task performance in heat: a review
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Task performance in heat: a reviewJERRY D. RAMSEY aa Texas Tech University, Box 42019, Lubbock, Texas, 79409-2019, USAPublished online: 27 Mar 2007.
To cite this article: JERRY D. RAMSEY (1995) Task performance in heat: a review, Ergonomics, 38:1, 154-165, DOI:10.1080/00140139508925092
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ERGONOMICS, 1995, VOL. 38, NO. I, 154-165
Task performance in heat: a review
JERRY D. RAMSEY
Texas Tech University, Box 42019, Lubbock, Texas 79409-2019, USA
Keywords: Perceptual motor performance; Hot work environments.
A wide array of variable conditions, tasks, subject populations, etc., have beenincluded in studies that have produced data on perceptual motor performance in theheat. This paper uses a methodology for comparing these studies, regardless of theinherent differences, which allows determination of whether thermal effects aredominant enough to persist through diverse combinations of variables. Approximately 160 individual studies of perceptual motor performance reported in theliterature were summarized based on thermal level, duration of exposure and thetype of task performed. Results indicated no dominanteffect of duration of exposureto the heat and no dominant effect of thermal level on mental/cognitive tasks. Forperceptual motor tasks other than very simple or mental tasks, an onset ofperformance decrement was noted in the 30--33°C WBGT range of temperature.This temperature level is consistent with the Recommended Exposure Limits forwork in the heat at low levels of metabolic heat.
1. IntroductionThe often contradictory findings of individual studies have led several authors tosummarize such research and attempt to generalize the findings. Wing (1965) was anearly and major contributor in this regard. Grether (1973) analysed over 50 studies,Ramsey and Morrissey (1978) reviewed over 100 studies and Kobrick and Fine (1983)summarized over 90 studies; each with the purpose of providing insight into the wayhumans perform tasks in different kinds of thermal environments. Other summaryreviews of human performance in the heat are found in the literature, but they alsoprovide minimal assistance toward general or standard interpretation of dominantrelationships (Pepler 1963, Poulton 1970, Griffiths 1975, Bell and Greene 1982,Ramsey 1983, Hancock 1984).
Upper limits of occupational exposure for unimpaired mental performance wereproposed by NIOSH (1972), but were not included in the revised version of this criteriadocument (NIOSH, 1986). These limits were not included because it was felt therelationship between performance and heat was not established well. enough to makesuch a presumption (NIOSH, 1980).
2. Variables affecting performance in the heatHuman performance is influenced by numerous psychological and physiological factorsand by differences in individual's response to specific environment and taskcombinations. Analysis of reported research results for performance in the heat wasbased in this paper on two types of variables: controlled and uncontrolled. Controlledvariables, where information from the studies could be quantified or categorized,include the thermal level, the exposure duration and the type of task. The uncontrolledvariables which were not controlled or identified in many of the reported studies, includethe level of acclimatization, degree of arousal, clothing worn, whether physical work
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is involved, skill and training levels, presence of other combined stressors, motivation,and numerous other individual, task and work environment factors. These factors havebeen shown to affect performance but tend to disguise the underlying thermalinfluences, rather than to explain them. There are, however, some basic relationshipswhich seem to be apparent, as discussed below.
2.1. AcclimatizationTask performance related to degree of acclimatization has been addressed by severalauthors, but is an area which is not well defined (Edholm 1963, Ramsey et al., 1975,Poulton 1976). Much of the reported literature does not specify whether subjects areacclimatized or unacclimatized. In general the acclimatized individual should be moretolerant and physiologically more able to work in a hot environment, and should showmore resistance to performance losses if the tasks involve motor responses; for simpleand mental tasks this difference is less likely to be observed.
2.2. ArousalSeveral studies have suggested that there is an optimal arousal level for any task beingperformed in the heat (Provins 1966, Poulton 1970, 1976, Wyon 1970), and it istypically lower for complex tasks than for simple tasks. Arousal can not only retardperformance losses, but has been shown to improve performance in many instances.Since temperature spans the continuum from cool, to comfortable, to hot, the idea ofan inverted U to describe performance is intuitively attractive. It is difficult togeneralize, however, due to the problems ofdetermining the level ofarousal at any givenpoint of exposure to the heat, as well as the interaction of arousal with the taskdifferences and other variables.
2.3. Skill/TrainingPerformance in the heat as a function of skill or training level of the worker and theskill requirements of the task has also been addressed (Mackworth 1961, Bell andProvins, 1962, 1966). In general, persons with high skill will have higher resistance toperformance loss unless they are already operating at a high perceptual motor loadwhich, with the addition of temperature, becomes an overload condition, and negativelyaffects performance.
