Verification of electronic device technology for measurement and evaluation of thermal exposure of fire fighters and members of rescue teams

Šárka Bernatíková1 Hana Tomášková2 Marek Bužga3 Zdeněk Jirák3 Petr Novák4 Ján Babjak4 Tomáš Kot4 Václav Krys4 Ladislav Jánošík1 1 VŠB– Technical University of Ostrava, Ostrava, Czech Republic Faculty of Safety Engineering, Laboratory for Risk Research and Management 2 University of Ostrava, Ostrava, Czech Republic Faculty of Medicine, Department of Epidemiology and Public Health 3 University of Ostrava, Ostrava, Czech Republic Faculty of Medicine, Department of Physiology and Pathophysiology 4 VŠB– Technical University of Ostrava, Ostrava, Czech Republic Faculty of Mechanical Engineering, Department of Robotics Abstract Background: The work of members of rescue teams could be associated with very high physical and thermal loads. If not timely interrupted, any extreme labour-thermal load may lead to afailure of the body and fatal collapse. This risk may be significantly reduced by devices that monitor the response of the body during the intervention and inform rescuers about the need to interrupt the exposure when the critical value of the reference indicator is achieved. The aim of the study was to test the correlation between the data of the newly developed device for signaling the strain of rescuers and the indicators of physiological response of the body. Material and Methods: The tests were performed on2 physically fit fire fighters dressed in aprotective rescue suit and using insulating breathing apparatus, over awide range of heat load under amodel load on abicycle ergometer in aclimatic chamber. Results: The study provided asignificant correlation between the body temperature measured in the ear canal and the temperature under the suit sensed by the tested device– the Safety Ambient Monitor(SAM) (R=0.9007). The temperature under the suit also correlated with the temperature of the chest skin (R=0.8928) and heart rate (R=0.8613). Conclusions: Astatistically significant correlation was proven between the temperature sensed by theSAM and the body temperature. The technical solution of sensing the temperature under the suit using the verifiedSAM technology does not affect or limit fire fighters in their work and minimizes the possibility of damage to the sensor and signaling failures. MedPr2018;69(1):1–11 Key words: heart rate, correlation, body temperature, fire fighter, physical strain, thermal load Corresponding author: Šárka Bernatíková, VŠB– Technical University of Ostrava, Faculty of Safety Engineering, Laboratory for Risk Research and Management, 17.Listopadu15/2172, 70833Ostrava-Poruba, Czech Republic, e-mail:[email protected] Received: January 2, 2017, accepted: August 17, 2017 Medycyna Pracy2018;69(1):1–11 http://medpr.imp.lodz.pl/en VERIFICATION OF ELECTRONIC DEVICE TECHNOLOGY FOR MEASUREMENT AND EVALUATION OF THERMAL EXPOSURE OF FIRE FIGHTERS AND MEMBERS OF RESCUE TEAMS ORIGINAL PAPER INTRODUCTION In dealing with emergency situations, the work of fire fighters as well as the work of mine rescuers and members of rescue teams is spasmodically associated with high and often even extreme physical and thermal loads. If not timely interrupted, any extreme labour-thermal load may lead to afailure of the body and fatal collapse[1,2]. https://doi.org/10.13075/mp.5893.00591 Funding: this work was supported by the Pre-seed activity project of VŠB – Technical University of Ostrava II– Safety (registration No.CZ.1.05/3.1.00/14.0316) and the project is co-financed by the European Regional Development Fund and the state budget of the Czech Republic. Project manager: Prof.Petr Novák,M.D.Eng. Š. Bernatíková etal.2Nr 1 During12real interventions (4interventions when fighting house or apartment fires, 3interventions regarding traffic accidents, trailer fire, 2car fires, cistern fire and freeing from an elevator), the measured average heart rate(HR) was100–120min–1 but fitfully it reached even180min–1. The corresponding average values of energy expenditure(EV) ranged between15–25kJ×min–1; on ashort-term basis, however, they reached net values of50–60kJ×min–1[3]. In the model situations, when carrying awounded person under difficult circumstances and when ascending quickly up the stairs with baskets containing1or2hoses to the6thfloor (measured using an integral method), the net EV was 48–130 kJ×min–1 (888–2167W) whereas theHR values at the ascent end were around180min–1[3]. Similarly are high values found by anumber of