Evaluation of the Impact of Exposure To Heat and to By-Products of Combustion on the Health of Firefighters

SUMMARY

Firefighters are often exposed to high temperatures and by-products of combustion, which can affect their health. In this study, we assessed the impact of acute exposure of firefighters in fire simulators. Twenty male firefighters were exposed to fire simulators, and observed in four phases: pre-exposure (group 0, control) and after the end of the first (group 1), second (group 2), and fourth (group 3) weeks of training. Blood samples were collected and dosed to evaluate the response of the immune, inflammatory (C-reactive protein, IL6, and IL10), and endocrine systems (cortisone, total testosterone, free testosterone, SHBG, bioavailable testosterone, TSH, and free T4). In groups 0, 1, and 3, a thermographic evaluation was also carried out to study the temperature and body heat flow of the participants. Regarding the inflammatory process, an increase in C-reactive protein and a reduction in IL-10 were observed. With respect to hormonal markers, an increase in cortisol and reduced levels of free T4 and bioavailable testosterone were found after exposure, with recovery of testosterone levels in the final week of training. Thermoregulatory adaptation of the organism has been associated with changes in heat flow in the organism in people subjected to extreme temperatures, with emphasis on the performance of the lower limbs. Our findings demonstrate an inflammatory response with hormonal changes after exposure to fire and an adaptive response of thermal balance, which could aid understanding of the physiology of the human body in extreme situations.

Introduction

The impact of exposure to heat and products derived from combustion on the firefighter is not yet fully understood. Hyperthermia, dehydration, respiratory complaints and cancer are associated with firefighter activity.

To understand fire behavior and extinction tactics, compartmentalized fire simulators are used for training purposes. This type of training is performed in metallic environments, with burning pallets and sheets of oriented strand board (OSB), and exposes firefighters to environments with high temperatures, greater than 400°C,¹ with high concentrations of gases and particulates (byproducts of combustion). It is only possible to remain in these environments while using personal protective equipment (PPE) and respiratory protective equipment (RPE) intended for firefighting activities. This training modality, commonly referred to by the acronym CFBT (compartment fire behavior training), has been used in several countries around the world, in order to provide firefighters with a realistic experience of structural firefighting activity, increasing the understanding of these professionals on the behavior of fire inside compartmented environments and providing them with the opportunity to train in techniques and tactics.

Even with the adoption of safety procedures and the use of protective equipment, training participants are subjected to increases in temperature, and physical (the protection kit alone weighs approximately 22 kg) and psychological efforts. In addition, although they remain protected from smoke by the self-contained breathing apparatus (SCBA), the firefighters are subject to inhalation of residual smoke in the training environment, as well as direct and indirect skin absorption (due to soot impregnated in the protective kit and equipment).

As a result of the above information, many studies have demonstrated physiological alterations and effects on health, resulting from the activity of compartment fire behavior training instructors, such as the increased possibility of cardiovascular and circulatory events,² impaired airways²², as well as as well as immunological alterations³ arising from the environmental temperatures inside the simulators, generating dehydration, physical^3-5 and psychological stress.⁶

From the analysis of the literature it appears that there are two main mechanisms of aggravation resulting from firefighting activities:⁷ (1) cardiovascular alterations, arising from exposure to heat and increased body temperature, causing vasodilation, dehydration, and, therefore, increased cardiac output; and (2) systemic and respiratory alterations resulting from chemical exposure to the combustion by-products (gases, vapors, and particulate solids) that can be absorbed by inhalation or cutaneous absorption (facilitated by the elevation of the epidermis temperature), with toxic effects related to the action of the absorbed components, as well as alterations in the airways and pulmonary system.

