International Journal of Industrial Ergonomics 25 (2000) 233}238

Climate under impermeable protective clothing

Phillip Bishop*, Dingliang Gu, Anthony Clapp

Department of Industrial Engineering, Human Performance Laboratory, University of Alabama, PO Box 870312,Tuscaloosa, AL 35487-0312, USA

Received 24 September 1998; received in revised form 24 November 1998; accepted 27 January 1999

Abstract

The actual microenvironmental wet bulb and dry bulb conditions were measured with a remote sensor underimpermeable encapsulating protective clothing (EPC) at two sites (shoulder and waist). Subjects (n"16) worked for30 min followed by 30 min rest up to 4 h at 350 W in three di!erent ambient environments, 18.23C, 22.33C and 26.93CWBGT. The mean ($S.D.) of the microenvironment for each of the three macro-environments was 27.9 ($1.6)3C, 30.3($1.6)3C, and 32.6 ($1.5)3C WBGT, respectively, yielding clothing adjustments of about 9.73C, 8.03C and 5.73CWBGT. Signi"cant di!erences (p(0.05) were seen between measurement sites, with the waist being slightly hotter thanthe shoulder. These data suggest that generally, adding 103C to the ambient WBGT re#ects the microenvironment forthis type EPC. For greater precision, the adjustment in WBGT is likely related to the actual ambient environment withthe microenvironmental di!erence decreasing with increases in ambient temperature.

Relevance to industry

An accurate estimation of environmental heat stress under protective clothing is needed. Presented is a practicalmethod for laboratory and "eld estimations of potential heat strain under protective clothing. ( 2000 Elsevier ScienceB.V. All rights reserved.

Keywords: Microenvironment; Ambient environment; Protective clothing; Clothing adjustment factors; Heat strain; Environmentalmeasurement

1. Introduction

When any material, such as encapsulating pro-tective clothing (EPC) covers the body, a microen-vironment is formed between the clothing and theskin. Generally, this microenvironment quickly be-comes warmer and more humid than ambient envir-onment because of the production of metabolic

*Corresponding author.E-mail address: pbishop@bamaed.ua.edu (P. Bishop)

heat and sweat by the worker. Consequently, anyheat stress to the worker is actually a result of theconditions of this microenvironment. In warm orhot (but not extremely hot) ambient environments,he total heat stress becomes higher in the microen-vironment than in the ambient environment. Themicroenvironment under impermeable EPC is es-pecially hot and humid when workers are workingat a high metabolic rate.

The American Conference of Governmental In-dustrial Hygienists, (ACGIH, 1987), in establishingthe threshold limit values for heat strain, also

0169-8141/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved.PII: S 0 1 6 9 - 8 1 4 1 ( 9 9 ) 0 0 0 1 3 - X

supplied an adjustment factor to account for theincreased stress provided by the clothing microen-vironment. The recommended adjustment factorhas varied over the years (Reneau and Bishop,1996), and the accuracy of the adjustment has beenevaluated in several manners (Kenney, 1987; Re-neau and Bishop, 1996a,b; O'Connor and Bernard,1999). However, direct assessement of the actualmicroenvironment under contemporary EPC hasnever been reported. The purpose of this study wasto assess the actual microenvironment. Temper-ature and humidity sensor were placed under theEPC while subjects worked in mild to hot environ-ments.

2. Methods

Sixteen healthy male subjects were recruited forparticipation in two tests in each of the three envir-onments. All subjects underwent all treatments ina repeated-measures design, counter-balanced forthe three ambient environments. This particularexperiment was part of a larger study of EPC.

2.1. Work conditions

First, subjects were familiarized with the testprotocol and signed voluntary informed consentforms. Then they were measured using indirectspirometric calorimetry to determine the speed andgrade of treadmill walking and the number of armcurl repetitions per minute needed to achievea time-weighted work rate of 350 W (300 Kcal/h).The work bouts consisted of 12 min (80% of totalwork cycle time) of walking at 141 m/s (3 mph) andat a grade to elicit a metabolic rate of 383 W(330 Kcal/h; oxygen uptake of 1.1 l/min). This wasfollowed by 3 min (20%) of arm curls at a curlrate that elicited a metabolic rate of 210 W(180 Kcal/h; 0.6 l/min). This work cycle was repeat-ed 4 times.

Subjects were tested on as many as six occasions(each of two types of under-clothing and threeenvironments) at least 48 h apart. On each occa-sion, subjects wore one of the two clothing en-sembles over underwear: (a) A 100% cotton T-shirt,100% cotton jeans, and cotton athletic socks, or (b)

A commercial undergarment. These two undergar-ments elicited very similar physiological responses.Subjects for all tests then donned a hooded Saranex23-P one-piece coverall with hood, glove liners andnitrile over-gloves. Subjects wore Tyvek shoecovers over their own shoes, and a full-face respir-atory protective mask.

