Effects of wearing aircrew protective clothing on physiological and cognitive responses under various ambient conditions

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Effects of wearing aircrew protective clothing on physiological andcognitive responses under various ambient conditionsHilde Færevik a; Randi Eidsmo Reinertsen a

a Department of Health and Work Physiology SINTEF Unimed N-7465 Trondheim Norway.

To cite this Article Færevik, Hilde and Reinertsen, Randi Eidsmo(2003) 'Effects of wearing aircrew protective clothing onphysiological and cognitive responses under various ambient conditions', Ergonomics, 46: 8, 780 — 799To link to this Article: DOI: 10.1080/0014013031000085644URL: http://dx.doi.org/10.1080/0014013031000085644

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Effects of wearing aircrew protective clothing on physiological

and cognitive responses under various ambient conditions

HILDE FÆREVIK* and RANDI EIDSMO REINERTSEN

Department of Health and Work Physiology, SINTEF Unimed,N-7465 Trondheim, Norway

Keywords: Protective clothing; Heat stress; Cognitive performance; Pilots.

Heat stress can be a significant problem for pilots wearing protective clothingduring flights, because they provide extra insulation which prevents evaporativeheat loss. Heat stress can influence human cognitive activity, which might becritical in the flying situation, requiring efficient and error-free performance. Thisstudy investigated the effect of wearing protective clothing under various ambientconditions on physiological and cognitive performance. On several occasions,eight subjects were exposed for 3 h to three different environmental conditions;08C at 80% RH, 238C at 63% RH and 408C at 19% RH. The subjects wereequipped with thermistors, dressed as they normally do for flights (includinghelmet, two layers of underwear and an uninsulated survival suit). During threeseparate exposures the subjects carried out two cognitive performance tests(Vigilance test and DG test). Performance was scored as correct, incorrect, missedreaction and reaction time. Skin temperature, deep body temperature, heart rate,oxygen consumption, temperature and humidity inside the clothing, sweat loss,subjective sensation of temperature and thermal comfort were measured. Rises inrectal temperature, skin temperature, heart rate and body water loss indicated ahigh level of heat stress in the 408C ambient temperature condition in comparisonwith 08C and 238C. Performance of the DG test was unaffected by ambienttemperature. However, the number of incorrect reactions in the Vigilance test wassignificantly higher at 408C than at 238C (p=0.006) or 08C (p=0.03). The effecton Vigilance performance correlated with changes in deep-body temperature, andthis is in accordance with earlier studies that have demonstrated that cognitiveperformance is virtually unaffected unless environmental conditions are sufficientto change deep body temperature.

1. Introduction

Human error is regarded as a contributing factor in 85% of all aviation crashes (Li etal. 2001). Identifying and reducing factors that have detrimental effects on humanperformance in the aircraft cockpit environment can therefore improve flight safety.The more specific task of helicopter flying includes a number of factors that degradehuman performance; these include vibration, noise, gravitation forces, uncomfor-table seats and high temperatures. Several studies have demonstrated an associationbetween ambient heat stress and pilot error outside the laboratory. A study of 500Israel military helicopter accidents and incidents found that pilot errors were fewestat ambient temperatures of 25 – 298C, with an increased risk at 30 – 348C and the

*Author for correspondence. e-mail: [email protected]

Ergonomics ISSN 0014-0139 print/ISSN 1366-5847 online # 2003 Taylor & Francis Ltdhttp://www.tandf.co.uk/journals

DOI: 10.1080/0014013031000085644

ERGONOMICS, 2003, VOL. 46, NO. 8, 780 – 799

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highest risk at 358C (Froom et al. 1993). Field trials of pilots flying the FR-4C atShaw AFB in South Carolina, USA also demonstrated that pilot error increasesduring flights in hot weather (Bollinger and Carwell 1975). Studies of fighter pilotsflying the A-10 fighter jet at Davis-Montham Air Force Base in Arizona havedemonstrated lower G-tolerance and increased general fatigue in low-level flying onwarmer flights (Nunneley and Flick 1981). These studies investigated problems thatarise when aircraft are operating in hot areas. This is not a problem in northernEurope. Sea King helicopters operating on the Norwegian coast have a relatively lowmean ambient cockpit temperature in winter (18.48C), but which occasionally risesto 408C (Færevik and Reinertsen 1998). This high ambient temperature is caused bythe helicopter’s large canopy, which produces a greenhouse effect, combined with aheating system, which is located by the pilots’ feet. In normal aircrew clothingassemblies, the resulting thermal strain might be physiologically acceptable.However, Sea King pilots are obliged to wear survival suits all year around becausethey are operating in areas with low sea temperatures. Wearing a survival suit resultsin higher discomfort ratings and significant rises in skin temperatures and sweat ratesin helicopter pilots during the winter (Færevik and Reinertsen 1998). Laboratorystudies have demonstrated that an ambient temperature of 10 – 148C is needed forthermoneutrality in subjects wearing survival suits (Færevik et al. 2001).Even though heat stress, protective clothing and performance have been