Systemic alterations include hormonal changes, as due to the peculiar characteristics of the thyroid and testicles, their functions can be impacted by the increase in temperature. The thyroid is located in the neck region, which receives less thermal protection and is subject to greater changes in skin temperature,⁸ while the testicles, in addition to presenting decreased metabolic activity during stress,⁹ are also especially sensitive to temperature rises.¹⁰ There is evidence of a higher incidence of testicular cancer in firefighters compared to the general population.¹¹

The thermal profile of the body may also be altered; the use of protective clothing decreases the intensity of heat exchanges, essential to protect the combatant. However, this equipment also hinders the loss of body heat during the activity, which can lead to intense thermal stress and altered thermoregulation, a crucial factor for maintaining physiological homeostasis in the body.

Thus, the aim of this study is to analyze the effects and the adaptive response of firefighter activity using structural firefighting simulators as model.

Material and methods

Study participants and ethical guidelines

The study was conducted by the Department of Otorhinolaryngology - Head and Neck Surgery of the Federal University of São Paulo, at the São Paulo State Military Police Superior School of Firefighters, in October 2019. The work evaluates the impact of this type of training on the health of the participants, through the analysis of hormonal and inflammatory markers, and body temperature variations in the 20 participants of the structural fire-fighting instructor course. All firefighters were male, aged between 18 and 50 years, and healthy, approved by the institution’s medical and physical inspections (Table I).

Table I.

Anthropometric data of the participants

	Min	Max	Mean	SD	Median
Age (years)	30	49	36.1	5.9	34.5
Height (cm)	166	189	177.2	7.5	176.5
Weight (Kg)	65	100	80.7	10.7	77.5
BMI (Kg/m2)	21.5	30.9	22.34	1.79	24.24

The study was approved by a research ethics committee (CAAE: 09843919.2.0000,5505). Written informed consent was obtained from all participants, making relevant data to the study, including infrared images, available for scientific studies and publication. Only after acceptance and terms subscription, did the participants have their data and infrared image collected. All the study protocols adhered to relevant ethical guidelines.

National Fire Protection Association guidelines

Firefighting training is based on the guidelines established by norm number 1403 of the National Fire Protection Association (NFPA),¹² which establishes strict safety procedures that include: the use of rapid intervention teams (for the need for real assistance to any of the participants); redundant sources of water supply; and instructors, in adequate numbers and specially qualified for this purpose (at least: one instructor in charge, one instructor for fire control, and one instructor designated as security officer). In addition, all participants (students and instructors) must use the complete kit of individual protection measures to combat the structural fire, consisting of capes, pants, balaclava, gloves, and boots fabricated in accordance with the NFPA 1971,¹³ structural firefighting helmet, in accordance with EN (European Norm) 443/2008,¹⁴ and a respiratory protection set in accordance with NFPA 1981¹⁵ or EN 137 certification.¹⁶

This type of training is as close as possible, while in controlled environments, to structural firefighting activities, making it a great opportunity to understand the effects of this activity, not only for training but for the entire firefighting service.

Study design

To carry out the research, we opted to follow the course of firefighting instructors, because we aimed to evaluate the effects of repeated exposure in simulators, which forms part of the dynamics of this training. During the course, which lasts 4 weeks (20 learning days), firefighters undergo training in these simulators on 10 of the days. The fires last approximately 25 minutes and each volunteer participates in 2 to 3 sessions of training in the simulators per day. The volunteers are divided into groups and all take part in the same number of activities (24 sessions per volunteer, over the entire period).

In order to conduct the study, 4 groups were formed (composed of the same 20 volunteers): group 0 (control), prior to exposure in the simulators; group 1 after acute exposure in the firefighting simulators carried out in the first week; group 2 after acute exposure in firefighting simulators carried out in the second week; and group 3 after acute exposure in firefighting simulators carried out in the final week of the course.

The exercises were carried out inside modular environments with a metallic structure (consisting of marine containers) specifically adapted for this purpose. The fire charge used to perform the fires is 5 sheets of OSB (Fig.1A), and around 3 pine pallets. The fire lasts approximately 30 minutes, and, during this period, the environment is saturated with real smoke with low visibility conditions.The students are only able to remain there due to the use of protective equipment (Fig.1B-C) (clothing to withstand heat and autonomous respiratory protection equipment). The temperature inside the container can exceed 600°C (Fig.1C).