2.2. Measurements

During work, the microenvironment wasmonitored by attaching a sensor (Davis WeatherMonitor II with an external Temperature/Humid-ity Sensor, product d7440, Davis InstrumentsCorp. 1995) to the subject's undergarment at theshoulder between the apex of the deltoid and theneck (12 di!erent occasions) or waist level about10 cm lateral from the umbilicus (48 di!erent occa-sions). This device uses a thin "lm capacitor ele-ment with a dialetric polymer layer which absorbswater molecules causing a change in capacitanceproportional to the vapor pressure and expressedas a relative humidity with a reported accuracy of$3%. The temperature was measured by a plati-num wire thermistor to $0.53C. The structure ofthe box prevented direct contact between the sen-sors and the skin or the EPC.

The shoulder and the waist were selected becausethey were believed to be representative of largeportions of the body and because they did notinterfere with subject movements. Remote readingsfrom this device were taken for each minute for the"rst 5 min, and at minutes 5, 10, 15, 20, and 25.Observations could not be made for every testsession because of equipment and support limita-tions. Simultaneous observations were also notpossible for the two measurement points. The ac-tual number of observations is shown in the Tables1 and 2.

Measurements were made under the EPC but ontop of the cotton work clothes or commercialundergarment because cotton work clothes are typ-ically the reference work clothing used in the AC-GIH clothing adjustment tables.

After 30 min of total work, subjects were seatedin the same environment as for work, but requiredto remove the outer PC coverall down to the levelof the ankles. Subjects recovered for 30 min in this

234 P. Bishop et al. / International Journal of Industrial Ergonomics 25 (2000) 233}238

Fig. 1. Mean WBGT di!erence between micro- and ambient environments for the 4 work cycles, in each of the three ambientenvironments.

ambient environment. This simulated a recoveryperiod in "eld conditions and allowed repeatedobservations of the microenvironment undersimulated "eld conditions. The 1996 ACGIH Thre-shold Limit Values indicate a 30-min : 30-minwork : rest paradigm for our metabolic rate at anambient WBGT of 29.43C unadjusted for clothing(ACGIH, 1997). No measures of microenvironmentwere made during rest. Total duration was 4 hinclusive of both work and rest. The three environ-ments tested were: WBGT"18.23C (¹

$3:"

21.63C, ¹8%5

"16.73C, or 70/633F); WBGT"22.33C(25.53C, 20.83C; or 77/703F); and WBGT"26.93C(30.93C, 25.23C; or 87/763F).

Descriptive statistics of all variables were cal-culated. The waist and shoulder readings werepooled for all mean values in the "gures. Meansfor microenvironmental WBGT were calculatedfrom the data at 5, 10, 15, 20, and 25 min of each

walk to avoid biasing the means. Repeated-measures ANOVA was used to compare microen-vironmental changes.

3. Results

The subject's mean age (#S.D.) was 24 ($2) yr,weight was 80 ($12) kg, and height was 179($7.3) cm. Fig. 1 is a record of the mean di!erencebetween the ambient and microenvironmentWBGTs for the 4 work cycles, in each of the threeenvironments. Table 1 shows the mean ($S.D.)temperatures for ambient and microenvironmentalWBGTs averaged across repeated walks at eachmeasurement site. Signi"cantly lower (p(0.05)values were seen for the shoulder compared to thewaist values, except for the "rst bouts at 22 and273C WBGT. The waist measure was hotter

P. Bishop et al. / International Journal of Industrial Ergonomics 25 (2000) 233}238 235

Fig. 2. Mean microenvironmental relative humidity across time (20 min) for all observations pooled, in each of the three ambientenvironments.

Table 1Mean ambient and microenvironmental WBGTs (3C) averagedover 25 min for repeated walks for each environment measuredat the waist (W) and shoulder (S). Number of total observations(obs) for each condition are shown

WBGT (3C) 18 22 27BoutObs (d) W S W S W S

35 102 70 15 63 306

1 28.7 25.5! 30.3 30.1 32.5 31.8(2.7) (3.5) (2.1) (2.5) (1.7) (2.1)

2 28.2 25.5! 31.8 29.7! 33.3 32.5!

(2.1) (1.9) (1.3) (1.5) (1.1) (12.1)3 28.5 26.7! 31.1 29.21! 33.3 32.5!

