Heat strain in professional firefighters: physiological responses to a simulated smoke dive in extremely hot environments and the subsequent recovery phase

Abstract

Abstract: Firefighters risk heat strain during occupational tasks when exposed to extremely hot environmental conditions and performing high-intensity work. Relevant training scenarios are therefore essential. This study investigated the effect of a single simulated smoke dive and the following recovery phase on physiological and perceptual responses. Nineteen professional male firefighters (43 ± 8 yr) performed a 2-min stair walk and a15-min simulated smoke dive in a two-floor heat chamber (110°C to 272°C) (HEAT), followed by a 5-min stair walk outside the heat chamber. Heart rate (HR), gastrointestinal temperature (T_gi) and skin temperatures were registered continuously during the test. The T_gi increased significantly from the start (37.5 ± 0.3°C) to the end of HEAT (38.4 ± 0.4°C) and further increased after the heat exposure (39.6 ± 0.5°C). The HR also increased significantly from the start (92 ± 14 bpm) to the end of HEAT (185 ± 13 bpm) and increased after the heat exposure to a maximum of 190 ± 13 bpm. The simulated smoke dive induced high physiological strain on the firefighters, and the increase in T_gi and HR after the hot exposure must be considered during live fire events when repeated smoke dives are required.

Introduction

At the fire scene, firefighters perform various tasks such as lifting, pulling, dragging, and carrying heavy objects¹⁾. During this high-intensity work, they are exposed to extremely hot environmental conditions.

Together with heavy and semipermeable protective clothing and equipment, significant levels of thermoregulatory and cardiovascular stress may occur^2,3,4,5,6,7), risking heat strain during their occupational tasks. Increasing core temperature (T_c) can negatively affect cognitive performance, reduce the person’s physical reactivity and ability to reason clearly and increase the risk of heat-related illnesses^8,9,10). Many firefighters also experience physical, cognitive, sleep-related, burnout and emotional fatigue, a pervasive problem among others in firefighting occupations¹¹⁾. The combination of high metabolic heat production, wearing protective clothing and equipment, and high ambient temperature leads to uncompensable heat stress wherein the heat production exceeds the heat loss potential, leaving the body in a state of continuous heat gain^{12, 13)}.

Firefighters typically utilise self-contained breathing apparatus (SCBA) rated for approximately 20 min of air, and bouts of firefighting activity may be repeated several times after short breaks for recovery and rehabilitation (i.e. cooling, hydration and change of air cylinder)²⁾. Short breaks of 20 min are not necessarily enough for the T_c to return to normal, and it is suggested that approximately 60 min is required for the firefighter’s heart rate (HR) and T_c to return to baseline levels^{2, 14)}. Thus, firefighters often return to activity at higher T_c levels for each bout of activity²⁾, which may lead to increasingly higher peak T_c for each bout. The T_c after a set time in an exposure is also affected by the baseline T_c¹³⁾. However, physiological data from actual firefighting activity or realistic training are scarce.

Laboratory tests¹⁵⁾ and work-simulated tests^{1, 16, 17)} have been developed to ensure sufficient physical abilities for firefighters. Physical employment standards differ between countries and often even between fire departments in the same country^{1, 18)}. In Norway, firefighters complete tests annually at room temperature to monitor physical fitness. To also ensure that the firefighters are capable of performing their duties in hot and confined places, often under high physiological and mental stress, it must be documented that an assessment has been made of the smoke and chemical diver’s tolerance for or ability to manage intense heat over time, work in tight spaces and to perform under high-stress levels¹⁹⁾. This is most often assessed through firefighter training, specific scenarios or drills, which gives valuable experience to both the individual firefighter and their leaders. Furthermore, it facilitates measuring their physiological responses during relevant environmental exposure and physical activity, a task considerably more challenging to accomplish during actual live events.

Personal protective clothing and equipment protect the wearer from serious personal injury. In firefighting, several layers of clothing are necessary to reach the needed protection level^{20, 21)}.

In Norway, there are no standard practices regarding the type of underwear and clothing layers worn under the firefighting outer garment, and preferences are different between firefighters and fire stations. Whereas some prefer the added protection of two layers, others prefer one layer underneath. Smith et al.²⁰⁾ examined the effect of clothing layers worn under firefighting turnout gear on physiological and perceptual responses during work and recovery cycles in a laboratory study at 21°C. The study demonstrated that the additional layer of clothing (station uniform) imposed no greater physiological or perceptual strain during moderate-intensity work bouts compared with the t-shirt and turnout gear ensemble. However, to our knowledge, no prior investigations have explored whether wearing one or two layers beneath turnout gear leads to differences in the physiological responses in more intense heat conditions like those encountered in real-life events.