Fig. 1.

A) Compartmented metallic environments; B) Fire charge; C) Protective equipment and environmental conditions of the exercise; D) Thermal image being collected - maximum temperature of the face - inner canthus of the eye; E) Thermal profile of the body.

In all groups (0, 1, 2 and 3), blood samples were taken to measure immune and inflammatory system markers (serology for Hepatitis B, C-reactive protein, IL6, and IL10), and endocrine markers (cortisone, total testosterone, free testosterone, SHBG, bioavailable testosterone, TSH, free T-4). In groups 0, 1 and 3, a thermographic evaluation was also carried out to collect temperature measurements of the participants.

For the control group, thermal images and blood collections were taken before the beginning of the course, in an air-conditioned environment and exempt from previous exposure to this type of training. Temperature images and blood samples from each of the three acute exposure groups were taken on the training ground, just after the participants left the simulators, in three separate weeks.

Serological sample analysis

The blood samples collected were subjected to centrifugation and separation of the supernatant and were stored in a refrigerator at -80C, in the ENT Research Lab of the Federal University of São Paulo.

Dosage of cytokines (IL6 and IL10): – the blood containing EDTA was centrifuged at 800 G for 8 minutes. Next, the collected plasma was stored and subsequently analyzed by ELISA test commercial kit Invitrogen by Thermo Fisher Scientific, (Vienna, Austria).

C-reactive protein (CRP): - dosed in Siemens (Munich, Germany) Dimension EXL 200” equipment, using the Immunoturbidimetry method.

Hormonal dosage: - cortisol, total testosterone, free testosterone, SHBG, bioavailable testosterone, TSH, and free T-4 were measured with Architect i2000SR equipment, an ABBOTT (Chicago, Illinois, USA) device, using the chemiluminescence microparticle immunoassay method (CMIA).

Infrared thermographic analysis

The infrared images were taken using an infrared digital camera model T540 (Flir Systems - thermal sensitivity <40mK, and a resolution of 464 × 348 pixels, allowing a matrix of 161,472 data for temperature) (Wilsonville, Oregon, EUA).

The quantitative variables evaluated were: temperature of the inner canthus of the eye (Tic - Highest temperature measured in the medial corner of the eye, right or left) (Fig. 1D), maximum total body temperature (highest temperature measured in the body, analyzing the anterior, posterior, right and left, taking images of 360°, including the soles of the feet and hands) (Fig. 1E). We used an emissivity of 0.98, as this value is used in biological tissues.

In addition, heart and respiratory rates and peripheral oxygen saturation were also measured in the groups in which thermographic measurements were taken, using a G-Tech portable oximeter.

Statistical analysis

The data obtained were tabulated in Excel. The thermographic collections resulted in an n=12, as some of the participants did not attend the collection point and/or removed their protective equipment ahead of time, resulting in a total of 12 firefighters who obtained valid data in the 3 groups for which the measurements were taken (0, 1 and 3). For serological markers, n=19 was obtained, since nineteen participants had valid collections in the 4 groups in which this collection was performed.

The data were evaluated by statistical comparison using JAMOVI software. The markers were evaluated analytically and graphically for distribution, in terms of mean, median, standard deviation, kurtosis and slope, and were also subjected to the normality test (Shapiro - Wilk) and homogeneity of variances (Levene).

The groups were compared using the Friedman test, with the paired comparison test of Durbin and Conover as a post-hoc test in order to verify which groups presented alterations.

Qualitative data, referring to temperature measurements, were compared using the chi-square test of independence (Pearson). In all analyses, p values lower than α = 0.05 (5%) were considered statistically significant. The results are presented as mean (μ) and standard deviation (σ), for parametric data, and as medians (M) and interquartile amplitudes (IQR), for nonparametric data.