(1.2) (1.4) (1.1) (1.5) (1.1) (1.3)4 28.4 26.8! 30.9 30.0! 33.2 32.5!

(1.4) (1.5) (1.4) (1.5) (1.3) (1.4)Mean (S.D.)

28.5 26.1! 30.8 29.9! 33.1 32.3!

(2.0) (2.3) (1.6) (1.8) (1.4) (1.7)

!Indicates W and S means were signi"cantly di!erent(p(0.05).

(p(0.05) than the shoulder measure, with the dif-ference being as great as 33C, but commonly about23C di!erent declining to about 13C over successivebouts. The mean relative humidity (RH) tended tobe, but was not signi"cantly (p'0.05) lower at thecooler temperatures (Fig. 2) and only reached100% after several walks at the hottest temper-atures. Fig. 2 shows the early rapid rise in RHfollowed by a plateau during the course of a walk.

4. Discussion

4.1. The microenvironment

The microenvironment is the primary environ-ment that impacts the body. Humans alter themicroenvironment by adding or removing clothingwith di!erent properties. That is, the worker canexperience mild heat strain even in a very coldambient environment if he/she is producing highmetabolic heat and is wearing heavy insulativeclothing.

236 P. Bishop et al. / International Journal of Industrial Ergonomics 25 (2000) 233}238

The purpose of the present study was to measurethe microenvironment under EPC for subjects ata moderate work rate in three di!erent ambientenvironments. In general, it was observed that themicroenvironment under impermeable (coated)Tyvekt was almost 103C WBGT hotter than theambient environment, especially around an envir-onment of 183C WBGT. But this di!erencedeclined to about 63C as the ambient WBGTincreased to 273C.

4.2. Clothing adjustment factors

Our "ndings generally support Reneau andBishop's 1996 review which concluded that thephysiological response suggested that a 103C ad-justment in WBGT was appropriate for imperme-able EPC (Reneau and Bishop, 1996a). Likewise,Reneau and Bishop (1996b) observed neither prac-tial nor statistically signi"cant di!erences (p'0.1)between an EPC trial and a non-EPC trial at 103CWBGT hotter (non-EPC ambient WBGT of 303Cand metabolic rate of 400 Kcal/h) for heart rate,sweat rate, or change in rectal or mean skin temper-atures during the work or recovery phases. In thatstudy, however, the semi-permeable military EPCwith a much higher system clo of 2.55 was used incontrast to impermeable industrial EPC used inthe present study. The present methods o!ered theadvantage of more e$cient and precise determina-tion of the environmental adjustment across di!er-ing ambient environments.

Kenney (1987) also found that the additionalheat stress attributable to impermeable Tyvekt wasapproximately that of an ambient environment10.63C hotter. However, his method involved adesign in which the environment was steadily in-creased until subjects lost thermal equilibrium. It isremarkable that there is a general agreementamong these di!ferent studies, given the very di!er-ent research designs utilized.

The present study generally supports the pre-viously published clothing adjustment observa-tions for impermeable Tyvekt clothing. However,the present study suggests that the adjustment tothe ambient WBGT may be related to environ-mental temperature, with a substantial decline inthe adjustment factor at higher temperatures, as

can be calculated from the data in Table 1. Such anambient-dependent adjustment is consistent withbasic thermodynamics.

An ambient-environmentally-based clothing ad-justment, although more complex, has practicalimportance, because the TLV's for EPC are cal-culated from the adjusted ambient environmentand the metabolic rate. The adjusted ambientWBGT determines the appropriate work : rest ra-tio. An environmental adjustment of 63C comparedto 103C WBGT could result in increased projectedsafe work time and consequent increased produc-tivity. The 26.93C WBGT used as the hottest envir-onment in this study is routinely exceeded duringthe summer in the US.

It may appear initially counter-intuitive that thedi!erence between micro- and ambient environ-ments, and hence a clothing adjustment factor, de-creases with increasing ambient temperatures. It iscounter-intuitive if one thinks of a microenviron-mental clothing adjustment factor in isolation. Infact, any clothing adjustment factor must always beused in conjuction with the ambient conditions.That is, if the ambient temperature is su$cientlycold (say 03C WBGT), adding even a 203CWBGT clothing adjustment CAF will not predicta heat stress situation at any common human meta-bolic work rate. At the opposite extreme, there areambient conditions so hot that they will prohibitsafe work even if the clothing adjustment factor iszero.