authors in testing fire fighters as well as mine rescuers[4]. For the model conditions in the mine, the measured values of energy expenditure ranged between184W and 966 W, while the average HR values were around 153±17 min–1; however, short-term HR values were180–200min–1. The core body temperature reached the average values of 38.1–38.3°C, individually even up to 38.9°C [5–7]. Working conditions are especially challenging if the work must be done with abreathing apparatus[5,8,9]. Health risk may be significantly reduced by devices that monitor the response of the body during the intervention and inform rescuers about the need to interrupt the exposure when the critical value of the reference indicator is achieved. In work-medical practice, evaluation of the labour-thermal load of workers in hot workplaces takes grounds in changes of physiological parameters, primarily the body core temperature, liquid loss by perspiration and respiration, heart rate, and skin temperature. The evaluation also may use computation programs to predict perspiration production and body core temperature stemming from accurately defined indicators on metabolic production of the organism, heat-humidity conditions of the environment, and thermal resistance of clothing[10,11]. Recently, Yokotaetal.[12] have published amodel for evaluation of the labour-thermal load, which enables to predict body core temperature, skin temperature, perspiration production and heat stress index (physiological strain index (PSI) [13]) based on input data including microclimatic conditions of the environment,HR, metabolic production, height, weight, and clothes characteristics. These methods have only limited use in monitoring load of fire fighters and rescuers in terrain conditions due to unpredictable and quickly changing heat-humidity conditions at the place of the intervention. Monitoring of the labour-thermal load of fire fighters and rescuers in terrain conditions requires using methods that put minimal load on the worker and are safe from the point of view of possible faults and failures of the early warning system. The best indicator of organism overheating is the body core temperature(tc) which reacts to metabolic and climatic load. Its direct measurement in terrain conditions is not possible. The only mini-invasive system for remote monitoring oftc is athermometer pill, although its routine use is limited due to high costs and results may be influenced by water and food intake[14]. This is the reason to search for methods that would be accurate enough (better than ±0.5°C) to determine the body core temperature from physiological indicators well-measured on the body surface under non-rest conditions. In the past methods were published to have been based on prediction of the body core temperature from temperature measured at selected places on the skin, thermal flow on the skin and heart rate[14–16] or only from the heart rate[17]. The methods were verified in the laboratory and terrain conditions in awide range of thermal-humid conditions, metabolic demands, and various clothes. The reliability of the calculated values compared with the body core temperature measured by some of the invasive methods (rectal temperature, esophageal, intestinal) was better than the required±0.5°C. Although the mentioned methods are maximally gentle from the monitored person’s point of view, they required scanning the monitored parameters directly from the surface of the monitored person, which might be the cause of faults and failures of the early warning system. The aim of the work was to validate the correlation between data measured using the technology of the Safety Ambient Monitor(SAM) and indicators of physiological responses of the body under experimental conditions of aclimatic chamber. MATERIAL AND METHODS The Safety Ambient Monitor device (Photo1) was developed at the VŠB– Technical University of Ostrava for monitoring and evaluation of those working conditions in order to minimize hazard by an early warning. Device for evaluation of thermal exposure 3Nr 1 The device records and evaluates the air temperature and humidity under the suit and outdoor temperature. Based on defined limits, it signals any excess of these limits and alerts wearers to impending critical conditions of body overheating. The device also registers and analyzes movements of the monitored person and alerts to loss of consciousness or to fall from aheight of that person. The unit weighs about130g, has asize of amobile phone and its