Although heat exposure and protective clothing have previously been studied extensively on physiological responses in laboratory settings (ambient temperature <50°C), less is known about work strain and physiological and perceptual responses during realistic training scenarios in extremely hot conditions. Since heat stress tests have been introduced as annual mandatory exercises for firefighters, and such tests differ between countries and even within countries, acquiring more knowledge about them is essential to ensure good health and safety for the firefighters. Hence, the primary objective of this study was to examine the impact of authentic heat exposure and physical exertion on firefighters’ heart rate (HR), core temperature, and skin temperatures throughout a 2-min stair walk and a 15-min simulated smoke dive in a heat chamber followed by a 5-min stair walk, along with the subsequent recovery period. Additionally, we examined whether wearing a single or double layer of woollen underwear during the smoke dive had any measurable influence on the participants’ physiological responses. This knowledge may be valuable for developing high-performance clothing, relevant protocols for training scenarios, implementing health promotion measures and improving performance and safety in high-temperature environments for firefighters.

Subjects and Methods

Participants

Twenty male professional firefighters from a fire brigade in Norway volunteered to participate in the study. Because of missing data (HR and T_gi) from one of the participants, nineteen subjects were included in the results chapter. They represented a wide range of ages (34–59 yr), body mass (76–108 kg), BMI (23–33 kg·m⁻²), and years of experience (1–36 yr) (Table 1). Consistent with the principles of the Declaration of Helsinki, the participants were informed about the aim of the study, the test protocol, and their rights to terminate their participation at any time before they provided written consent. The Norwegian Centre for Research Data (NSD) approved the study.

Table 1. Participant characteristics (n=19).

	Mean ± SD	Range
Age (yr)	43 ± 8	34–59
Height (cm)	179 ± 5	169–190
Body mass (kg)	84.6 ± 7.4	75.7–107.6
BMI (kg·m−2)	26.3 ± 2.2	23.4–32.7
Total weight with PPE (kg)	107.6 ± 7.2	98.1–129.7
Experience (yr)	14 ± 9	1–36
Estimated HRmax	183 ± 5	173–189

Estimated maximal heart rate (HR_max): 211 − 0.64*age²⁴⁾.

BMI: body mass index; SD: standard deviation; PPE: personal protective equipment.

Experimental design

The study was part of the firefighter’s yearly training schedule in hot conditions. The test took place over three days at the training facilities to the participating fire brigade during one of their regular work shifts. The firefighters met at the fire station at 09:00 a.m. to be given information about the test and sign the written consent. They ingested the telemetric pill for gastrointestinal temperature (T_gi) measurements and were equipped with sensors for HR and skin temperature measurements. Anthropometric data (age, height and weight) were registered, and new under- and middle-layer clothing were handed over. Four hours later, the firefighters met at the training facility, fully dressed in firefighter clothing, to be given further information about the test schedule by the staff at the fire brigade. The weight of the firefighters, including full PPE, was registered, and sensors for measuring the ambient temperature outside each firefighter’s jacket were attached. A questionnaire about thermal sensation, comfort²²⁾ and self-perceived exertion²³⁾ was answered before start of the 2-min stair walk and again immediately after finalising the 5-min stair walk.

Personal protective clothing

All firefighters used the same flame-retardant woollen shirts and pants (Devold^® Spirit, Norway). Nine of the firefighters were selected to also wear a middle layer consisting of a wool jacket and wool pants (Devold^® Shield, Norway). To ensure equal distribution of sizes in the two groups, the ones who wore the extra undergarment were selected based on the preferred sizes used by the firefighters (3 of 5 medium sizes, 6 of 13 large sizes, 0 of 1 extra-large size). Personal undergarments and socks were used. The firefighters used their personal protective outer garment provided by the employer; jacket and trousers of two different brands (Wenaas Workwear AS, Norway or Viking Life-saving Equipment AS, Denmark). Dependent on the number of fires and wear and tear, the firefighters ask for new outer garments when needed, which implies that the age of the participant’s outer garment varied between 0 and approximately ten years. In addition, a fire hood/balaclava (Viking), helmet (Rosenbauer or Magma), gloves (Granqvist) and shoes (Haix Fire Hero, Alfa or Kofra) were used. During the heat exercise drill, the firefighters also used masks and carried a self-contained breathing apparatus (SCBA), communication equipment and light.