Results

None of the laboratory markers (obtained by serological tests) presented normality in all groups; only the temperature data, obtained through thermographic measurements (face temperature and total body temperature) showed normal distribution, however, they did not present homogeneity of variances (Levene’s test). For this reason, considering also the low numbers in the samples (12 for the thermographic data), it was decided to treat these data as non-parametric, using the Friedman test and the Durbin and Conover post-hoc.

Serological testing results

IL-6 and IL-10 cytokines:

In the dosage of IL-6, no statistically significant differences were observed between the groups.

With respect to the dosage of IL-10, there was a statistically significant difference between the groups (Friedman test: p = 0.007), with reductions being identified in group 1 (M = 5.9 pg/ml, IQR = 11.5 pg/ml) and group 2 (M = 3.08 pg/ml, IQR = 5.49 pg/ml), when compared to group 0 (M = 9.62 pg/ml, IQR = 14.4 pg/ml), with p<0.01 and p<0.02, respectively (Fig. 2A).

Fig. 2.

Quantification of Fig. 3: A) Interleukin 10; B) T4 C-Reactive Protein; C) Cortisol; D) Free T4; E) Total testosterone; F) SHBG; G) Free testosterone; H) Bioavailable testosterone; I) Facial temperature; J) Total body temperature.

C-reactive protein (CRP):

The CRP presented a statistically significant difference between the groups (Friedman test: p = 0.037), with increases in group 1 (M = 2.96 pg/dl, IQR = 0.91 mg/dl) and in group 3 (M = 3.01 pg/dl, IQR = 1.18 mg/d l), when compared to group 0 (M = 2.65 mg/dl, IQR = 0.97 pg/dl), with p <0.05 and p < 0.01, respectively (Fig. 2B).

Hormonal markers:

Cortisol presented a statistically significant difference between groups (Friedman test: p <0.001), with increases in group 1 (M = 19.5 μg/dL, IQR = 9.3 μg/dl), group 2 (M = 14.7 μg/dl, IQR = 3.0 μg/dl), and group 3 (M = 12.5 μg/dl, IQR = 5.0 μg/dl), when compared to group 0 (M = 9, 4 μg/dl, IQR = 4.45 μg/dl), with p<0.01, p<0.01, and p<0.03, respectively. The differences observed in the comparisons between the other groups showed the following levels of significance: (p<0.01) between groups 1 and 3; (p<0.01) between groups 1 and 3; and (p<0.03) between groups 2 and 3 (Fig. 2C).

Free T4 presented a statistically significant difference between groups (Friedman test: p<0.017), with a reduction in group 3 (M = 0.93 ng/dl, IQR = 0.1 ng/dl), when compared to group 1 (M = 0.98 ng/dl, IQR = 0.16 ng/dl) and group 2 (M = 1.0 ng/dl, IQR = 0.15 ng/dl) with p<0.01 and p< 0.03, respectively (Fig. 2D).

The thyroid-stimulating hormone did not present significant differences between the groups.

Total testosterone presented a statistically significant difference between groups (Friedman test: p< 0.001), with an increase in group 3 (M = 564 ng/dl, IQR = 276 ng/dl), when compared to group 0 (M = 452 ng/dl, IQR = 147 ng/dl), group 1 (M = 395 ng/dl, IQR = 211 ng/dl), and group 2 (M = 393 ng/dl, IQR = 248 ng/dl) with p<0.01 in the three comparisons described (Fig. 2E).

The sex hormone binding globulin (SHBG) presented a statistically significant difference between the groups (Friedman test: p=0.035), with an increase in group 2 (M = 33.9 nmol/L, IQR = 13.5 nmol/L), when compared to group 0 (M = 29.8 nmol/L, IQR = 11.7 nmol/L) and group 1 (M = 30.6 nmol/L, IQR = 15.0 nmol/L), with p<0.03 and p<0.01, respectively (Fig. 2F).