battery lasts48h. Charging is done via amicro-USB connector and the unit parameters are set from aPC. For testing purposes, we selected2professional fire fighters(FF) aged32 and42years old, well adapted for the profession of afire fighter (seniority as afire fighter, respectively:10years and13years) and physically fit (maximal oxygen consumptionper minute (VO2max) = 43.6 ml/min/kg and 42.7 ml/min/kg), who, prior to testing, had successfully passed the stress test in afunctional laboratory of the Department of Physiology and Pathophysiology, Faculty of Medicine, University of Ostrava, Ostrava, the Czech Republic. First of all, those experimental persons (fire fighter1–FF1, fire fighter2–FF2) completed acontinuous load test on abicycle ergometer (Ergo900, Ergo-Line), in the case of which the load, after an initial warm-up, rose from the start value of75W by25W every minute up to maximum. Fans’ parameters were found and the gas analysis was done using the instrument OxyconPro (Jaeger– CareFusion, Germany). The achieved values of the maximum oxygen consumption per minute, heart rate (HR) andVO2 at the level of anaerobic threshold(AT) are listed in theTable1. After acomplete 2-hour recovery in optimal microclimate conditions, they underwent asubmaximal exercise test according to the design by Hollmann[18], in which the load was increased for the start value of30W every3min by40W. TheFFs underwent this test in the laboratory microclimatic conditions in the rescue suit with the same equipment as during the subsequent testing in the climatic chamber. TheFFs were dressed in astandard rescue suit TigerPlus. Under the suit, they had short underpants, aT-shirt with short sleeves and socks. Both of them had rescue leather shoes afoot and standard helmets for fire fighters on their heads. On their backs, they were carrying abreathing apparatus weighing14.1kg (Photo2). During the test theFFs did not use abreathing apparatus, but OxyconPro analyzer. OxyconPro has automatic calibration and its accuracy of measurement is3% or0.05l/min. The results were used for calculating individual equations for the relationship between the magnitude of the load and strain physiological indicators. The technology for signaling the load of rescuers was verified in aclimatic chamber of the Department of Work Physiology at the Health Institute in Ostrava. The climatic chamber has an inner space of2×3×2m. Conditioned air is conveyed to the chamber from an engine room through a duct hole sized 1×1 m and located in the wall along the chamber longitudinal Photo1. Safety Ambient Monitor(SAM)– the tested device measuring thermal exposure of fire fighters and members of rescue teams, and signaling the strain of rescuers and the indicators of physiological response of the body Table 1. Continuous load* test of professional fire fighters on abicycle ergometer in the climatic chamber in suit under normal heat-humidity conditions Fire fighter Age [years] Body weight [kg] Parameters VO2max [l/min] VO2max/kg [ml/min] HRmax [bpm] Wmax/kg [W/kg] AT HR [bpm] VO2 [l/min] FF1(N =1) 32 75.8 3.30 43.6 184 4.0 130 1.6 FF2(N=1) 42 86.0 3.67 42.7 160 3.8 134 2.4 * Load (after an initial warm-up) rises from the start value of75W by25W every minute up to maximum value. FF1– aperson with 10-year-exposure as afire fighter, FF2– aperson with 13-year-exposure as afire fighter. VO2max– maximum oxygen consumption per minute, VO2max/kg– oxygen consumption perkg of weight, HRmax– maximum heart rate, Wmax/kg– maximum work rate achieved during an incremental cycle ergometer test perkg, AT– the values of the heart rate with the corresponding oxygen consumption at anaerobic threshold. Š. Bernatíková etal.4Nr 1 axis and removed in the wall on the opposite side (Figure1). The back wall of the chamber is created by aradiating panel. The source of radiation is12heating units (each of250W) connected serially and placed in 3 superposed rows. There are 3 metal rods and ametal net to protect the heating units from adirect contact. The chamber allows for experimenters to adjust the air temperature in the range20–60°C, airflow speed in the rangeof 0.2–2.0m×s–1, relative humidity in the range of30–90%. Intensity of heat radiation from the back wall of the chamber(IA) can be gradually increased up to700W×m–2. The load on the bicycle ergometer in the climatic chamber was set uniformly at100W for all phases. The calculated values of the oxygen consumption per minute, heart rate, and energy expenditure(kJ, W), which theFFs reach after the100W