“Heat exercise drill”

The “heat exercise drill” was designed by the safety officers of the fire brigade to approximate a fire-suppression response and took place in the burn facility at the fire brigade. All subjects wore full turn-out gear SCBA and were consented in firefighter teams of four (three groups), five (one group) or two (one group). A safety officer accompanied all groups. The heat exercise drill consisted of three main parts: (1) the firefighters moved to “the scene of fire” (stair walk ~2 min), (2) physical work at “the scene of fire” (~15 min) and (3) retreat from “the scene of fire” (stair walk ~5 min). The firefighters had a resting period (baseline) and a recovery period before and after the heat exercise drill (Fig. 1). Before the start, the SCBA was checked, the air pressure was recorded, and the firefighters answered questions about thermal sensation and comfort.

Fig. 1.

Schematic of the test setup.

Part 1: Move to “the scene of fire”

The firefighters started with stair climbing (~2 min) in the stairwell, from the ground floor to the 4th floor, with objects (fire hoses and cans) in each hand. The objects were left on the 4th floor, and the firefighters returned to the 1st floor and entered the corridor into the warm environment.

Part 2: Physical work at “the scene of fire” (HEAT)

When entering the hot environment, the firefighters crawled through a 2.3 m long tunnel into room one. Then, each participant carried a “Leca block” (15 kg) through the corridor (room two) and into room three. Ten firehose couplings were connected or disconnected by one firefighter each time this room was entered, and the participants were encouraged to alternate on this task. Then they walked up to the 2nd floor, where the “Leca-blocks” were put aside. The firefighters continued walking around this floor before returning to room one (1st floor), where this exercise was repeated three times. Then the firefighters entered room three and carried two fire and rescue training manikins (55 and 67 kg) across the room before connecting/disconnecting the firehose couplings. The manikins were then carried around in the 2nd floor before being transported to the 1st floor where the couplings of the firehoses were repeated. The firefighters returned the manikins to room three and repeated the firehose couplings. Then the first part was repeated to all Leca blocks were back in room one before crawling through the tunnel and exiting the hot environment into the stairwell.

Part 3: Retreat from “the scene of fire”

In the stairwell (cool environment), they walked up to the 4th floor and brought the objects back to the ground-level starting point. Then the firefighters were encouraged to continue walking up and down the stairs to exhaustion, and to the SCBA of air was depleted.

Immediately after the 5-min stair walk, the firefighters removed their SCBA, helmets and balaclava. The fire brigade defined no guidelines for further undressing during recovery, which entailed various degrees of undressing the outer garment.

Measurements and instruments

To measure HR, the firefighters were equipped with a Firstbeat Bodyguard 2 (Firstbeat Technologies Oy, Finland) with two self-adhesive, disposable, pre-gelled surface Ag/AgCl electrodes (Ambu BlueSensor L, Ambu, Ballerup, Denmark). Maximal HR was estimated according to this formula:

211 − 0.64*age²⁴⁾

Thermal stress during work was quantified by core temperature measurements and obtained using an ingestible telemetric capsule (BodyCap, Caen, France). Participants were given the pill to swallow approximately four hours before the heat exercise drill, which allow sufficient time for the sensor to pass from the stomach to the gastrointestinal tract avoiding the confounding effect of food and fluid²⁵⁾. Skin temperatures were continuously measured by wireless thermistors (iButton^® type DS1921H, Maxim/Dallas Semiconductor Corp., Sunnyvale, CA, USA) at three locations (chest, upper arm and front thigh) of the body. For measurements of ambient temperature around each firefighter, a temperature logger (LASCAR EL-USB-TC-LCD/LASCAR EL-USB-TC, Lascar electronics Module House, Wiltshire, UK) was put in the chest pocket of the outer jacket on the firefighters, with the external thermoelement placed outside of the pocket.

Before and after the heat exercise drill, participants were asked to evaluate their perceived thermal sensation (PTS) and thermal comfort by answering questions modified from Gagge et al²²⁾. Questions are scaled from 5 to 13, where 5 is “cool”, and 13 is “unbearably hot”. The sensation of shivering and sweating (SS) was assessed using the numerical verbal anchors from 1 “heavy shivering” to 7 “heavy sweating”²⁶⁾. Participants were also asked about thermal comfort (TC) using numerical verbal anchors between 1 “comfortable” and 4 “very uncomfortable”²²⁾. To evaluate self-perceived exertion, a 15-point Borgs scale, where 6 is no exertion, and 20 is maximal exertion, was used (RPE)²³⁾.