Free testosterone presented a statistically significant difference between groups (Friedman test: p=0.001), with an increase in group 3 (M = 10.7 ng/dL, IQR = 5.35 ng/dl), when compared to the group 0 (M = 8.7 ng/dl, IQR = 4.33 ng/dl), group 1 (M = 7.09 ng/dl, IQR = 4.91 ng/dl), and group 2 (M = 6.66 ng/dl, IQR = 5.36 ng/dl), with p<0.01 in the three described comparisons (Fig. 2G).

Bioavailable testosterone showed a statistically significant difference between groups (Friedman test: p <0.001), with an increase in group 3 (M = 250 ng/dL, IQR = 134 ng/dl), when compared to group 0 (M = 204 ng/dl, IQR = 106 ng/dl), group 1 (M = 166 ng/dl, IQR = 115 ng/dl), and group 2 (M = 156 ng/dl, IQR = 126 ng/dl), with p <0.01 in the three described comparisons. There was also a reduction in group 2 when compared to group 0, with p<0.05 (Fig. 2H).

Infrared thermographic analysis

In the evaluation of the temperature of the (T_IC) (thermographic records obtained in the inner canthus of the eye, analyzing both eyes, with the highest temperature being recorded), an increase in temperature was observed in group 1, compared to groups 0 and 3; the values verified in the groups were: 0 (M = 35.5, IQR = 0.42), 1 (M = 37.3, IQR = 0.65), and 3 (M = 35.4, IQR = 1.3). Levels of significance (p <0.01) were obtained between groups 0 and 1 and between groups 1 and 3 (Fig. 2I).

In the evaluation of the total body temperature (thermographic record of the point of highest temperature in the body surface, regardless of its location), there was an increase in temperature after exposures in groups 1 and 3, with the following values being observed in the groups: 0 (M = 35.3 IQR = 0.65), 1 (M = 37.2 IQR = 0.77), and 3 (M = 37.0 IQR = 1.63). Levels of significance (p <0.01) were obtained between groups 0 and 1 and between groups 0 and 3 (Fig. 2J).

From a qualitative aspect, with respect to body thermoregulation, differences were observed in the thermoregulatory flow of the organism.

In group 0 (control), the highest measured body temperatures were found in the region of the head and upper limbs (58% of the individuals had the point of highest body temperature in the inner canthus of the eye (Tic), 25% in the acoustic meatus region, and 17% in the armpit region) (Fig. 3A).

Fig. 3.

Full Body Hot Spot: A) Group 0; B) Group 1; C) Group 3; D) Thermal profile of the body, pre and post-exposure; E) Distribution of the highest total body temperature points.

In group 1 (after exposure in the 1st week), the highest measured body temperatures were distributed as follows: 50% in the medial corner of the eye, 17% in the acoustic meatus region, 8% in the shoulder region, 8% in the umbilical region, and 17% in the feet (Fig. 3B).

In group 3 (after exposure in the 4th week), the highest measured body temperatures were distributed as follows: 25% in the feet, 17% in the posterior region of the head, 17% in the knee region, 9% in the medial corner of the eye, 8% in the acoustic meatus region, 8% in the auricular pavilion, 8% in the chest region, and 8% in the umbilical region (Fig 3C).

It was noted that after the exposures the body heat flow presented different dynamics; the measurements of group 0, taken in an air-conditioned environment and without exposure to the simulators, show that the points of greatest heat are concentrated in the region of the head and upper limbs, while after exposures the measurements show the lower limbs standing out (including the extremities) as heated points, sometimes being the hottest in the body (Fig. 3D).