thoroughly investigated, the relationships between environmental parameters,physiological status and cognitive performance are still not fully understood.Studies that have looked at the effects on cognitive performance of wearingprotective clothing at different ambient temperatures often lack information abouthow the physiological status of the subject correlates with decrements inperformance (Fine and Kobrick 1987, Thornton and Caldwell 1993, Reardon etal. 1998b). Other studies have focused primarily on the physiological consequencesof wearing protective clothing in different ambient temperatures, or on limitationson the ability to perform physical work, rather than on mental performance as such(Thornton et al. 1985, White et al. 1991, Scott et al. 1994, Rissanen and Rintamaki1997). Reviews of cognitive performance under hot conditions have also revealedseveral shortcomings, such as inadequate pretraining of the subjects on theexperimental tasks, the use of unrealistic tasks for assessing performance, poormethodological designs for statistical implications and insufficient duration ofexposure to the heat (Kobrick and Fine 1983, Ramsey 1995).On the background of these shortcomings and the problem of Norwegian Sea King

pilots described above, the aims of this study were to investigate the effects of wearingaircrew protective clothing under various ambient conditions on physiological andcognitive parameters. Any observed changes in performance should be related notonly to the ambient conditions involved but also to changes in physiological measuresof heat strain. Such information will aid assessments of aircrew efficiency undervarious ambient conditions while wearing protective clothing. It was hypothesized thattypical cockpit temperatures cause heat stress, which will have detrimental effects oncognitive performance of pilots wearing aircrew protective clothing.

2. Materials and methods

Eight male volunteers (including three RNAF pilots) participated in the study. Themean age, weight, stature, percent body fat and surface area (ADu) of the subjectswere 26.8+ 6.5 years, 74.5+ 10.5 kg, 178.9+ 0.01 cm, 14.3+ 2.7% and

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1.92+ 0.12 m2, respectively. The subjects were asked to retire to bed at their usualtime on the night before each exposure. They abstained from taking exercise andfrom consuming caffeine, alcohol or snuff for 24 h before exposure. None of thesubjects were smokers. All subjects were in good health and had recently undergonean electrocardiogram test. The Ethical Review Committee of the Faculty ofMedicine at the Norwegian University of Science and Technology approved theexperimental procedure. The subjects were free to withdraw from the chamberenvironment at any time. The experiment was cancelled if either of the followingphysiological safety limits were exceeded: core temperature above 398C or skintemperature above 428C.

2.1. Experimental protocolOn three different occasions, the subjects were exposed for 3 h to three differentambient conditions in an environmental chamber; 08C and 80% relative humidity(RH), 238C and 63% RH, 408C and 19% RH, respectively. The subjects wereexposed in a random order to the three ambient conditions so as to avoid any effectsof order. The tests were carried out at the same time of the day on different days withat least a 1-day pause between the tests. Total time of exposure to the test conditionswas 3 h. Subjects were not permitted to drink or eat during the experiment.Subjects reported to the preparation room at least 1 h before testing. They were

equipped with thermistors and heart rate recorder, and then dressed as they normallydo for flights, in long legged/long sleeved underwear (200 g Ullfrotte), a woollen wholebody cover all, helmet and the uninsulated British Mark 10 survival suit (Beaufort).This survival suit consists of a double-layer of cotton Ventile which permits thetransmission of water vapour; once wetted, the fibres expand such that the interfibrespaces no longer transfuse liquid. The insulation value of the whole clothing ensemblewas 2.20 Clo, measured on a thermal manikin. The subjects sat quietly outside theclimatic chamber for 20 min in order to provide baseline measurements of oxygenconsumption (VO2), skin and rectal temperature and subjective evaluations.In order to identify changes associated with the development of thermal stress,

performance was measured at intervals throughout the exposure. During the 3 h ofexposure in the climatic chamber, subjects carried out an experimental scheduleaccording to the timetable described in table 1.

2.2. Cognitive variablesIt was not possible to devise a test procedure capable of fully simulating the actualworking conditions of helicopter pilots in an operational situation. Cognitiveperformance was therefore assessed using the computerized Vienna Test Battery(Schufried 1990) (figure 1). This test battery has been designed to test vigilance andmultiple-choice reactions, and is used in the selection of pilots for the RoyalNorwegian Air Force. The subjects were required to perform two cognitive tests; theVigilance test and the Vienna Determination Unit test (DG test) to measure reactivestress tolerance. The Vigilance test requires subjects to exercise continuous vigilanceover a lengthy period of time. A bright dot moves in small jumps along a circularpath. Occasionally, the dot jumps twice the usual distance, to which event subjectsmust react by pressing a reaction button. The following variables were scored;increase/decrease in reaction times, total number of incorrect/correct reactions andmissed reactions. The DG test is intended to evaluate the ability of subjects toperform multiple-choice reactions to rapidly changing stimuli and to detect

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impairments in attentive capacity. Subjects are required to respond to visual oracoustic stimuli by pressing a button or stepping on a foot pedal. The visual stimuliconsist of white, yellow, red, green and blue signals, the acoustic stimuli of high andlow tones. Performance is measured in terms of the number of correct (timely/delayed), incorrect and missed reactions as well as reaction time. The test lasts for13 min and consists of four subtests with the same stimuli but presented in differentorders and at different speeds. Before the actual study in the environmental chamber,all subjects did 10 pilot test runs on the Vienna test battery in a thermoneutralenvironment to ensure that a learning plateau was reached and minimize the learningeffects during the three environmental exposures.