Data calculations

The typical time course for the test with HR and T_gi data from a single participant is shown in Fig. 2. The following calculations from the tests were made on individual data:

Fig. 2.

A typical time course of a firefighter’s heart rate and core temperature (T_gi) before, during, and after the heat exercise drill.

One hour before start the firefighters prepared for the heat exercise drill (security information, dressing and resting in room temperature). Approximately two min before entering the hot environment, the firefighters started with stair walking, from the ground floor to the 4th floor, with objects (fire hoses and cans) in each hand. The objects were left on the 4th floor, and the firefighters returned to the 1st floor and entered the corridor into the warm environment for a ~15 min simulated smoke dive (crawling through tunnels, carrying blocks and manikins, connecting firehoses) before retreat from the heat and ~5 min stair walk and recovery phase.

1) Duration HEAT: Time spent in the heat for each participant.

2) Duration stair walking: Time spent walking stairs after the heat exposure for each participant.

3) Start: The starting value for the physiological data is the average of two min from when they were seated and received their final instructions before starting the heat exercise drill.

4) Average HEAT: Data averaged over the entire heat exposure time.

5) Peak HEAT: Calculated as the maximum value measured during the heat exposure.

6) Peak: Calculated as the maximum value attained during the entire heat exercise drill and the post-test resting time.

7) Time rate of change in T_gi (ΔT_gi/Δt) for the heat exposure: Change in T_gi divided by the duration of HEAT.

8) Time rate of change in T_gi for the decrease in T_gi: Change in T_gi during 15 min following peak T_gi divided by 15 min.

Statistical analysis

IBM SPSS Statistics version 28.0 (IBM Corp., Armonk, NY, USA) was used for statistical analyses.

Individual HR and T_gi data are presented over time for one participant. Bivariate analysis was conducted between start T_gi and peak HEAT T_gi and the time rate of change in T_gi (ΔT_gi/Δt).

Differences between start and peak HEAT and between peak HEAT and peak for T_gi, HR and skin temperatures were analysed by Student’s t-test for paired samples. Differences in physiological data between single and double layers of underwear were assessed using an independent samples t-test. Wilcoxon signed-ranks test was used for paired samples of nonparametric data (PTS, SS, TC and RPE) between the start and end of the simulated smoke dive.

Results

No significant effects for undergarment layers were detected for any physiological measurements, and data for all participants are reported combined. There were no significant differences in age, height, experience, or duration of HEAT between the one- and two-layer groups. The average body mass for the one-layer group was 87.9 ± 8.6 kg and 80.9 ± 3.5 for the two-layer group, a significant difference of 6.9 kg (95% CI, 0.5 to 13.4, p<0.001).

Heart rate, core- and skin temperatures

Table 2 describes the T_gi, HR and skin temperature data at the start of the fire drill, average and peak values in HEAT (15 ± 1 min), and the peak values, including the time spent walking in the stairs (5 ± 2 min) and the time resting afterwards. Figure 3 shows the individual values of T_gi for the fire drill. The T_gi and HR for the firefighters increased significantly from the start to a peak during the HEAT and then further increased to reach a peak after the HEAT. All skin temperatures increased significantly from the start to the end of HEAT but with no further increase after the heat exposure. Temperatures measured outside the jacket on the chest of each firefighter during the HEAT were, on average, 97 ± 11°C (range: 76 to 119°C), and the maximal temperatures measured was 134 ± 28°C (range: 98 to 216°C). No effects were seen between the highest jacket temperatures (148, 184, 216°C) and individual skin temperatures.

Table 2. Physiological data from the heat exercise drill.

