Wildland firefighting entails demanding physical activity over extended periods (1). These challenges are exacerbated by contextual factors such as high ambient temperatures, heavy manual tool handling, and steep terrain, all while wearing personal protective equipment (PPE) (2, 3). However, the cumulative effect of these factors can lead to a non-compensable heat load (3, 4), compromising physical performance and increasing the risk of heat-related disorders or even heatstroke (5, 6). To mitigate these risks, various cooling strategies have been explored, including personal cooling garments (7–9) and passive cooling techniques (10).

The scientific literature has extensively investigated the efficacy of personal cooling garments, such as water-perfused suits, air-perfused suits, ice vests, and phase change material vests, in mitigating heat strain and extending working durations in thermally demanding environments among various groups, including military personnel (11), military firefighters (12), and structural firefighters (8, 13). These garments have been shown to alleviate physiological and perceptual strain by facilitating heat transfer from the body’s core to the periphery, consequently attenuating the rise in central temperature (14). Given that a substantial body surface area is required for effective heat transfer, cooling garments typically focus on the torso, which constitutes approximately 24% of the total body surface area (15). Cooling vests, a subset of personal cooling garments, employ active cooling mechanisms to lower skin temperature through heat conduction (16). Extensively studied in sports contexts (17, 18), cooling vests have demonstrated their capacity to mitigate core temperature elevation and enhance performance when utilized before (pre-cooling) or during exercise (per-cooling), as well as expedite recovery post-exercise (post-cooling) (18). In occupational settings, employing an ice vest or a phase change material vest under firefighter gear (7, 19) and chemical, biological, radiological, and nuclear (CBRN) suits (8, 20) has been found to alleviate physiological and subjective strain during strenuous work in hot conditions, potentially extending tolerance times by 10–20%.

While PPE aims to shield first responders from diverse hazards, it can exacerbate metabolic and thermal responses, especially in hot environments (21, 22). Studies have showed that components such as helmets, gloves, and boots significantly impede body heat dissipation and heighten the thermophysiological strain on wildland firefighters (WFFs) (3) and structural firefighters (22). Consequently, removing one or more PPE components has been proposed as a heat mitigation strategy in occupational literature (10, 23). This passive approach enhances body heat dissipation by facilitating sweat evaporation and improving heat transfer through radiation and convection, thereby reducing body temperature and enhancing perceived comfort (24). Notably, this strategy has been observed to reduce thermal strain to levels comparable to active cooling during breaks in moderate environments (24°C) (10), making it a common choice among structural firefighters and WFFs (9, 25).

Despite the critical impact of heat strain on WFFs’ health and safety, there is a paucity of research investigating the effectiveness of cooling strategies, both active and passive, in mitigating the thermal load experienced by WFFs. This gap leaves unclear whether employing cooling strategies during wildland fire suppression task effectively mitigates the adverse effects of prolonged exposure to high temperatures. It is also crucial to note that while improving aerobic fitness and heat acclimatization have proven to be efficient strategies to combat heat stress over time (26, 27), they require long-term planning to realize benefits. Therefore, alternative measures or strategies may offer more immediate effectiveness. The aim of this study was to address this gap by examining the impact of both active (cooling vest) and passive cooling interventions (PPE removal) on WFFs’ physiological strain. We hypothesized that employing either a cooling vest or removing PPE would enhance heat dissipation in WFFs, leading to decreased core temperature and reduced cardiovascular strain, potentially increasing tolerance time.

Tolerance time was similar across all conditions (74.1 ± 14.9, 75.0 ± 16.7, and 75.7 ± 18.7 min in PPE, PASSIVE, and VEST, respectively). However, only T_skin (F = 12.9, p < 0.01, η_p² = 0.62), PSI (F = 6.0, p < 0.05, η_p² = 0.50) RPE (χ² = 9.3, p < 0.05, η_p² = 0.47) and thermal sensation (χ² = 12.3, p < 0.01, η_p² = 0.60) were significantly affected by the study conditions. Lower (p < 0.05) T_skin, RPE, thermal and wet sensation were observed in the VEST condition (Table 1). Notably, significant differences (p < 0.05) in PSI were found only between the PASSIVE and VEST conditions. Furthermore, the thermophysiological response of the subjects varied with the test time (F = 26.9–325.3, χ² = 77.4–101.3, p < 0.001, η_p² = 0.61–0.96) (Figures 2–4). A significant interaction (p < 0.05) was observed between the study condition and test time in T_skin (F = 2.8, p < 0.05, η_p² = 0.32) and PSI (F = 2.5, p < 0.05, η_p² = 0.30).

