimated increases in Tc for light, moderate and high activity tasks of average duration were 0.07°C (95% CI 0.02 to 0.11°C), 0.19°C (95% CI 0.15 to 0.23) and 0.44°C (95% CI 0.39 to 0.49°C), respectively. In addition, we found evidence of interaction between task type and duration.

Conclusions This study supports the hypothesis that occupational exposure to physical exertion increases Tc, highlighting the need to update recommendations to protect WLFFs from heat-related illness.