2.4. ClothingIncreases in the amount of clothing affect insulative and permeability characteristicsand can interfere with a person's heat dissipation abilities. Tables have been developedfor estimating the equivalent thermal load based on different levels of clothing (Ramsey1993). Studies of perceptual motor performance in protective clothing, during and afterphysical work, have been reported by Robinson and Bishop (1988) and Ray et al.(1991). No significant decrements were reported for cognitive or vigilance tasks at thetemperature and workloads, but fine motor skills were impaired and also partiallyameliorated by cyclic cooling in a protective clothing garment.
2.5. Physical workStudies involving physical work effects on the performance of perceptual motor taskshave been reported (Macworth 1961, Benor and Shvartz 1971, Shvartz et al. 1976).Exposure to work and heat which will generate localized or general fatigue will
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normally have a negative effect on muscular based perceptual motor performance, butwill have less direct effect on tasks which are primarily cognitive.
2.6. Elevated core temperatureWork and thermal environments which elevate the body temperature have also beenshown to have adverse effects on task performance (Carpenter 1946, Pepler 1958,Mackworth 1961, Fox et al. 1963, Bell et al. 1964, Wilkinson et al. 1964, Poulton 1970,Nunneley et al. 1982, Hancock 1982). Although body temperature has not been reportedin most studies, there are means of estimating body temperature as a function ofenvironmental temperatures, exposure time and level of metabolic activity. Thisestimation ability allows a more comprehensive assessment of body temperature effectson task performance. Hancock and Vercruyssen (1988), after review of the relevantliterature and, based upon the concept of elevated deep body temperature, proposed azone of tolerance for unimpaired cognitive and neuromuscular performance. Thisrelationship between body temperature and task performance appears to be a dominantfactor for some types of tasks.
2.7. Combined stressorsReported studies of the combined environmental stressors of heat and noise have beenreported (Dean and McGlothlen 1965, Poulton and Edwards 1974, Bell 1978). Someinstances show the combined effects of the two stressors to be smaller and some reportthem to be additive; further, these differences are seen in some tasks and not in othertasks. High altitude and heat are commonly investigated areas (Lahiri et al. 1976, Fineand Kobrick 1978) and other combined stressors have also been reported (Loeb andJeantheau 1958, Poulton and Edwards 1974). Performance results with combinedstressors are highly variable and appear to be a direct function of the stressor levels andthe specificity of the conditions utilized in the study.
2.8. ComfortComfort level is a function of numerous variables, but for the sedentary, normallyclothed worker (0·5-0·7 c10) with low air movement (less than 0·3 m1swc), the comfortrange is approximately 23-27°C ET (21-26°C WBGT) (ASHRAE 1977, Fanger 1970).Comfort for a person doing work slightly above the sedentary level would likely beI-2°C lower. Numerous studies of task performance near the range of comfort havebeen conducted (Wyon 1970, Griffiths and Boyce 1971, Wyon et al. 1975, 1979, Reddyand Ramsey 1976, Hafez and Beshir 1987) and performance decrements havesometimes been noted; if so, the lower temperatures are most often associated with bestperformance.
3. MethodologyThe approach used in this paper was to summarize and evaluate performance in the heatbased on the controlled factors of thermal characteristics, exposure time and thegroupings of task type. A more detailed description of methods used to develop meansof predicting performance loss in the heat has been reported to the Commission of theEuropean Communities (Ramsey and Kwon 1988).
3.1. Thermal characteristicsA complete definition of thermal characteristics is important in the interpretation ofresearch about temperature/performance effects. However. many reported studies
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either do not specify the precise levels of humidity, air movement and/or radiant heat,or they define the thermal environment using a variety of thermal units and indices, e.g.,effective temperature, air temperature and relative humidity, and air temperaturewithout relative humidity. A common basis for interpretation of results was developedin this study by interpolating, transferring or estimating the thermal characteristics interms of a single set of units. In this analysis, all data concerning the thermalenvironment were converted into an approximate or calculated Wet Bulb GlobeTemperature (WBGT) in order to'compare in a uniform manner the different types ofrecorded temperatures. The thermal characteristics were either extracted directly fromthe recorded studies or were estimated, using generally accepted procedures (Ramseyand Kwon 1992).
Much recent research on performance in the heat (Ramsey and Morrissey 1978) andmost current recommendations concerning limiting heat stress conditions for workersin the heat (NIOSH 1986, ISO 1989, ACGIH 1992) are expressed in WBGT units, sothis index was selected. Unless specifically noted, it was assumed that the reportedresearch was conducted indoors with no appreciable radiation or additional airmovement. For conversion purposes, this then meant the air and globe temperatureswere approximately equal, as were the natural and psychometric wet bulb temperatures.