2.3. Physiological variablesRectal temperature (Tre) was measured by a thermistor probe (YSI-700, YellowSprings Instrument, USA, accuracy + 0.158C) inserted 10 cm beyond the analsphincter. Skin temperatures were measured using thermistors (YSI-400 YellowSprings Instrument, USA, accuracy + 0.158C) at six different locations (forehead,chest, upper arm, left hand, front thigh and front leg). Mean skin temperatures(MST) were calculated using the formula of Ramanathan (1964). Heart-rate (fc) wasrecorded by a Polar Sports Tester (Polar Electro, Finland). All parameters wererecorded at 1-min intervals, and graphically and numerically displayed on acomputer screen to have continuous information about the thermal state of thesubject while the experiment was running (software: Templog 3.1). The means of10 min were used for statistical analysis. Oxygen consumption (VO2) was logged for5 min using a Cortex MetaMax Portable Metabolic Test System (Cortex BiophysicGmbH, Germany). A questionnaire developed by Nielsen et al. (1989) was used to

Table 1. Experimental schedule followed during exposure to the three ambient conditions,08C at 80% RH, 238C at 63% RH and 408C at 19% RH.

Session Elapsed time (min) Event

Before start 7 20 Subjective assessment of thermal comfortBaseline values of skin and rectal temperatures

0 Subjects enter the environmental chamber1 2 – 15 DG test

15 – 17 Subjective evaluation17 – 22 Oxygen consumption22 – 55 Vigilance test55 – 57 Subjective evaluation57 – 62 Oxygen consumption

2 62 – 75 DG test75 – 77 Subjective evaluation77 – 82 Oxygen consumption82 – 115 Vigilance test115 – 117 Subjective evaluation117 – 122 Oxygen consumption

3 122 – 135 DG test135 – 137 Subjective evaluation137 – 142 Oxygen consumption142 – 175 Vigilance test175 – 177 Subjective evaluation177 – 182 Oxygen consumption

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obtain information about overall thermal sensation, sensation of skin and clothingwetness, ambient temperature preference and thermal comfort. Total body weightloss was obtained by weighing the subjects before and after the experiment. Bodysurface in square metres (ADu) was calculated using the following formula (DuBoisand DuBois 1916): ADu=0.202 �Wb

0.425 �Hb0.725, where Wb is the body weight in

kg, and Hb is the body height in m. Percentage body fat was calculated using theDurnin and Wommersley 4-site skinfold thickness measure (Durnin and Wommers-ley 1974). Ambient conditions (air temperature, radiation and relative humidity)were measured continuously with an Indoor Climate Analyser T1213 (Bruel & KjærA/S, Denmark). Microclimate inside the clothing was measured using sensors (typeHIH-3605-B-CP, Honeywell, USA,) located in the upper back region. The sensorswere attached inside the inner layer of clothing, between the inner and the middlelayer and between the middle and the outer layer. All logged data were transferred toa computer and the results were displayed every minute on the computer screen tohave continuous information about the microclimate of the clothing while theexperiment was running (software: Templog 3.1).

2.4. Statistical analysisThe difference in performance response under the different ambient conditions wasassessed by two-way analyses of variance (ANOVA) for repeated measures. SPSS

Figure 1. Set-up of the Vienna test battery in the laboratory.

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10.0 (SPSS Inc. Chicago, USA) was used to process the statistical material. A withingroup study design was used. All performance data were tested for effects of time(session 1,2,3), ambient condition, and the interaction between these two. WhenANOVA revealed a significant main effect, a contrast test was used as a post hoc testto locate significant differences between temperatures. For the DG test there werefour subtests in each session. The subtests were collapsed in the analysis, since anytime effect from test duration of only 13 min was not expected. Time-dependentchanges in rectal temperature, mean skin temperatures, oxygen consumption andheart rate were also evaluated by two-way analyses of variance (ANOVA) forrepeated measures, using a contrast test as a post hoc test. Differences in sweatingrates and the individual ratings of thermal comfort, thermal sensation, degree ofshivering or sweating, sensation of skin wetness, temperature and humidity in theclothing and ambient temperature preference were assessed by Student’s t-test forpaired samples. The Shapiro Wilk’s test was used to test for normal distribution.Spearman’s test was used to find correlations between changes in performance andthe physiological measurements/subjective ratings. Correlation tests were performedwithin each series. Results are presented as means+ SD for eight subjects. Alldifferences reported are significant at the p4 0.05 level.

3. Results

3.1. Rectal temperature (Tre)The rectal temperature (Tre) fell slightly at both 08C and 238C ambient temperature(Ta), by 0.68C and 0.38C respectively (figure 2) when the start and end values of the

Figure 2. Development of mean rectal temperatures in the three test conditions: 08C, 238C and408C.� * Indicates significantly higher rectal temperature at 408C than at 08C after 40 min,and after 70 min than at 238C. The difference lasted throughout the test period (n= 8).