load in afire fighter’s suit under normal heat-humidity conditions, are stated in the Table2. Ambient conditions were continuously measured at aheight of110cm above the floor, at the level of the chest of experimental persons. The measured physical parameters, dry air temperature(ta)(°C), air relative humidity(RH)(%), air velocity(va)(m×s–1), radiation temperature in the direction from the chamber front wall (trA) (°C) and from the opposite wall (trB) (°C), and the intensity of radiation from the chamber front wall(IrA)(W×m–2) and from the opposite wall(IrB) (W×m–2), were measured using the instrument– the Indoor Climate Analyzer, type1213, made by the company Bruel and Kjaer. The resulting temperature (tg) (°C) was measured with aspherical thermometer made by Ahlborn company, having aball diameter of 150 mm. Monitored values were recorded at 3-minute intervals. For each phase, the measured values were used for calculating mean values and standard deviations (SD). Detailed heat-humidity conditions measured during each phase are given in the Table3. In all stages, the FFs were forehead-oriented to the radiating panel. To be able to communicate, theFFs were not connected to abreathing apparatus and they breathed atmospheric air from the chamber. Within2testing days, theFFs passed5phases Photo2. Fire fighter in astandard rescue suit with breathing apparatus on a bicycle ergometer in the model work (load value:100W) inside the climatic chamber 1– vertical radiant panel, 2– air flow direction, 3– control room window, 4– entrance to the chamber. Inner space: 2×3×2m. Arrows – direction of conditioned air, which is conveyed to the chamber from an engine room through aduct hole located in the wall and sized1×1m, along the chamber longitudinal axis, and removed in the wall on the opposite side. Fig. 1. Climatic chamber used for the professional fire fighter load test Table 2. Professional fire fighter parameters after the load of100W on the bicycle ergometer in the climatic chamber in suit under normal heat-humidity conditions Fire fighter VO2 [l/min] HR [bpm] Energy expenditure kJ/min (gross) kJ/min (net) W (net) FF1(N=1) 1.48 140 30.28 25.0 416.8 FF2(N=1) 1.67 132 33.51 28.2 470.6 FF1– a person with 10-year-exposure as afire fighter, FF2– a person with 13-year-exposure as afire fighter. VO2– oxygen consumption per minute, HR– heart rate. 1 2 34 Device for evaluation of thermal exposure 5Nr 1 Table 3. Heat-humidity conditions and the time in the climatic chamber during professional fire fighter load test on the bicycle ergometer in suit Phase No. Exposure time [min] Temperature [°C] (M±SD) Intensity of radiation [W×m–2] (M±SD) RH [%] (M±SD) v [m×s–1] (M±SD) tatgtrA trB IrA IrB 1 FF1 30 22.6±0.4 23.9±0.5 24.3±1.2 24.0±0.4 without theradiant without theradiant 41.2±0.7 0.29±0.11 FF2 30 22.9±0.2 23.7±0.3 24.3±0.3 24.4±0.4 panel panel 38.5±0.0 0.32±0.04 2 FF1 30 31.7±1.2 35.9±2.4 56.1±2.0 33.5±1.9 660.0±8.3 509.0±6.4 29.9±1.4 0.33±0.01 FF2 30 31.7±1.2 35.9±2.4 56.1±2.0 33.5±1.9 639.0±8.7 511.5±5.1 29.9±1.4 0.33±0.01 3 FF1 30 40.8±2.1 43.7±2.3 59.3±0.4 40.5±2.5 695.0±7.1 537.1±8.5 41.5±2.8 0.32±0.02 FF2 30 42.4±0.1 45.5±0.0 58.3±0.4 41.6±0.3 680.0±2.6 550.0±3.5 27.5±9.9 0.27±0.04 4 FF1 25 50.4±0.2 50.0±0.6 56.8±0.5 45.6±1.6 672.3±4.9 585.7±11.3 27.6±2.8 0.21±0.02 FF2 27 50.8±0.3 50.8±0.1 57.0±1.1 47.7±1.1 674.6±3.5 592.3±7.8 28.9±0.7 0.22±0.02 5 FF1 30 33.5±0.4 34.2±0.3 34.1±1.1 33.5±0.3 505.5±6.4 501.4±2.1 71.6±1.4 0.24±0.03 FF2 25 32.9±0.1 33.6±0.0 33.8±0.4 33.0±0.1 502.9±2.8 498.3±1.4 72.4±2.8 0.22±0.01 FF1– a person with 10-year-exposure as afire fighter (N = 1), FF2– a person with 13-year-exposure as afire fighter (N = 1). M– mean, SD– standard deviation. ta– dry air temperature, tg– resulting temperature, trA– radiation temperature in the direction from the chamber front wall, trB– radiation temperature from the opposite wall, IrA– intensity of radiation from the chamber front wall, IrB– intensity of radiation from the opposite wall, RH– air relative humidity, v– air velocity. Š. Bernatíková etal.6Nr 1 (P1–P5) with different air temperature, radiation intensity and relative humidity (RH). During phases P1–P4, the thermal load was increased from avery modest load inP1 (ta=22°C, trA=24°C, RH=30–40%) up toP4 (ta=50°C, trA=57°C,RH=30%). InP5, the relative humidity was very high (RH=70%) at relatively low air temperature (ta=33°C) and temperature from the chamber front wall trA=34°C. Intensity of radiation was adjusted to achieve the desired air temperature(ta) in