3.2. Exposure timeThe duration of exposure is intuitively an important factor for evaluating or specifyinglimits to performance in the heat. Although time in the heat is not always reported, thispaper identified and interpreted those studies where performance data were availablefor specific exposure times and temperature combinations.
3.3. TasksThe type of task used in a study is always reported and represents a major variableinfluencing performance. Typical task categories include tracking, reaction time,vigilance, eye-hand coordination, mental, psychomotor, sensory, time estimation,monitoring, cognitive, complex and dual tasks (Grether 1973, Ramsey and Morrisey1978, Kobrick and Fine 1983). Some ofthe apparent contradiction observed in researchresults can be removed or explained by task categorization.
4. ResultsTasks in this paper have been grouped for purposes of evaluation into only twocategories:
(I) mental, cognitive, very simple perceptual motor, sensory, time estimation,reaction time, etc.;
(2) other perceptual motor tasks, including tracking, vigilance, vehicle or machineoperation, complex or dual tasks, etc.
These groupings were selected based on results from earlier review papers whichindicate distinct performance difference between these two general groupings (Ramseyand Morrissey 1978). Procedurally, studies from the literature which providedperformance results at different combinations of temperatures and exposure time weregraphically displayed for each of these two task categories, as shown in figure I andfigure 2. Task performance as a function of temperature and time is shown in figure Ifor mental or simple tasks and in figure 2 for other perceptual motor tasks. Each pointon these figures represents a reported combination of temperature and exposure time,
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along with an indication of statistically significant performance decrement orenhancement at that point. Those instances where the author reported decrement wasnot statistically significant, but was partial, slight, and/or otherwise apparent are alsoincluded in an attempt to provide additional insight into the performance/heatrelationship. As an example for interpreting data in figure I, the study at 160 minutesexposure time reported performance on a visual acuity task during three temperatures(22°, 32°, 34°C WHGT) (Hohnsbein et al. 1984). This study indicated significantdecrement at the two higher temperatures when compared to performance scores at thelower temperature.
4.1. Mental or simple tasksFigure I summarizes a number of individual studies involving mental, reaction timeand other simple tasks. A few studies indicate significant performance decrement atelevated temperatures and short exposure times for reaction time tasks (Frazer 1955,Bursill 1958). However, most studies of reaction times to visual or auditory stimuli have
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Task performance in heat 159
shown little or no effect and some have even shown high temperature levels to improveperformance on these types of tests (Pepler 1959, Grether et al. 1971, Azer et al. 1972,Chiles et al. 1972, Poulton and Edwards 1974, Ramsey et al. 1975, Poulton 1976).
Performance in the heat for a variety of mental tasks such as arithmetic, coding,multiplication, writing, and short term memory tasks has been reported by variousauthors to show minimal performance losses due to heat (Chiles 1958, Fine et al. 1960,Poulton and Kerslake 1965, Grether et al. 1971, Chiles et al. 1972, Ramsey et al. 1975,Ramsey and Pai 1975, Nunneley et al. 1978, Messe et al. 1981, Lewis et al. 1983). Afew studies report significant performance decrement to occur at elevated temperaturesand longer exposure times (Wing and Touchstone 1965, Givoni and Rim 1962,Hohnsbein et al. 1984).
Figure I, which includes a large collection of research results for mental tasks andsimple perceptual motor tasks in hot environments, indicates that a loss in performanceis commonly not observed with these types of tasks. Further, an enhancement ofperformance is frequently observed for mental tasks in the heat during brief heatexposures.
4.2. Other perceptual motor tasksFigure 2 summarizes a large number of individual studies involving perceptual motortasks, such as tracking, vigilance, operating vehicles or machines and other complextasks. Results from several tracking studies which are included in figure 2 show adefinite loss of performance which tends to begin in the 30°-33°C WBFT range(Mackworth 1950, Teichner and Wehrkamp 1954, Bursill 1958, Pepler 1958, 1959,1960, Grether et al. 1971, Azer et al. 1972, Chiles et al. 1972, Poulton et al. 1974,Ramsey and Pai 1975, Ramsey et al. 1975, Lewis et al. 1983). These decrements havebeen observed even in short exposure times of less than thirty minutes.
Performance in the heat during vigilance tasks has been reported by Pepler (1958),Bell et at. (1964), Colquhoun (1969), Grether (1973), Mortagy and Ramsey (1973), andPoulton and Edwards (1974). These data depict performance decrements similar tothose noted for tracking. Complex or dual tasks are more difficult and demandingperceptual motor skills and thus come closest to representing typical industrial andmilitary work tasks (Mackworth 1950, Bursill 1958, Provins and Bell 1970, Gretheret al. 1971, Azer et al. 1972, Chiles et al. 1972, Poulton et al. 1974, Mackie andO'Hanlon 1976, Fine and Kobrick 1978). These data, as shown in figure 2, also depictan onset of performance decrement in the same 30°-33°C WBGT, as noted for the otherperceptual motor tasks.