In order to better assess these alterations, the distribution was simplified by subdividing the points of highest temperature into two categories, the first of which is made up of individuals with the hottest points on the body in the region above the xiphoid appendix (a situation more similar to that of the individuals not subjected to exposure), while the second was composed of individuals who presented regions under this line as the hottest points. The groups differed when compared with each other using the chi-square test for independent samples, with statistical significance regarding the distribution of points in these two categories (χ² = 8.00, p = 0.018)

Using this categorization, no individuals presented the hottest spots in regions below the xiphoid appendix in group 0, while three individuals (25%) in group 1 and six individuals (50%) in group 3 presented the hottest spots in this region (Fig. 3E).

The observed respiratory rate data were higher in group 2 (μ = 35.8;σ = 7.6), compared to group 3 (μ = 25.3; σ = 4.3), which, in turn, were higher than in group 0 (μ = 17.6; σ = 2.3). Heart rate data were higher in group 3 (μ = 150.2; σ = 19.4), compared to group 1 (μ = 121.8; σ = 13) which, in turn, were higher than in group 0 (μ = 121.8; σ = 13). Peripheral oxygen saturation presented higher values in group 1 (μ = 97.1; σ = 1.4), compared to group 2 (μ = 95.9; σ = 1.9), which, in turn, were higher than in group 3 (μ = 92; σ = 5.3) (Table II).

Table II.

Heart rate, respiratory rate, and saturation

	Group 0		Group 1		Group 3
	Mean	SD	Mean	SD	Mean	SD
Respiratory rate (per minute)	17.6	2.3	35.8	7.6	25.3	4.3
Heart rate (per minute)	67.8	8.2	121.8	13	150.2	19.4
Peripheral oxygen saturation %	97.1	1.4	95.9	1.9	92	5.3

Discussion

The results showed an increase in inflammatory markers: IL-10 (an anti-inflammatory interleukin) reduced in groups 1 and 2, compared to group 0 (control) and C-Reactive Protein increased in groups 1 and 3, compared to group 0. Still in this sense, it should be noted that, among the hormonal markers, cortisol presented an increase after exposure during firefighting in all phases, corroborating findings in the literature that establish cortisol as an index of heat intolerance.¹⁷

CRP is an acute-phase protein and a marker of systemic inflammation, which presented elevations in groups 1 and 3. Cortisol is a potent anti-inflammatory which presented statistically significant elevations in all post-exposure groups (1, 2 and 3). IL-10 is an anti-inflammatory cytokine and showed a fall after exposure in groups 1 and 2. The analysis of these markers together suggests an acute phase inflammatory process in post-exposure measurements (elevation of CRP and cortisol). A reduction in IL-10 after smoke inhalation during firefighting activities is reported in the literature,¹⁸ corroborating our findings. A plausible explanation for this is the negative association found between IL-10 and cortisol production, which may explain the low IL-10.¹⁹

Among the hormonal markers associated with testosterone, there was an increase in SHBG, which was matched by a reduction in bioavailable testosterone in phases 1 and 2. The male reproductive organ is sensitive to temperature rises,¹⁰ which can lead to temporary infertility,^9,20 which is perhaps related to the greater propensity for the development of testicular cancer¹¹ in firefighters, when compared to the general population. In the particular case of an increase in sex hormone-binding globulin levels in group 2, accompanied by a reduction in bioavailable testosterone levels in the same group, this phenomenon can be explained by the fact that sex hormone-binding globulin has a high affinity for testosterone, binding both free testosterone and albumin-bound testosterone (weak binding), consisting of these two markers, therefore the increase in SHBG corresponds to a higher level of testosterone linked to this globulin and the reduction in testosterone available for ready use. There was also an increase in all forms of testosterone after prolonged exposure (fourth week, group 3), which seems to indicate an adaptation of the organism to training.