Measure	Start		Average HEAT		Peak HEAT		Peak		n
	Mean ± SD	Range	Mean ± SD	Range	Mean ± SD	Range	Mean ± SD	Range
Tc (°C)	37.5 ± 0.3	37.0–37.9	37.9 ± 0.3	37.4–38.4	38.4 ± 0.4*	37.8–39.3	39.6 ± 0.5†	38.5–40.2	19
HR (bpm)	92.2 ± 13.7	70–119	162.8 ± 12.3	136–182	185.4 ± 12.6*	154–205	190.0 ± 13†	154–207	19
Tchest (°C)	34.0 ± 0.8	32.6–35.8	38.5 ± 0.7	37.1–39.6	41.5 ± 0.8*	39.8–42.7	41.7 ± 0.7	40.6–42.7	18
Tthigh (°C)	32.2 ± 1.0	30.3–34.5	39.2 ± 0.8	37.4–40.4	43.3 ± 1.1*	41.1–44.7	43.3 ± 1.1	41.1–44.7	18
Tarm (°C)	33.4 ± 1.1	30.6–35.2	39.4 ± 0.6	38.4–40.5	42.6 ± 0.7*	41.6–43.9	42.6 ± 0.7	41.6–43.9	18

*Significantly different between Start and Peak HEAT, p<0.001. †Significantly different between Peak HEAT and Peak, p<0.001.

T_c: core temperature; HR: heart rate; T_chest: chest skin temperature; T_thigh: thigh skin temperature; T_arm: arm skin temperature. HEAT is when the firefighters are inside the heated building. SD: standard deviation.

Fig. 3.

Individual (n=18) core temperature (T_gi) values from the heat exercise drill and recovery.

The figure follows the guidelines set by Watkins et al.^{47, 60)} adopting a color-coded continuum for interpreting heat tolerance. Instead of categorizing individuals as simply heat tolerant or intolerant, the continuum uses light grey to denote high heat tolerance, white for moderate tolerance, and dark grey to indicate low heat tolerance.

The average T_gi increase in HEAT was 0.9 ± 0.3°C with individual values ranging from 0.5 to 1.4°C, with a rate of change at 0.06 ± 0.02°C·min⁻¹ (range: 0.03 to 0.10°C·min⁻¹). The peak T_gi during HEAT was 0.9°C (95% CI, 0.8 to 1.1, p<0.001) higher than at the start. It took on average 13.5 ± 4.5 min (range: 6.5 to 22.2 min) for T_gi to peak after HEAT, with an increase in T_gi of 1.2°C (95% CI, 1.0 to 1.3, p<0.001). The rate of change during 15 min following peak T_gi was −0.06 ± 0.3°C·min⁻¹ (range: −0.02 to −0.15°C·min⁻¹). The peak HR during HEAT was 93 bpm (95% CI, 88 to 98, p<0.001) higher than at the start and further increased by five bpm (95% CI, 3 to 6, p<0.001) after the heat exposure. HR showed no correlation between age and Peak HR (R²=0.1821). The SCBA air was used within the duration of the drill for all participants (22 ± 3 min), and the average used during the HEAT was 238 ± 37 bar (range: 190 to 300 bar).

Bivariate analyses of the data of all participants revealed a positive correlation between T_gi at start and peak T_gi during HEAT (r=0.621, p=0.005). Moreover, the ΔT_gj/Δt was not affected by the T_gi at start (r=0.035, p=0.886) (Fig. 4).

Fig. 4.

Peak T_gi during HEAT (a) and time rate of change (ΔTgi/Δt) during HEAT (b) plotted against start T_gi.

Subjective evaluation

The RPE and the perceptual responses of TS, TC and SS increased significantly from before to after the training session (Table 3). RPE increased from a mean of “very, very light” (7) before start to “very hard” (17) after the 5-min stair walk with maximum perceived exhaustion of “very, very hard” (19). The subjective ratings of TS increased from “slightly warm” (8) to “very hot”, and “extremely hot” (12) was the highest value voted. TC increased from “comfortable” (1) to “uncomfortable” (3) from before to after the heat exercise drill, and SS changed from “neither shivering nor sweating” (4) to “moderate sweating” (6).

Table 3. Perceptual responses before and after the heat exercise drill.

	Before	After
RPE	7 [6–11]	17 [15–19]*
TS	8 [7–9]	11 [10–12]*
TC	1 [1–2]	3 [2–3]*
SS	4 [4–5]	6 [5–7]*

Values are median and range [min–max]. n=19.

*Significantly different between before and after the training session, p<0.05.

RPE: rate of perceived exertion; TS: thermal sensation; TC: thermal comfort; SS: shivering/sweating sensation.