Finally, the body heat storage (58.4 ± 7.6, 52.7 ± 9.7, and 45.3 ± 15.0 W·m²), fluid intake (855.0 ± 345.5, 750.0 ± 523.8, and 738.7 ± 365.3 mL), total sweat loss (1922.7 ± 511.3, 1895.0 ± 578.5, and 1585.0 ± 502.4 gr), sweat rate (1,317 ± 363, 1,413 ± 457, and 1,229 ± 349 g·h⁻¹) and evaporative efficiency (49.5 ± 4.7, 46.3 ± 13.1, and 46.5 ± 14.6%) were similar between PPE, PASSIVE, and VEST conditions.

The present study aimed to assess the impact of wearing a cooling vest or removing the top part of the PPE ensemble during intermittent rest periods on the thermal strain and tolerance time of WFFs. The primary finding of this investigation was that while the cooling vest led to reductions in mean T_skin, PSI, RPE and thermal sensation throughout most of the test, it did not have a significant effect on exercise tolerance time. However, neither cooling strategy proved effective in reducing the thermal or cardiovascular load or enhancing the tolerance time of WFFs under the experimental conditions tested.

Overall, the results from all three conditions revealed a substantial thermoregulatory and cardiovascular strain, as evidenced by final values of T_gi nearing 39°C (38.9 ± 0.4, 38.8 ± 0.3, and 38.6 ± 0.6°C in PPE, PASSIVE and VEST condition, respectively) and average HR values reaching 95% of maximal individual HR (93.8 ± 4.4, 96.9 ± 2.5, and 94.0 ± 4.8% in PPE, PASSIVE and VEST condition, respectively). These findings are consistent with previous laboratory-based studies investigating the physiological response of individuals wearing personal protective equipment (PPE) in hot conditions (3, 21, 22). The combination of environmental conditions and the high metabolic rate induced by the incremental protocol led to an uncompensable heat strain, irrespective of the cooling intervention, resulting in similar metabolic rates, cardiovascular loads (Table 1) and performance durations (~75 min).

The cooling vest did mitigate the increase in T_gi, particularly in the second half of the trial. However, this effect was insufficient to counter the high thermal load, as indicated by the similar rate of increase in T_gi across all conditions (0.025 ± 0.004, 0.023 ± 0.004 and 0.017 ± 0.008°C·min⁻¹ in PPE, PASSIVE and VEST condition, respectively). Nevertheless, the lower PSI observed in the VEST condition suggests a beneficial impact of the cooling vest on overall physiological load. This effect may be attributed to the lower T_skin (Figure 2) in the VEST condition (PPE vs. VEST = 0.94°C, p < 0.05; PASSIVE vs. VEST = 0.79°C, p < 0.05) which may have resulted in reduced cutaneous blood flow and cardiovascular effort (40). However, the lack of significant effects of the cooling vest in our study is consistent with the findings of Teunissen et al. (7) and Carter et al. (41), who also examined the impact of cooling vests on structural firefighters’ heat strain during exercise (per-cooling). Specifically, this cooling intervention had no effect on core temperature or HR when firefighters performed steady-state exercise in controlled laboratory conditions (30°C and 50% relative humidity) (7), simulated an intervention in an underground tunnel wearing gas-tight suits, or engaged in a simulated rescue activity in either a hot (170°C) or neutral (15–20°C) environment (41).

While many studies in the literature have reported that cooling vests effectively decrease heat strain and increase tolerance time (8, 19, 20, 42), discrepancies in study protocols, particularly in terms of the net heat load imposed on participants, may explain the differing results. Moreover, the intensity of the cooling treatment might have been insufficient to adequately alleviate the uncompensable heat strain (23). Previous studies utilizing ice vests and phase change material vests with melting points above 10°C have also failed to demonstrate reductions in HR and core temperature (43, 44), indicating the necessity of substantial cooling power to impact core temperature. The effectiveness of cooling vests is largely influenced by factors such as mass, coverage area, and cooling medium (7, 19). Thus, the results obtained suggest that the cooling vest tested might not have had sufficient cooling power to adequately mitigate the thermal stress imposed by the experimental conditions.