In the current study, the decrease in bioavailable testosterone was observed very occasionally in phase 2, and it can be conjectured that this decrease is related to the elevation of SHBG as the body’s metabolic response to control homeostasis, in an attempt to reduce metabolic activity. This reduction was statistically significant, and although for the group (group 2), both the mean and the median remained within the reference values, for 4 participants, the levels of free testosterone and bioavailable testosterone fell below the reference minimum (3.03 and 71 ng/dl, respectively). These findings demonstrate the need for more studies with a larger sample size and a longer evaluation period to better assess the dynamics of testosterone production and availability when faced with thermal stress, with a strong elevation of this hormone throughout the exposures after the body adapts to this new thermal reality.

When we analyzed thyroid-related hormones, free thyroxine (T4) showed a significant reduction in group 3 (fourth week), when compared to group 0, while TSH did not demonstrate significant alterations.

The thyroid is located in the neck region, which is less thermally protected.⁸ Internal effects in this region were demonstrated by alterations in the larynx after exposure to firefighting simulators,^21,22 which could explain the occurrence of interferences in this gland. In addition, thyroxine (T4) has a regulatory function on the body’s metabolic processes, influencing the control of its internal temperature, with indications that reductions in free thyroxine levels may be associated with adaptive processes to high temperature environments in humans²³ and even to acute exposures, as seen in studies with mice,²⁴ and this adaptive response mechanism does not involve changes in TSH levels. In this context it is likely that the reduction in T4 in group 3 is an adaptive effect because its decrease serves to reduce metabolism and, consequently, temperature, representing a desirable mechanism for maintaining thermal balance and avoiding hyperthermia and its consequences, ranging from muscle fatigue, altered state of consciousness, multiple organ failure, and death.²⁵

Thermographic measurements were not taken in group 2 due to technical limitations on this date. Here, too, testosterone and T4 levels are suggestive of a compensatory mechanism of adaptation to the firefighter’s activity to avoid hyperthermia, given that there are indications that the organism becomes more efficient in dispersing heat, especially in peripheral regions, improving homeostasis control. A reverse process occurs, with a decrease in blood circulation in the extremities, when the body is exposed to cold, focusing on preservation of vital regions.

The T_IC temperature, taken at the inner canthus corner of the eye, offers a good idea of the internal temperature^26-28 or the brain temperature or brain injuries.²⁹ This marker presented a significant increase in group 1 (after exposure in the first week) both in relation to group 0 (control) and in relation to group 3 (after exposure in the fourth week), with the latter showing no statistically significant change in these measures in relation to group 0 (control).

On the other hand, the measurements of total body temperature, which consider the hottest point in the body, regardless of where it is located, were significantly higher in both group 1 and group 3, when compared with the control group, showing that the energy transmitted in the form of heat by physical exercise and exposure to high temperatures continued in group 3, although apparently directed to peripheral portions due to alterations in blood flow. This result reinforces the hypothesis of the organism’s adaptive response to thermal stress, in developing efficient mechanisms to maintain homeostatic control.

As can be seen by the analysis of the hottest point on the body performed in group 0 (control), in 83% of the individuals the highest point of body temperature was in the head region; 58% in the medial corner of the eye, 25% in the acoustic meatus region, and the remaining 17% in the armpits, and in all individuals the most heated point was on the head or upper portion of the trunk. After exposure in the first week, this distribution changed; 50% of the individuals continued to present the medial corner of the eye as the most heated point, 17% the acoustic meatus, 16% parts of the trunk, and another 17% the feet. After the exposure in the fourth week, this distribution was even more altered: only 9% of the participants presented the medial corner of the eye as the hottest point and 8% the acoustic meatus, 17% the region of the knees, 25% the feet, and the rest were distributed in the upper part of the body. This gradual and progressive change in the points of higher body temperature to the lower regions during exposures, demonstrated in this study, indicates a change in blood flow to peripheral regions, increasing the dispersion of heat.