Ambient conditions

The measurements took place in December 2021, and the ambient temperatures outside at the test site were −2.0, −5.5 and −13.5°C on the three test days, respectively. The temperature in the final preparation room varied between 10.0 and 14.0°C on the three test days. Ambient temperatures in the test building ranged from 110 to 130°C on the ground floor and between 150 and 272°C on the second floor, measured 2.3 m above floor level. To reach the highest temperatures and to make the scenario realistic, additional heat was generated by burning propane between 2 and 6 min after the start and again between 11 and 14 min and 30 s of the 15 min heat exposure.

Discussion

One simulated smoke dive in extremely hot environments resulted in high physiological strain on the firefighters, manifested as a rapid increase in T_gi, HR and skin temperatures. T_gi and HR continued to increase after the heat exercise drill, increasing HR to maximal levels and elevating T_gi for an extended period in the recovery phase. No significant effects for undergarment layers were detected for any of the physiological measurements.

Physical work, such as strenuous firefighting activities, increases the body’s heat production, elevating the deep-body temperature, and depending on the work intensity, the T_c rises and may stabilise at different levels²⁷⁾. In our study, the firefighter training increased the T_gi for the firefighters significantly from a mean start value of 37.5°C to a mean peak of 38.4°C during the 15 min heat exposure, with individual variations for peak measurements ranging between 37.8 and 39.3°C. Corresponding results were found by Horn et al.¹⁶⁾, where T_c increased from approximately 37.5°C to a peak at 38.4–38.7°C. We measured on average a 0.9 rise °C in T_gi during the 15 min heat exposure (ΔT_gi/Δt = 0.06°C·min⁻¹). As Horn et al.²⁾ summed up, other published findings on single bouts of short-term firefighting activities have reported T_c rises during live-fire activities ranging from 0.4 to 1.4°C^{6, 14, 28, 29)}. However, the firefighters in our study continued with a 5 min stair walking which further raised the T_gi, and a peak was measured between 6.5 to 22.2 min after stopping exercising with individual variations between 38.5 and 40.2°C.

The study was part of the firefighter’s yearly heat exposure training, which we followed with the least possible interference, and except for the resting period during the operator’s security review (before dressing the jacket, helmet and SCBA), no defined resting period was included to stabilise the T_gi and HR before and after the heat exercise drill. The start values of T_gi (37.5 ± 0.3°C) and HR (92 ± 14 bpm) are higher in this study than the normal range of resting values for healthy individuals as reported in other studies among firefighters, e.g., Horn et al.²⁾ (36.9 ± 0.3 / 69 ± 10 bpm). However, in a real-world setting, a slightly increased T_c and HR before a fire emergency may be the reality for firefighters as they often engage in other work activities prior to the onset of a fire incident. Both baseline and absolute T_c during firefighting activity may therefore be of importance. Absolute T_c has a high clinical value in predicting and treating exertional illnesses³⁰⁾, and reducing baseline T_c before firefighting might be a valuable preventive measure to maintain the T_c below pathological levels³¹⁾.

The rise in T_c is proportional to relative rather than absolute workload and provides the central stimulus for sweating and cutaneous vasodilatation to increase heat loss³²⁾. During exercise the sweating response is determined by changes in thermal and non-thermal factors^{33, 34)}, and during work at high ambient temperatures sweating is the primary physiological mechanism for heat loss³⁴⁾ whereas important modifiers of sweat production are fitness level, acclimatization status, environmental conditions, clothing and evaporative efficiency^{35, 36)}. The extent of heat acclimatization among the firefighters in this study is uncertain. Given the winter conditions in Norway, with outdoor temperatures around or below freezing, sauna use is the primary means of achieving heat acclimatization. Nonetheless, the firefighters have probably undergone partial acclimatization due to the physical training required to meet employment standards and job demands. Ravanelli et al.³⁷⁾ present evidence that partial heat acclimatization can be attained through repeated exposure to thermal stress in training sessions, which improves sweating efficiency and core temperature control during conditions of uncompensable heat stress. However, water vapour permeability is typical low, and insulation is high in the PPE worn by the firefighters³⁸⁾. This reduces the capacity to dissipate heat by both evaporative and non-evaporative mechanisms, and the rate of heat storage for a given rate of heat production will increase. Furthermore, PPE also increases metabolic heat production during physical activity^{13, 39,40,41)}. The combination of high ambient temperature and high metabolic heat production leads to uncompensable heat stress wearing PPE, wherein the heat production exceeds the heat loss potential, leaving the body in continuous heat gain¹³⁾. Thus, an increased sweating response will not necessarily improve heat loss when wearing PPE and working at high metabolic rates but may increase dehydration¹²⁾. Increased core temperature leads to a strain on the cardiovascular system, which is further exacerbated by dehydration⁴²⁾. This situation is especially concerning because it is reported that more firefighters die or are injured due to overexertion and stress, leading to sudden cardiac deaths⁴³⁾.