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whole test period were compared. At 408C (Ta) Tre rose significantly by 1.28C to38.48C. The time-dependent change in Tre revealed a significant effect of ambientcondition. When the time courses of the changes in Tre were compared, these weresignificantly higher in the 408C (Ta) condition than at 08C (Ta) after 40 min. After70 min the Tre at 408C was also significantly higher than at 238C (Ta). Thesedifferences lasted throughout the period of testing. There were no differences inrectal temperature between 08C and 238C.

3.2. Mean skin temperature (MST)The change in MST from the initial value on entering the climatic chamber wasgreater when subjects were exposed to 408C than to 238C or 08C (figure 3). MST wassignificantly different after only 20 min and throughout the rest of the test in allthree series. After 3 h in the climatic chamber MST rose by 1.98C from its initialvalue when subjects exposed to 408C (Ta). MST was almost unchanged (0.38C) at238C (Ta), and fell significantly at 08C (Ta), by 2.98C.

3.3. Heart rate (fc)Heart rate also demonstrated significant effects of both time and ambient condition.The increase in mean fc was significantly higher at 408C (Ta) than at 238C or 08C (Ta)(figure 4). Heart rate rose significantly in the whole 408C series, ending at 120beats �min71. In both 23 and 08C (Ta), fc decreased slightly, ending at values of 76and 65 beats �min71. The heart rate was significantly higher after 30 min andthroughout remainder of the test at 408C (Ta) than at either 0 or 238C (Ta). Therewere no differences in fc between 08C and 238C.

Figure 3. Development of mean skin temperature in the three test conditions. �*Indicatessignificantly higher MST at 408C after 20 min than at 238C or 08C. The differences lastedthroughout the test period (n = 8).

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3.4. Sweat production and body water lossLosses of body water through sweat and evaporation were considerably higher at408C (Ta) than 238C or 08C, with values of 1145.2+ 290 g, 384.6+ 175 g and246.2+ 113 g, respectively (figure 5). The subjects were significantly dehydrated inthe 408C condition, with a mean weight loss of 1.2 kg, equivalent to 1.5% of theirtotal body weight.

3.5. Oxygen consumptionMean oxygen consumption (VO2) under the three ambient conditions was on average0.46+ 0.05 l �min7 1 at 08C, 0.41+ 0.02 l �min7 1 at 208C and 0.39+ 0.01 l �min7 1

at 408C (Ta). This corresponds to mean heat production rates of 82.9+ 9.1 W �m7 2,73.8+ 5.1 W �m7 2 and 68.9+ 2.4 W �m7 2 respectively. The change in VO2 fromthe initial value was significant only at 08C and 408C (Ta). VO2 rose by 0.16 l �min7 1,0.03 l �min7 1 and 0.10 l �min7 1 at 08C, 208C and 408C (Ta), respectively. Thechange in VO2 from the baseline level at 08C (Ta) was after 137 min and for theremainder of the test. The change in VO2 from baseline that was observed at 408C (Ta)was not significant until 177 min (the last measure). The time-dependent rise inoxygen consumption did not differ between the three exposure conditions.

3.6. Subjective assessments and microclimate in the clothingAfter only 15 min, subjects felt significantly warmer when exposed to 408C (Ta)than at 238C or and 08C. While they felt only slightly warm when exposed to

Figure 4. Development of heart rate in the three test conditions. �*Indicates significantlyhigher heart rate at 408C after 30 min than at 238C or 08C. The observed differenceslasted throughout the test period (n = 8).

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238C, they stated that they were hot at 408C (Ta). Thermal sensation of the feet,hands and head were significantly warmer at 408C (Ta) than at 238C or 08C (Ta)after 15 min and during the remainder of the test. The subjects felt significantlycolder at 08C than at 238C (Ta), and were thermally more uncomfortable at 408Cthan at either 08C or 238C (figure 6). This was noted after only 15 min in theclimatic chamber. They remained more uncomfortable throughout the test.Subjects stated that they started sweating after only 15 min when exposed to408C; after 55 min they were sweating moderately, and their clothing felt damp.At 155 min the clothing felt wet and they stated that they were sweating heavily.This is in contrast to the 238C (Ta) condition, where it was not until 135 minthat the subjects stated that they were sweating slightly, and that their clothingfelt slightly damp. At 08C (Ta) the clothing felt dry throughout the tests, and nosensation of sweating was felt at all. This is in accordance with the results of theclothing moisture measurements. In the 408C series, the inner and middle layersof the clothing were 100% saturated with moisture after 110 min, and thiscondition persisted throughout the rest of the experiment. The outer layerreached 80% saturation after 121 min. In the 238C series, less moistureaccumulated in the inner layer, but even so 80% was reached after 130 min.In the 08C series the moisture was transported from the inner layer outwards inthe clothing ensemble, resulting in a fall in moisture content in the inner layerfrom 60% at the beginning of the test to 30% at the end.

3.7. Performance dataThe mean scores of the performance tasks are summarized in table 2. In theVigilance test ANOVA showed no effects of time or ambient condition or interaction

Figure 5. Total body water loss during the 3 h of exposure in the three test conditions 08C,238C and 408C. �*Indicates considerably higher water loss at 408C than at 08C or 238C(n= 8).