individual phasesP1–P5. The period of exposure in various phases was set to30min or until reaching criticalHR or body temperature values. The exceeded limit value was the reason to interrupt the test and to set it toHR200-age and the body core temperature– 38.5°C. On the first testing day, theFFs underwent phaseE1 andE2, on the second day – stage E3 to E5, and between individual phases, there was a2-hour break which theFFs spent in light clothes in optimal climatic conditions in arest room. Monitored physiological parameters, heart rate(HR) (bpm) and the skin temperature at the inferior sternum (tsk,chest) (°C) were measured using the multivariable monitoring device – MLE 120X BioHarness. TheHR was captured through electrode sensors housed within the chest strap with the TEAM System station for BioHarness device. Skin temperature data was collected through an infrared sensitive sensor behind aclear window on the apex of the monitoring device; values were displayed on amonitor and continuously recorded at 30-second intervals in the memory of the computer. Precision of measurement for HR data is < ±1 beat/min; for methods involving temperature measurement, a threshold of accuracy stands at0.1°C[19]. The body core temperature was measured continuously in the left ear canal(tac)(°C) using the thermocouple typeT sensor with accuracy of±0.2K. Having otoscopically checked the cleanliness of the ear canal, the temperature sensor was introduced deep into the ear canal and thermally shielded by a 5-centimeter foam insulation and helmet plastic skeleton. The measured values were continuously displayed on the monitor and saved in the computer memory. The device for signaling the load, theSAM, was placed in the inner pocket of the rescue suit on the right chest side. Through the hole in the pocket, the sensors of theSAM for the continuous measurement of the microclimate temperature (tsuit) and relative humidity(RHsuit) measured microclimate in the space between the outer and inner layers of the suit. The suit surface temperature(tsurface) was measured contactlessly shortly after entering the chamber and just before leaving the chamber. Loss of water due to sweating and breathing(SR)(ml) was measured by weighingFFs before entering and immediately after leaving the climate chamber. During their stay in the climatic chamber, EPsdid not eat and drink. At the end of each phase, EPsevaluated their subjective feeling of physical strain and fatigue using a 5-point scales created by authors (Table4 and5). Table 4. Scale of subjective evaluation of the amount of labour-thermal load Load intensity Labour-thermal load 1 very light load with no uncomfortable subjective feelings 2 light load with minor uncomfortable subjective feelings, which can be tolerated for avery long time 3 medium load with mid-intensive uncomfortable feelings, which can be tolerated for quite along time 4 intensive load with uncomfortable feelings, which can be hardly tolerated even for avery short time 5 very intensive load with very uncomfortable subjective feelings leading to the feeling of exhaustion after avery short time with the need to interrupt the work Table 5. Scale of subjective evaluation of the tiredness level after experiments in the climatic chamber Tiredness level Feelings after the labour-thermal load 1 pleasantly tired 2 slightly tired 3 quite tired 4 very tired 5exhausted For purposes of statistical evaluation, authors used correlation and regression analyses with the chosen significance level of5%. Processing was performed using Stata software, version13. RESULTS In various phases, the average production of sweat per hour did not vary too much and ranged around 1.38±0.13l/h, 3 were no statistically significant differences among the phases. The temperature on the suit Device for evaluation of thermal exposure 7Nr 1 surface at the end of the load reached28.7°C inP1 up to49.8°C inP4 on the back (in the direction of radiation) and26.4°C inP1 up to44.7°C inP4 on the chest. At the end ofP3 andP4, the heart rate exceeded the limit values of SFmax, approaching to the maximum values; the body core temperature measured in the ear canal also exceeded the limit value of38.5°C during the stated phases. DuringP5, all the indicators ofFFs reached lower values than inP4. Experimental persons(FFs) assessed the load magnitude with grade1–3 (very slight to medium load causing moderately intensive unpleasant subjective feelings which can be tolerated for quite along time) withinP1 toP3, and with grade4 (intensive load associated with unpleasant sensations which can be tolerated only with great effort for ashort time) withinP4 andP5. They assessed the feeling of fatigue with grade1 (pleasantly tired) withinP1 andP2, and with grade3 (rather tired) withinP3toP5. Minimum and maximum values of basic physiological indicators, i.e.,the body temperature measured in the ear canal(tac), heart rate(HR) and the skin temperature on the chest(tsk,chest), and the values measured by theSAM device, i.e.,the air temperature measured under the suit(tsuit) and air relative humidity measured under the suit(RHsuit), found during individual experiments and their differences are shown in theTable6. The relationship between basic physiological indicators (HR, tac,tsk,chest) on the one hand and the values measured by the verifiedSAM technology (tsuit,RHsuit) Table 6. Physiological indicators for professional fire fighters(FF1,FF2) during individual experiments within5 phases in the climatic chamber Experiment No. tac [°C] HR [bpm] tsk,chest [°C] tsuit [°C] RHsuit [°C] FF1 FF2 FF1 FF2 FF1 FF2 FF1 FF2 FF1 FF2 1 min. 35.8 35.5 94 61 33.4 34.6 28.0 27.4 62.9 53.4 max 37.1 37.0 163 137 35.4 36.3 33.6 34.0 81.8 89.8 increase 1.3 1.5 69 76 2.0 1.7 5.6 6.6 18.9 36.4 2 min. failure 34.7 65 61 32.9 35.0 28.4 28.6 70.9 59.0 max 37.6 157 158 37.0 37.1 35.4 36.1 91.7 88.4 increase 2.9 92 97 4.2 2.1 7.0 7.5 20.8 29.4 3 min. 35.6 33.1 110 61 34.4 35.1 29.5 29.0 66.9 58.2 max 38.5 37.7 196 170 38.0 37.7 38.1 38.2 89.7 88.6 increase 2.9 2.3 85 109 3.5 2.5 8.6 9.2 22.8 30.4 4 min. 34.6 35.4 78 61 34.5 35.2 28.5 31.0 58.2 53.6 max 38.6 38.8 173 178 38.2 38.6 39.3 39.7 85.1 83.2 increase 4.1 3.5 94 117 3.6 2.9 10.8 8.7 26.9 29.6 5 min. 34.4 35.4 84 61 34.9 35.6 29.7 31.7 58.8 55.2 max 37.8 37.9 173 175 37.2 37.2 37.9 37.3 83.1 82.2 increase 3.3 2.6 89 114 2.3 1.6 8.2 5.6 24.3 27.0 FF1– a person with 10-year-exposure as afire fighter(N=1), FF2– a person with13-year-exposure as afire fighter(N=1). min.– minimal value, max– maximal value. tac– body temperature measured in the ear canal, HR– heart rate, tsk,chest– skin temperature measured on the left side of the chest using glued thermistor sensors, tsuit– air temperature measured between the outer and inner layer of the suit by the verified technology for signaling the load (safety ambient monitor–SAM), RHsuit– air relative humidity measured between the outer and inner layer of the suit by the verified technology for signaling the load(SAM). Other abbreviations as in Table 3. Š. Bernatíková etal.8Nr 1 Table 7. Relationships between the air temperature(tsuit) and the air relative humidity(RHsuit) measured under the suit of professional fire fighters by the verified technology for signaling the load (safety ambient monitor–SAM) and the load physiological indicators: body temperature(tac), heart rate(HR) and the temperature of the chest skin(tsk,chest) Relationship Equation Correlation coefficient* tac:tsuit tac=27.32+0.28×tsuit [°C] 0.9007 tac:RHsuit tac=0.06×RHsuit+31.98 [°C] 0.6651 HR:tsuit HR=–77.65+6.6×tsuit [bpm] 0.8613 HR:RHsuit HR=28.22+1.47×RHsuit [bpm] 0.6292 tsk,chest:tsuit tsk,chest=18.31+0.5×tsuit [°C] 0.8928 tsk,chest:RHsuit tsk,chest=25.00+0.13×RHsuit [°C] 0.7512 * p<0.05. Abbreviations as in Table6. Correlation between tsuit and tac (R)=0.907. Fig.2. Relationship between the professional fire fighters’ body temperature measured in the ear canal(tac) and the air temperature measured between the upper and the inner layer of the suit by the verified technology for signaling the load(tsuit) Correlation between RHsuit and tac (R)=0.6651. Fig.3. Relationship between the professional fire fighters’ body temperature measured in the ear canal(tac) and the air relative humidity measured between the upper and the inner layer of the suit by the verified technology for signaling the load(RHsuit) Correlation betweentsuit andHR (R)=0.8613. Fig.4. Relationship between the professional fire fighters’ heart rate(HR) and air temperature measured between the outer and inner layer of the suit by the testing device(tsuit) Correlation betweenRHsuit andHR (R)=0.6292. Fig.5. Relationship between the professional fire fighters’ heart rate(HR) and relative air humidity measured between the outer and inner layer of the suit by the testing device(RHsuit) Correlation betweentsuit andtsk,chest (R)=0.8928. Fig.6. Relationship between the professional fire fighters’ skin temperature measured on the chest (tsk,chest) and air temperature measured between the outer and inner