A wide normal range of T_c measurements has been demonstrated outside laboratory settings and in real-life field studies⁴⁴⁾. Overlapping high T_c between heat illness patients and healthy individuals engaged in peak physical performance has also been shown⁴⁵⁾. Explanations for large individual differences in T_c, HR, skin temperatures and sweat rate are diverse and may include aerobic fitness, clothing used, heat acclimation status⁴⁶⁾ and individual differences in heat tolerance^{13, 47)}. In addition, a significant variability may be explained by the type of activity undertaken by the firefighters and the location of the participants within the burning structure²⁾. Even though all firefighters in our study were well trained and had passed the NLIA test, the firefighters represented a wide range of ages (34–59 yr), body mass (76–108 kg), BMI (23–33 kg·m⁻²), and years of experience (1–36 yr) which partly may explain the large differences in T_gi and HR. The work was performed in pairs of two to four firefighters, providing an opportunity for some individual adjustments in work intensity (for example, carrying the manikins of different weights). The requirements of performing equal amounts of work will give a higher relative workload for those with poorer physical fitness, hence a higher and faster increase in T_gi. HR responses to firefighting also vary depending on the type of work, as well as multiple other factors such as ambient temperature, length of time engaged and fitness level⁷⁾.

During strenuous firefighting and training activities, HR increase and rises to maximal or near maximal levels^{14, 48)}. In our study, the average HR for the firefighters significantly increased from the start (92 ± 14 bpm) to the peak of the 15-min heat exposure (185 ± 13 bpm) and to 190 ± 13 bpm after the heat exposure and stair walking. As discussed for the T_gi measurements, large individual variations in peak HR (154−207 bpm) were observed, explained by the difference in workload, age (34–59 yr) and genetic predispositions, contributing to variations in HR_max. Firefighters’ HR also vary throughout a given emergency because of the intermittent nature of the work²⁾. In our study, the lowest peak HR was registered in a 57-yr-old firefighter (154 bpm), a peak HR of 192 bpm were measured in the oldest participant (59 yr), whereas the highest HR was registered in a 52-yr-old firefighter (207 bpm). Regarding HR_max, a linear relationship of R²=1 is shown between age and the decrease in HR_max at the population level²⁴⁾ (age-estimated HR_max). However, in our study, the measured HR showed no correlation between age and peak HR, and the firefighting activities increased the peak HR to higher than the estimated age-predicted HR_max in 11 firefighters. Even though estimations of HR_max are not as precise as direct measurements of HR_max, it demonstrates that extreme heat exposure and heavy physical work lead to significant cardiovascular strain, which may trigger a cardiovascular event in firefighters with underlying diseases⁷⁾. This underscores the critical importance of annual physical and health evaluations for firefighters.

This study included the use of one firefighter training scenario using one cylinder of air, where the SCBA air was used within the duration of the drill for all participants (22 ± 3 min), which caused an increase in T_gi of 1.2 ± 0.3°C (range: 0.7 to 1.7°C). In a real incident, however, firefighters may re-enter the building following a break, and may be exposed to conditions resulting in more significant increases in core temperature²⁹⁾. The starting T_c influences the T_c at the end of a firefighter activity¹³⁾, and studies on long-term firefighting activities that require more than one cylinder of air have shown that the rate of increase in T_c is augmented in later bouts of activity. This may lead to a higher HR and further exacerbate cardiovascular strain²⁾.