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between them in the responses for reaction time, number of correct reactions ornumber of missed reactions. The only significant effect of ambient condition in theVigilance test was in the total number of incorrect reactions for the whole time series(figure 7). The average number of incorrect reactions was 1.44 in the 408C series; thiswas significantly higher compared to 0.80 at 08C (p=0.03) and 0.89 at 238C(p=0.006). There was no difference between 08C and 238C (p=0.62). Incorrectreactions were unaffected by time and there was no interaction between time andambient condition.In the more complex cognitive DG test there was no significant effect of ambient

condition, but a clear effect of time on some parameters (table 2 and figure 8). Therewere no interactions between temperature and time. The total effect of time onperformance parameters was observed in terms of fewer incorrect reactions inSession 1 than in Sessions 2 (p=0.032) or 3 (p=0.009), and faster reaction times inSession 1 than in Session 2 (p=0.007) or 3 (p=0.012). Performance measured asmissed and delayed reactions in the DG test was unaffected by time.Performance scores were examined not only as they were affected by ambient

condition and time, but also in terms of how changes in performance correlatedwith physiological and subjective changes. The only significant correlation wasin the Vigilance test. Spearman’s test showed that the increase in the number ofincorrect reactions in the Vigilance test in the 408C series clearly correlated withthe change in core temperature (1.28C increase in Tre) (Spearman’s correlationcoefficient: 0.907, p=0.002) (figure 9). There was no such correlation at 238Cor 08C. The increase in incorrect reactions did not correlate with any of theother physiological parameters or with subjective assessments of thermalsensation and comfort.

Figure 6. Time course of subjective ratings of thermal comfort. �*Indicates significantdifferences in comfort between 408C and 238C conditions, and between 408C and 08Cafter 15 min and for the remainder of the test period (n = 8).

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Table2.

Cognitiveperform

ance

intheVigilance

andDG

tests.Meannumber

ofreactionsandmeanreactiontimeare

shownforeach

session(n=

8).

*1Indicatesaneffectofambientcondition,observed

inahighertotalofincorrectreactionsintheVigilance

testat408C

thanat08C

(p=

0.03)or238C

(p=

0.006).*2Indicatesatotaleffectoftimein

theDG

test;fewer

totalincorrectreactionsin

Session1thanSession2(p=

0.032)or3(p=

0.009).

*3Indicatesaslower

reactiontimein

Session1thanSession2(p=

0.007)orSession3(p=

0.012).

Ambienttemperature

andrelative

humidity

08C

,80%

RH

238C

,63%

RH

408C

,19%

RH

Task

Testvariable

Session1

Session2

Session3

Session1

Session2

Session3

Session1

Session2

Session3

Vigilance

Meannumber

ofcorrectreactions

27.3+

4.9

27.9+

4.3

27.4+

6.1

27.7+

5.7

26.6+

6.6

27.5+

5.4

27.4+

4.1

27.3+

.3.9

28.7+

2.1

Meannumber

ofincorrectreactions

0.89+

0.83

0.67+

1.03

0.84+

0.98

0.89+

1.13

0.51+

0.82

1.27+

0.67

1.52+

1.20*1

1.02+

1.07*1

1.78+

1.16*1

Meannumber

ofmissedreactions

4.64+

4.90

4.20+

4.31

4.68+

6.06

4.40+

5.66

5.39+

6.59

4.52+

5.37

4.65+

4.10

4.76+

3.96

3.40+

2.20

Meanreactiontime(second)

0.57+

0.19

0.54+

0.10

0.53+

0.09

0.51+

0.09

0.55+

0.08

0.53+

0.08

0.51+

0.07

0.53+

0.07

0.54+

0.10

DG

test

Meannumber

ofcorrectin

timereactions

586.4+

1.3

583.9+

5.1

582.9+

5.9

585.1+

5.3

581.9+

11.0

584.8+

3.9

586.5+

2.1

584.5+

4.9

583.4+

3.0

Meannumber

ofcorrectdelayed

reactions

1.0+

1.3

1.3+

1.3

1.4+

1.7

2.6+

2.5

2.8+

3.6

2.0+

1.8

1.5+

2.1

2.3+

2.3

2.6+

2.0

Complex

Meannumber

ofincorrectreactions

3.1+

3.1*2

4.9+

3.3

7.3+

.31

5.6+

3.3*2

6.6+

4.4

6.5+

4.7

5.1+

3.4*2

7.5+

4.7

8.4+

6.1

Cognitive

Meannumber

ofmissedreactions

1.6+

2.0

3.8+

3.9

4.1+

5.1

1.5+

2.4

4.0+

6.9

2.5+

2.9

1.4+

1.5

1.6+

1.4

2.3+

2.5

tasks

Meanreactiontime(second)

0.60+

0.09*3

0.59+

0.10

0.59+

0.10

0.60+

0.07*3

0.59+

0.09

0.58+

0.11

0.60+

0.10*3

0.58+

0.11

0.58+

0.12

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Figure 7. Total number of incorrect reactions, missed reactions and reaction time in theVigilance test. �*Indicates significantly more incorrect reactions at 408C than at 08C(p=0.03) or 238C (p=0.006) (mean+SD) (n=8).