Normally, the highest body temperatures are registered in the head region (as seen in group 0). The change was noteworthy during the weeks in which it was observed that a large part of the firefighters started to present the highest body temperatures in their lower limbs, including their extremities (feet). This is a region of lower blood flow and, therefore, of lower expected temperature, suggesting some kind of thermoregulatory modification (acclimatization) resulting from the activity, which uses the feet to cool the body more efficiently.^30-33

The human body has a thermal comfort zone for maintaining homeostasis and physiological processes in environments for which humans are thermally adapted. In this zone, heat exchanges involve a minimum of energy expenditure to prevent loss and facilitate heat absorption (response to cold), or to hinder entry and facilitate loss (response to heat).³³ In both processes, the individual expends energy executing conduction, convection, and radiation processes, or using evaporative water loss cooling (sweat - evaporative cooling), which depends on an environment not saturated by water vapor,³⁴ in addition to other metabolic and behavioral alterations. These complex processes are involved in body thermoregulation and allow the maintenance of the optimum temperature for the physiological and biochemical processes of our species. Heat stress generates rapid responses, such as the closure of arteriovenous anastomoses, causing the dilation of the arterioles of the vascular vein of the skin, increasing capillary blood flow, and circulation of peripheral regions, which are further away from vital organs and have higher ratios between surface and volume (thermal windows) favoring heat loss.^30-33

This is consistent with the observations of body temperature flux, which show that in group 3 there was lower participation of the head region and increased participation of the lower limbs (including their extremities - feet) as regions of higher body temperature in thermographic images. It is emphasized that this phenomenon can be observed to a lesser extent in group 1. It can also be noted that group 3 presented a higher mean heart rate than group 1, but a lower mean respiratory rate, suggesting that a homeostasis maintenance mechanism is being employed, since increased circulation helps to eliminate heat, while reduced metabolic rates, directly related to oxygen consumption,³³ decreases thermogenesis.

The fact that group 1 recorded the highest median of face temperature, which better reflects the core body temperature, while group 3 had the lowest mean, seems to indicate that the organisms have become more efficient in dealing with heat.

The observation of this behavior of alteration in the body thermal flow, using the lower limbs to dissipate excess heat in people submitted to extreme temperature rises, may help in understanding the physiology of the human body in extreme situations, and this new knowledge can be used to optimize the lower limbs, including the feet, as a cooling point (thermal window). In this direction, it could be interesting to guide these professionals who, in addition to removing their firefighting suit, should also remove their boots to take advantage of this phenomenon for heat loss and rapid reestablishment of the basal temperature,³¹ as these professionals use longer, rubberized, and thermally insulated special boots which are not as easy to remove from contact with the body as easily as the rest of the firefighting suits. It should be noted that there is evidence in the literature that cooling the feet by immersion in cold water is an efficient way to reduce thermal stress in users of chemical protective clothing.³²

The use of the clothing protects the firefighter by thermally isolating them from the external environment. This equipment also features an impermeable layer, so whoever wears it is temporarily protected from high external temperatures, but their ability to dissipate body heat is also limited since the same barrier that restricts the thermal radiation from the environment to the fireman, also restricts the thermal radiation of body heat to the environment (even if the external temperatures are not high). This phenomenon will favor the process of body sweating and also dehydration, up to a certain limit, because the saturation of steam resulting from the retention of moisture by the waterproof barrier can limit the cooling mechanism by evaporative loss. This high-lights the importance of the firefighter removing the protective equipment as soon as possible, in order to mitigate thermal stress.

Although the removal of the equipment from the upper body (helmet, balaclava, cape and gloves) is already included as part of firefighter usual practice, they often keep the equipment for the lower body on (pants and boots). The findings of the current study suggest that complete removal of the equipment could improve thermal recovery.

Conclusion

The results of the present study suggest that the firefighter activity, in compartmentalized environments and with real fire, seems to be associated with inflammatory processes and hormonal alterations, such as testosterone and thyroxine. They also point to a possible thermoregulatory adaptation of the organism, after successive exposures to high temperatures, in which the lower limbs, including the extremities (feet), seem to function as an effective body thermal window.