4.3. Physiological limits for work in the heatThe proposals for Recommended Exposure Limits (REL) and Recommended AlertLimits (RAL) for workers exposed to a combination of environmental heat (WBGT)and metabolic heat from the work activity being performed are shown in figure 3(NIOSH 1986). These limiting curves represent one-hour, time-weighted averages forboth environmental heat and metabolic heat exposures. The REL is consistent withlimits specified by the International Organization for Standardization (ISO 1989) andearlier work by Lind (1963). The REL curve is based on the premise that a standardworker who is acclimatized, healthy, and normally clothed will have a deep bodytemperature no greater than 38°C. The RAL curve also relates to a standard worker whois healthy, normally clothed, and limited to 38°C deep body temperature, but who isunacclimatized.
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200 300 400600 1200 1600233 349 465
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Figure 3. Recommended ExposureLimits (REL) and Recommended Alert Limits (RAL) forworkers in hot environments. Adapted from: NIOSH (1986).
Most perceptual motor tasks, whether seated, standing or involving some hand/armmovements, are performed at a sedentary or very light work metabolic heat level, i.e.,approximately 116-168 W/l1O-145 Kca\. The points A, Band C indicated on figure 3represent the environmental heat in WBGT at which the onset of physiological heatstrain will occur for workers engaged in sedentary or very light work. These WBGTtemperatures of 30°-33° WBGT also are consistent with the levels where onset ofperformance decrement occurs in a large number of individual perceptual motor studies,as shown at points A, Band C in figure 2.
5, ConclusionsThe literature relating hot environments to perceptual motor performance has notpresented clear findings and, indeed, has often been contradictory. This has been duein large part to the specific array of factors, controlled or not controlled, which arecharacteristics of an individual study.
This paper summarizes in a graphical format a large number of individual studieswhere information is available concerning exposure time, temperature and type of tasks,so that the results can be reviewed as a group or array. Individual study differencesrelating to the levels of acclimatization, skill, clothing, motivation and other previouslymentioned factors are not differentially categorized, and this may result in an individualdata point presenting a finding which appears to be inconsistent with the other studies.It is likely, however, that each of the reported studies has some level of uniqueness andit is only in the composite display of research results that the dominant factors oftemperature differences would be apparent.
Performance decrements during work in the heat for mental or simple tasks as shownin figure I are distinctly different from those shown in figure 2 for perceptual motortasks. This suggests that categorization of task type should be a first step in anyassessment of heat effects on task performance. Mental or simple tasks would most
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likely have minimal or no performance loss in the heat and, for brief exposures, mayeven have enhanced levels of performance.
Tasks requiring perceptual motor skills other than strictly mental or simple taskperformance most often appear to show the onset of a statistically significant decrementto occur in the 30°-33°C WBGT range. Extensive data as reported in the literature andshown in figure 2 indicates there are consistent losses above and infrequent losses belowthese levels. This relationship persists, regardless ofthe exposure time including shorterexposures (less than thirty minutes) and longer exposures (4-8 hours). The longerexposures typically include the effects of some rest or meal breaks in the schedule, ratherthan representing continuous heat exposure and work activity. The short exposuresshow many instances of performance loss, although it is unlikely that deep bodytemperatures are a major influence on these losses. At a sedentary work task and heatexposures less than 30 minutes, changes in deep body temperature would be expectedto be minimal for most workers.
A summary of the data for perceptual motortasks, otherthan those which are strictlymental or simple, as shown in figure 2, depicts the range of WBGT where the onsetof performance decrement is most likely to occur. The three lines, A, B, and C, in the30°-33°C WBGT range are consistent with the recommended heat stress limits (NIOSH1986, ISO 1989) and with the concept of the prescriptive zone (Lind 1963). The topline, A, represents the acclimatized standard worker performing sedentary perceptualmotor tasks. The lower line, C, represents the unacclimatized standard workerperforming perceptual motor tasks at a very light metabolic heat rate. The middle line,B, represents both the acclimatized standard worker at very light tasks and theunacclimatized standard worker at sedentary tasks. The levels of environmental heatwhich create onset of physiological heat stress risk for the worker performing sedentaryor very light work tasks are also the same levels where perceptual motor performancewill deteriorate for the all but the strictly mental or simple tasks.
Although a general relationship between heat and performance has been presented,some qualifications should be noted. Individual situations may not follow these generalrules since there are many other variables or factors which can affect task performanceand can easily dominate the effects of heat. Further reports of statistical significancerelate to the statistics of a study but do not necessarily relate to a practical or meaningfuldifference which would affect the overall level of productivity or safety in a particularwork situation.
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