Figure 8. Total number of delayed, incorrect and missed reactions and reaction time in theDG test. (mean+ SD) (n = 8).

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4. Discussion

The aim of this study was to investigate the effect of wearing aircrew protectiveclothing under different ambient conditions on physiological and cognitiveresponses. There was a clear effect of wearing protective clothing in the hot(408C) condition on physiological parameters, as was demonstrated by raised rectaland skin temperatures, heart rate and degree of dehydration. The subjectivediscomfort was also high at 408C. Wearing protective clothing at 408C alsolowered performance in some tasks. The study aimed to offer insight into thephysiological or subjective parameters with which such performance decrementcorrelates. In support of current theories, it was demonstrated that performancedoes not deteriorate with time unless there is a significant change in deep-bodytemperature. The actual physiological thermal state of the subject’s body seems tobe of more importance than the thermal sensation of the body or thermal comfort.Furthermore, decrements in performance were dependent on the type of taskperformed. In the following paragraphs, decrements in performance are discussedin terms of the physiological status of the subject, subjective evaluations, theclothing worn, subject skills, the nature of the task, and the relevance of the testresults to ‘real life’ conditions.Higher ambient temperature in the 408C series is capable of statistically explaining

the increase in incorrect reactions seen in the Vigilance test, but more interesting isthe question of the physiological mechanisms on which this performance decrementis based. Wearing protective clothing tends to hinder evaporative cooling, resultingin an increase in heat storage in the body (Sullivan and Mekjavic 1992). The

Figure 9. Correlation of total incorrect reactions with change in rectal temperature.Spearman’s Correlation Coefficient (0.907) indicates a clear correlation (p=0.002). (n = 8).

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physiological responses of the body of this uncompensable heat stress situation wereclearly demonstrated in raised rectal and skin temperatures, greater cardiac output,high rate of sweating and dehydration. The mean metabolic rate varied between 68.9and 82.9 W �m7 2 in the study, but was not significantly different between theambient conditions. The contribution of metabolic rate to the increase in coretemperature during the experiment is regarded as being relatively slight. Metabolicrate measured during flight varies, depending on the different phases of flight andtype of aircraft, between 48 and 116 W �m7 2 (Thornton et al. 1984). Meanmetabolic rates in Sea King helicopter pilots have been measured at 88.8 W �m7 2

(Færevik and Reinertsen 1998). Rectal temperature rose to 38.48C when subjectswere exposed to 408C, and the upper limit of core temperature recommended forpilots (388C) was exceeded after only 125 min in the climatic chamber. Even thoughthe test lasted for 3 h, ANOVA revealed no effect that could be exclusively attributedto time on performance in the Vigilance test, but did reveal a clear effect of ambientcondition. The correlation between the change in Tre in the 408C ambient conditionand the total number of incorrect reactions in the Vigilance test suggests that it is thedynamic change in Tre , not the duration of the test, which causes performancedecrements. This observation confirms the suggestion that the value of the ambienttemperature is not the key factor that dictates whether a breakdown in performancewill occur. Rather, it is the combination of ambient temperature with exposure time,sufficient to change deep body temperature, which affects performance (Grether1973, Hancock 1981). Hancock (1981) claimed that higher levels of heat stress arerequired before any decrements in performance can be observed when exposure isless than 1 h. In support of this, Grether (1973) found that the ambient temperatureat which individuals can maintain adequate performance is very close to thethreshold temperature at which the body could compensate physiologically for thethermal strain. The relationship between thermoregulatory changes and performanceis complex, and several hypotheses have been advanced that try to explain reductionsin mental performance on the basis of thermoregulatory changes. Reviews of earlystudies have explained the effects of heat on performance on the basis of heatbalance theory (Hancock 1986). According to this theory, performance should bebetter in a neutral environment when the body is in heat balance and there is a highgradient between core and skin temperatures. When this gradient is shallow, as inhot conditions, performance would be poor. Carlson (1961) proposed a theory ofinformation overload when skin and core temperatures are both high, and suggeststhat physiological and psychological sources of input sum a single structure througha common channel, and that high skin and core temperature causes informationoverload. This is in agreement with the present study; in which rises in the number ofincorrect reactions in the Vigilance test were observed only when core and skintemperatures were high in the 408C ambient condition. Other studies havedemonstrated that both the rate and direction of change in deep body temperatureaffect performance (Allan et al. 1979, Gibson and Allan 1979). These studiesdemonstrated that decrements in performance were greater when body temperatureswere high and rising than when they were low and falling. This is in agreement withthe present study, which demonstrated no decrements on performance when Tre fellby 0.38C or 0.68C, but clear decrements when Tre rose by 1.28C. Although vigilancedecrement is a common finding, in this study vigilance was not affected by time,subjective reports of thermal discomfort or dehydration. This absence of decrementin vigilance might indicate that thermal discomfort is not a very good measure of