During the breaks the passive recovery had a significant effect on T_skin, but not on T_gi or HR, being the cooling effect dissipated over subsequent exercise bouts. This observation may be attributed to the short duration of the breaks and the challenges associated with donning and doffing sweat-soaked PPE (45), which can impede the effectiveness of passive cooling during rest periods (4, 46, 47). Furthermore, the lack of an effect of the passive recovery might be linked to the fact that this strategy was implemented in the same hot environment as the treadmill walking bouts (30°C and 30% relative humidity), which might have limited its efficacy. In contrast to our findings, Hostler et al. (10) found that when performed during 20-min breaks in moderate environments (24°C), passive recovery strategy might reduce thermal strain to the level of active cooling in structural firefighters. These results suggest that for passive recovery to be effective, it should be conducted in thermoneutral environments and for a significantly longer duration than that applied in our study.

The significantly lower thermal sensation observed with the cooling vest compared to the other conditions may be linked to the lower mean T_skin achieved in the VEST condition. These findings support previous research indicating that thermal sensation is closely correlated with T_skin in warm environments (4, 7). However, it’s worth noting that a low thermal sensation without a concomitant reduction in T_gi, as observed in the VEST condition, might lead wildland firefighters to perceive a false sense of comfort, potentially exacerbating heat strain and the risk of heat-related illnesses if work continues (4).

No differences were found in total sweat loss or evaporative efficiency between conditions. However, the volume of sweat produced was lower in the VEST condition compared to PPE (~18%) and PASSIVE (~16%). This reduction in sweat loss might be attributed to the visibly lower T_skin in the VEST condition (7, 11). Alternatively, the cooling vest might hinder sweat evaporation from the body (43) and fail to contribute positively to the heat balance, as evidenced by the approximately 3% lower evaporative efficiency in the VEST condition compared to the PPE condition. However, the lack of statistically significant differences in total sweat produced and evaporative efficiency across conditions suggest that evaporation may have been equally restricted across conditions by the WFFs’ PPE (7).

The present study had several potential limitations. One of these was the small sample size, which might restrict the study’s conclusions and limit the ability to observe the effectiveness of the cooling strategies examined. Future studies should consider the expected variability in measures of thermoregulation and include a larger sample size (~20 subjects) to improve the robustness of the assessment of the cooling strategies. Additionally, all participants were male WFFs, meaning the results cannot be extrapolated to female WFFs. The different anthropometric characteristics and thermophysiological response of females compared to males (48) could condition the impact of the examined strategies. Another limitation was the hydration protocol used. Participants were allowed to drink water ad-libitum during rest periods, which could have been a confounding variable in this study. However, we believe this did not substantially affect the results, as there were no significant differences in the amount of liquid consumed by WFFs under the different conditions. Furthermore, this strategy may be appropriate for ensuring optimal fluid replacement (49). Lastly, the use of the cooling vest resulted in an approximate 2 kg increase in weight compared to the other conditions, which could have increased the workload for the subjects, particularly in the final minutes of the test (~2%) and raised the metabolic cost of walking in the VEST condition, potentially limiting the effectiveness of this strategy. However, since this weight was distributed around the torso, its impact on the metabolic rate was small (50).

In summary, despite the main effects observed in T_skin, PSI, RPE and thermal sensation due to the study conditions, as well as the interaction effect between conditions and test time on T_skin and PSI, these did not translate into an increase in tolerance time or a reduction in core temperature and cardiovascular effort among WFFs. While these findings suggest a more pronounced effect of using cooling vests on WFFs’ responses, the high metabolic rate of the protocol or the duration and environmental conditions during recovery periods might have influenced the results obtained. Therefore, future studies should investigate whether passive cooling strategies, coordinated with appropriate work:recovery protocols, could prove effective. Additionally, although practical limitations may exist regarding the use of cooling vests, further research should explore their potential as per-cooling measures or as a post-fire intervention recovery strategy for WFFs facing high thermal loads.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving humans were approved by Ethics Committee of the University of León. The studies were conducted in accordance with the local legislation and institutional requirements. The participants provided their written informed consent to participate in this study.

JG-A: Conceptualization, Data curation, Investigation, Methodology, Writing – original draft. JAR-M: Formal analysis, Funding acquisition, Methodology, Writing – review & editing. FG-H: Data curation, Writing – original draft. JR-M: Data curation, Investigation, Visualization, Writing – original draft. PC: Methodology, Writing – review & editing. JV-C: Conceptualization, Funding acquisition, Methodology, Writing – review & editing. BC-L: Conceptualization, Formal analysis, Methodology, Writing – review & editing.