If ACGIH TLV's are observed, many PC worksituations will result in 30-min work bouts. Ourdata is only applicable to these work bouts of about30 min duration. Inspection of Fig. 1 shows that thedi!erences between micro- and ambient environ-ments increase about 103C over 30 min. Our aver-age WBGT di!erences re#ect the mean data overa 30 min period. Thus, our micro-to-ambient di!er-ences underestimate for longer work duration inEPC, and should not be considered for longer workdurations. Longer duration studies are needed todetermine the micro-to-ambient environmental dif-ferences for longer work bouts. Most importantly,in "eld-work conditions, it is critical to recog-nize that workers probably seldom reach steadystate with regard to metabolism, or thermoregula-tion, or microenvironment.

P. Bishop et al. / International Journal of Industrial Ergonomics 25 (2000) 233}238 237

4.3. Microenvironmental variations

We were surprised at the magnitude of di!erencein temperature of the two measurements (waist andshoulder) since they were only 35 or 50 cm apart.We expected that the air inside the EPC wouldbe well mixed during our work simulation becausethe subjects were moving both the arms and legswhile walking, and the arms were moving in a dif-ferent motion during the arm curl portion. Thedata of the present study suggest that this mixingmay not be complete, resulting in di!erent temper-atures at di!erent sites.

There is a need then, for measurements at severalsites to precisely characterize the EPC microen-vironment. It should be noted that we were limitedto measurements at either the shoulder or the waist,hence simultaneous measurements were not pos-sible, although desired. However, even if our waist-level measurements were assumed to be too high,the variable nature of the microenvironment withrespect to the ambient environment still remains.

An additional relevant issue in determining theappropriate environmental adjustment for EPC isthe role of vapor pressure in determining the micro-environmental heat stress under EPC. Theoret-ically, under impermeable EPC, the vapor pressureof the microenvironment chie#y in#uences evapor-ation of sweat from the skin, relatively independentof the ambient vapor pressure and wind speed. Ourdata show rapidly rising humidity (vapor pressure)levels under EPC.

Logically then, the dry and globe temperaturesmay require much more weighting than in theconventional ambient WBGT. These observationssupport the work of Antunano and Nunneley(1992) who reported that 0.5 wet bulb and 0.5 drybulb temperature weightings, as opposed to theconventional WBGT calculation (0.7 * Wetbulb#0.2 * Globe#0.1 * Dry bulb), was a bet-ter predictor for heat strain in military EPC. Theexact role of ambient vapor pressure in determiningthe heat stress of industrial EPC remains to beexplained. This is a question worthy of carefulinquiry because accurate estimation of heat strainis crucial to maximizing safety and productivity.However, the change in microenvironmental vaporpressure across time is logical and suggests that the

heat stress under EPC does change over time, re-gardless of what clothing adjustments are used.

4.4. Summary

An adjustment of 103C to the ambient WBGT isa roughly accurate estimation of the microenviron-ment for impermeable EPC. It is most accuratewhen the ambient temperature is around 183CWBGT, and an over-adjustment at warmer tem-peratures. When the temperature is higher, weexpect that a smaller adjustment would be moreaccurate. The data also show a variable di!erentialbetween ambient WBGT and microenvironmentalconditions over time, which would be expected.A 103C addition to the ambient WBGT to estimatethe microenvironment under impermeableEPC may be insu$cient for work durations sub-stantially longer than 30 min. It is hypothesizedthat the di!erence in microenvironment acrossvarying ambient environments would be smallerfor permeable EPC, but this remains to be deter-mined. Additional research is needed to fully char-acterize the nature of the EPC microenvironmentwith di!erent ambient conditions, clothing typesand work durations.

References

Antunano, M.J., Nunneley, S.A., 1992. Heat stress in protectiveclothing: validation of a computer model and the heat-humidity index (HHI). Aviation Space and EnvironmentalMedicine 623, 1087}1092.

American Conference of Governmental Industrial Hygienists(ACGIH), 1997. Threshold limit values for chemical substan-ces and physical agents and biological exposure indices,ACGIH, Cincinnati, 1996.

American Conference of Governmental Industrial Hygienists(ACGIH), 1987. Threshold limit values and biological expo-sure indices for 1987}1988. ACGIH, Cincinnati, OH.

O'Connor, D.J., Bernard, T., 1999. Continuing the search forWBGT clothing adjustment factors. Applied Occupationaland Environmental Hygiene 14, 119}125.

Kenney, W.L., 1987. WBGT adjustments for protective clothing.American Industrial Hygiene Association Journal 48, 576}577.

Reneau, P., Bishop, P., 1996a. A review of the suggested WBGTadjustment for encapsulating protective clothing. AmericanIndustrial Hygiene Association Journal 57 (1), 58}61.

Reneau, P., Bishop, P., 1996b. Relating heat strain in CPC to theambient environment. Military Medicine 161 (4), 210}213.

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