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distraction, or that the test subjects are a selected cohort of highly motivated personstrained to be more able to withstand such distraction over time.The effects of increased thermal strain at 408C (Ta) did not have the same

detrimental effect on performance in the more complex cognitive DG test as wasobserved in the Vigilance test. This might indicate that change in body temperaturerepresents a form of distraction affecting simpler tasks, but increasing the level ofarousal in more complex tasks (DG-test). The cognitive load of the DG battery isregarded as high, and a long period of training was required to achieve a consistentlevel of competence. This may have contributed to the relatively minor performancechanges found. It is possible that the performance of such a well-learned test isdifficult to disrupt, and this is supported by the fact that the total number of missed,incorrect and delayed reactions was generally very low. Hancock et al. (1995) haveindicated that a key element in performance during heat stress is that the moreexperienced subjects are with the task they perform, the less likely they are to bedisturbed by thermal stress. It is also possible that subjects may be able to put extraefforts into maintaining adequate performance, particularly since they were aware ofthe duration of the experiments. Another possible explanation is that the higher levelof arousal and concentration required in this test might counteract the effect ofincreased body temperature. However, further studies will be needed to deal withthese questions more thoroughly.Even though there were no effects of ambient condition in the DG test, there was a

clear effect of time in some parameters. Fewer incorrect reactions and a slowerreaction time in Session 1 than in Sessions 2 and 3 might be explained by the initialarousal effect of entering the climatic chamber. Such arousal or stimulating effectsmay very well cause an initial improvement in performance (Ramsey 1995). Thehypothesis of an initial arousal effect is supported by that time-dependent changeswere only observed in the DG test performed immediately on entering the chamberand not in the Vigilance test which was performed 20 min later.Much of the literature is concerned with the effects of thermal stress on mental

performance in subjects not wearing protective clothing, and indices of heat stress inthe surroundings (air temperature, radiant temperature, and relative humidity andair velocity) are frequently used to estimate the heat stress experienced by workers.However, wearing protective clothing will lower the external limits for safe ambientconditions. For a fully suited worker with only the head exposed (relative surfacearea of the head is approximately 7%) as much as 93% of the body surface area willbe exposed to the suit microenvironment (Sullivan and Mekjavic 1992). This greatlyimpedes the exchange of moisture between the environment and the skin. Heat andmoisture accumulate inside the clothing and, Sullivan and Mekjavic (1992)emphasized, the microenvironment inside the clothing is therefore of moreimportance than ambient conditions only. Sullivan and Mekjavic (1992) comparedfour different protective garments for helicopter pilots. The important finding of thisstudy was that regardless of the heat load imposed, the true level of heat stress wasdependent on the evaporative properties of the clothing, as was demonstrated bydifferent increases in Tre for the four different protective garments (varying from0.28C to 1.28C). This further emphasizes the importance of the design andevaporative properties of the protective clothing worn. The double layer of cottonVentile fabric in the British Mark 10 used in the present study is intended to permitthe transmission of water vapour. When exposed to 08C the moisture wastransported outwards in the underclothing. This moisture transport is favourable,

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in that it lessens the thermal sensation of wetness of the skin (Bakkevig and Nielsen1994). However, the evaporative heat loss at 408C was not sufficient, resulting in100% saturation in the inner and middle clothing layers, heat accumulation andincreased physiological stress. It is not unlikely that a protective garment with betterevaporative properties would have alleviated the thermal stress and lowered Tre. Thisin turn could have had consequences for the cognitive performance, with fewerincorrect reactions in the Vigilance test. Detrimental effects of heat stress whenwearing protective clothing on flight performance have been demonstrated in theUH-60 helicopter simulator (Reardon et al. 1998a). Caldwell et al.(1997) showedthat aviators could not safely fly a single standard mission in the UH-60 helicoptersimulator wearing protective clothing at 418C, without some type of cockpit orindividual cooling garment.Most of the body water loss in the 408C ambient condition accumulated in the

clothing layers. The total mean weight loss of 1.2 kg represented 1.5% of thesubjects’ mean body weight, a level of dehydration that is associated with a reductionin physical work capacity (Saltin 1985). More important for the terms of this study, a1.2% level dehydration has been shown to decrease G tolerance and significantlyaffect pilot performance (Bollinger and Carwell 1975, Gillingham and Winter 1976,Nunneley and Stribey 1979, Nunneley et al. 1995). In contradiction to these results,this study found no association between the dehydration level and performanceeffects. A possible explanation for this could be differences in the study design andthe tasks to be performed.Even though it was not found that dehydration had an effect on performance,

the discomfort caused by sweating or increases in the subjective sensation ofwarmth might well induce performance decrements. If the performance measureswere a measure of comfort, a correlation between performance scores and ratingsof environmental comfort would be expected. Spearman’s correlation coefficientshowed no such association. Enander (1984) stated that the relationship betweensubjective sensation, comfort and performance is by no means clear, and there islittle reason to expect the optimum levels of any of these measures to coincide.This is in accordance with the findings of Griffiths and Boyce (1971), whospecifically studied the relationship between performance, thermal sensation andcomfort. They found that correlations between performance and subjectiveassessments were lower than between performance and temperature. Theyconclude that there is no evidence of a linear relationship between performanceand subjective assessment of thermal comfort. This finding suggests that eventhough the feeling of thermal discomfort and sensation of warmth during SeaKing helicopter flights is high (Færevik and Reinertsen 1998), this is not a criticalfactor for flight performance. However, the transfer of results from laboratorytesting of cognitive tasks during heat stress to ‘real life’ situations must not bedone without due consideration.Several authors have emphasized the difficulty of comparing studies on cognitive