In their regular duties, firefighters in the participating fire brigade were free to select flame-retardant underwear based on their preferences, while the employer selected the outer garments from two different brands. In this study, all firefighters were provided with flame-retardant underwear by the research team. Half of the participants were allocated to wear a single layer of woollen shirts and pants, while the other half were assigned to wear an additional, thicker set of flame-retardant woollen jacket and pants as a second layer. No significant effects for undergarment layers were detected for any of the physiological parameters. Although not directly comparable because of different ambient conditions and methods used, our results are in line with Smith et al.²⁰⁾ who found that an additional layer of clothing ensemble for the upper body did not imposed a greater level of physiological or perceptual strain during moderate-intensity work bouts compared to wearing one less layer of clothing. However, a controlled laboratory study by Renberg et al.⁴⁰⁾ demonstrated a 2% increase in metabolic rate by adding a second layer for both the upper and lower body beneath outer garments during inclined walking, an effect not observed during level walking. This suggests that in our study, including a middle layer might have similarly elevated metabolic rates and, consequently, metabolic heat production during certain physical activities. However, since metabolic rates were not directly measured and considering the variance in activities and the firefighters’ diverse outerwear, these factors likely overshadowed a possible impact of the additional layer. Because protective clothing is typically bulky and encapsulating, water vapour permeability and the rate of heat exchange are impaired^{12, 49)}. The combination of high ambient temperature and protective clothing in our study led to uncompensable heat stress, where the addition of a middle layer had no measurable effect on physiological strain.

This study did not assess the impact of using multiple layers of clothing on the risk of skin burns. Firefighters can be burned by radiant heat energy produced by a fire or by a combination of radiant energy and flame contact exposure. Vaporisation and condensation of hot moisture also induces skin burn, and some injuries occur because of compressing the protective garment against the skin⁵⁰⁾. The highest skin temperatures registered in our study were measured during the 15-min heat exposure with a mean of approximately 43°C on the front thigh and the upper arm, with peak values of 44.7°C and 43.9°C, respectively. Studies have shown that the threshold skin temperature for skin burn occurrence is 44°C, and once skin temperature reaches 55°C for only 10 s, skin injury will occur⁵¹⁾. Moisture in protective clothing can also significantly change the garment’s protective performance, and skin burns may occur under the conditions of moisture accumulation in protective clothing^{52, 53)}. None of our participants experienced skin burns, but one of the firefighters reported that heat transfer through the protective clothing was close to occurring. Smith et al.²⁰⁾ concluded that material performance testing indicates that the three-layer ensemble enhances protection against thermal injury but does not impose any greater thermal strain on the wearer. Since our study was a training scenario, the ambient temperature was regulated to avoid heat transfer, thus preventing both thermal injury and someone having to interrupt the test.

T_gi, HR and skin temperatures were reflected by the subjective feedback on the questions of TS with answers “very hot” and “extremely hot” right after the 5-min stair walk. At this time point, they also voted themselves as “uncomfortable”. The RPE was also voted “very hard”, demonstrating that the firefighters exerted themselves close to the maximum. Several factors influence thermal comfort, such as metabolic rate, air temperature, mean radiant temperature, air speed, relative humidity and clothing insulation⁵⁴⁾. Although not an objectively quantifiable measurement, many firefighter participants commented that the exertion and heat exposure level closely resembled what they encountered during house fires.

While this study offers a characterization of the thermal conditions and physiological responses associated with a firefighter training scenario, it is essential to acknowledge certain limitations when interpreting these findings. The firefighters wore outer garments provided by their employers, reflecting a range of ages and potential protection levels. Additionally, incorporating their standard clothing and equipment, except the layers supplied by the research team, introduced natural variations in the total weight carried, which probably influenced the workload. The work pace was also not standardized, allowing for potential discrepancies between teams. However, since these differences mirror real-world firefighting conditions, the study’s findings remain relevant to practical situations. Also, immediately after the 5-min stair walking, the firefighters removed their SCBA, helmets and balaclava. The fire brigade defined no guidelines for further undressing during recovery, which entailed various degrees of undressing the outer garment. Therefore, the restoration towards physiological equilibrium was most likely different between the firefighters⁵⁵⁾. Finally, the firefighter’s hydration status was not measured. It is well established that firefighters may experience the adverse effects of dehydration without adequate fluid intake^{56, 57)}, such as decreased T_c control⁶⁾, heat tolerance time⁵⁸⁾, cardiac output with higher HR⁵⁹⁾ and reduced muscle endurance⁷⁾. Hence, variations in hydration status could account for some of the observed differences in physiological responses to the simulated smoke dive and recovery phase.

Conclusions

The 15-min simulated smoke dive, followed by five minutes of stair walking, imposed a high physiological strain on the firefighters. This caused a rapid increase in T_gj and HR, with a continued increase in the following recovery phase. The results are expected to provide a better understanding of physiological responses, the impact of protective clothing, and the work strain experienced by firefighters during training and operations at high ambient temperatures. Additionally, these results can be valuable for implementing health promotion measures and improving the safety of the firefighters by adjusting work routines such as cooling strategies, hydration, and work/rest schedules.