layer of the suit by the testing device(tsuit) tsuit [°C] RHsuit [°C] 32 34 36 38 40 25 30 35 40 50 100 150 200 50 60 70 80 90 tac [°C] HR [bpm] RHsuit [%] tsuit [°C] 32 34 36 38 40 50 60 70 80 90 30 32 34 36 38 4030 35 tac [°C] tsk,chest [°C] tsuit [°C] 50 1 00 1 50 2 00 25 30 35 40 HR [bpm] Device for evaluation of thermal exposure 9Nr 1 on the other hand for both experimental persons is provided in theTable7. Graphical representation of the correlations of the measured quantities is presented in the Figure2–7. The multifactor regression analysis analyzing tac– tsuit andtac–RHsuit relationships did not lead to improvements of observed correlations. Proportion ofRHsuit in the studied relationship proved to be insignificant. DISCUSSION The permissible operating thermal load for the industrial population in the Czech Republic is based on the Government Regulation [20]. Nevertheless, the given limits do not apply to workers of rescue brigades in fire-fighting or liquidation of emergency situations if such persons have been recognized for this work as medically fit. In the Czech Republic the limit values for workers of rescue brigades are not legislatively determined. With regard to the safety of rescuers, however, HRshould not exceed the value of200-age even temporarily and the body core temperature should not be higher than38.5°C. Our selected load for testing the signaling device– theSAM was determined in away that the heat-humidity conditions would cover as wide range as possible and the resulting load in phases4and5would correspond to or be higher than the average load in regular interventions. This condition was met and theEPs in the last three attempts achieved or exceeded the recommended limit values ofHR and the body core temperature so the exposure had to be timely interrupted due to safety reasons. In order to measure the body core temperature in the laboratory conditions, most authors use some of invasive methods, mostly measured in anus, which is considered to be the “golden” standard[14,21]. Values measured by direct methods are considered little reliable as they can show striking differences against values acquired by invasive methods[21,22]. With respect to our used way of load (bicycle ergometer), the temperature in the ear canal was used for measuring the body core temperature. We have long experience in using this method and when strict conditions are followed, it provides very reliable results. Direct measurement of the body core temperature by both invasive methods and in the ear canal is not possible in terrain conditions due to technical or safety reasons. Promising results are from works which calculate the body core temperature from physiological indicators easily measurable in non-rest conditions[14–16,23]. However, these methods require scanning the monitored parameters directly from the surface of the monitored person, which might be related with higher or lower level of discomfort and primarily may be the cause of faults and failures of the early warning system. Our experiments had avery good correlation betweentsuit andtac (R=0.9007). The air temperature measured under the suit(tsuit) correlated both withtsk,chest (R=0.8928) andHR (R=0.8613) very well. On the contrary, RHsuitcorrelated with the measured physiological indicatorstac andHR very weakly. It may be explained that at the beginning of exercisesRHsuit achieved the maximum around90% in10–15min and did not increase in the following minutes. A good correlation between the temperature measured under the suit and the body temperature was apromising solution of this problem. Our results were achieved on asmall set of persons, which does not allow deeper statistical processing and evaluation of the tightness of the found relationships. The usability of the air temperature measured under the suit as the indicator of organism load during work in protective suits will require asufficiently large set of persons in awide range of labour and thermal load in the laboratory and terrain conditions. CONCLUSIONS Based on our experiments, the air temperature measured under the suit proved to be avery good indicator of the overall load of fire fighters or rescuers. The Correlation betweenRHsuit andtsk,chest (R)=0.7512. Fig.7. Relationship between the professional fire fighters’ skin temperature measured on the chest(tsk,chest) and relative air humidity measured between the outer and inner layer of the suit by the testing device(RHsuit) RHsuit [%] 30 32 34 36 38 50 60 70 80 90 tsk,chest [°C]