performance because of differences in the tasks performed, prior training, severityand duration of exposures, and differences in the characteristics and motivation ofsubjects (Kobrick and Fine 1983, Ramsey 1995). Enander and Hygge (1990) claimedthat setting a standard for limitations to heat stress must be done on the basis ofindividual differences. The subjects used for this study were highly motivatedmedical students or pilots with 3 to 6 years of experience. This group may be anextremely select cohort that was better able than most to withstand the detrimental

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effects of heat stress. Still, the choice of test subjects is relevant to the actual workingsituation. Even though a high level of motivation may to some extent counteract thedetrimental effects of heat stress, performance motivation might well be reduced overseveral days of testing. Differences in levels of motivation between individual testsubjects on different days were counteracted by the randomized test design.Furthermore, all subjects performed the test at the same time of the day on eachoccasion in order to counteract any effect of the time of the day the tests wereperformed.This study demonstrated that performance deficits were related not only to

time and temperature conditions, but also to the type of task that subjectswere asked to perform. The Vigilance test turned out to be more vulnerable toheat than the DG test. This is consonant with other findings that havedemonstrated that certain cognitive tasks are more profoundly affected by hightemperature than others; so called ‘heat stress selectivity’ (Grether 1973,Hancock 1982). Performance decrement effects of heat on simple cognitive andperceptual motor tasks are rarely reported, even with rises in body temperature(Ramsey 1995). More complex tasks, particularly involving vigilance, haveproved to be more vulnerable to thermal stress (Hancock 1986). Vigilancetasks have traditionally been characterized as tedious; however investigationshas revealed that such tasks can be quite demanding and induce considerablestress on those who perform them (Hancock and Warm 1989). The Vigilancetest used in the present study required the subjects to monitor a display forthe appearance of a critical signal to initiate an important response. This lowactivity monitoring is much like the issue in the context of helicopter flying,where monotonous, low-level, sustained periods of attention are interspersedwith highly stimulating, high-activity periods. Vigilance tasks represent animportant class of functions in aviation and are especially important forhelicopter pilots (Hancock et al. 1995). Sea King helicopters pilots have searchand rescue operations that may last for up to 12 – 14 h, and this requirescontinued sustained attention. The decrement in Vigilance response couldpossible be more pronounced in the Sea King helicopter, where pilots face acombination of interacting factors such as vibration, noise, gravitation forces,uncomfortable seats and high temperatures.Although Vigilance has been shown to be of relevance to helicopter flying, the next

question is whether real-life ambient conditions are sufficient to cause the samephysiological heat stress demonstrated in the 408C ambient condition in thelaboratory. In a winter field study of helicopter pilots, Tre of the pilots fell by 0.38Cduring 115+ 11 min-long flights when wearing the same clothing as in the presentstudy at an ambient temperature of 18.6+ 1.38C (Færevik and Reinertsen 1998). Itis not likely that ambient temperature in the Sea King cockpit during winter flights issufficient to cause a 1.28C rise in Tre. The results of this study are therefore morerelevant to flights in warmer climates and seasons.This study suggests that tolerance limits for heat stress ought to take into account

the specific skills required in a given working situation, as well as the maximumchanges in physiological and psychological responses that can be regarded asharmless. Furthermore, this study suggests that an ambient condition of 238C whilewearing protective clothing does not cause any detrimental changes in psychologicalor physiological parameters and therefore could be used as a safety guideline forhelicopter flight operations.

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5. Conclusions

This study demonstrated clear effects of protective clothing during 3 h exposuresto ambient condition of 408C, on physiological parameters, as shown by increasesin rectal and skin temperature, heart rate and degree of dehydration. Thisphysiological heat stress caused decrements in vigilance, observed in terms of arise in incorrect reactions to the test stimulus. The dynamic change in coretemperature was the only physiological parameter that correlated with decreasedperformance, which supports the hypothesis that performance is largelyunaffected unless there is an increase in deep-body temperature. Furthermore,performance deficits were dependent on the complexity of the performance test,and the Vigilance test proved to be the most vulnerable to heat stress. The resultsare important in the context of flying, where sustained attention over longperiods of time is required, and suggest that a rise in core temperature as a resultof wearing protective clothing in hot ambient conditions might mean an increasedrisk of pilot error. The results of this study challenge designers and producers ofprotective equipment to put greater efforts into reducing thermal stress duringflight in order to prevent a rise in core temperature.

Acknowledgements

The authors wish to thank Vigdis By Kampenes for her help in statistical analyses.We also wish to thank the Royal Norwegian Airforce for their co-operation in thisproject and the Norwegian Research Council for the financial support.

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