Sweatirg in hot baths'''" [, trf ,fbJ:ieJ,lg: G,/ z - 1, s-t.

BRUCE A. HERTIG, MAR\IIN L. RIEDESEL,3 ANDHAR\\'OOD S. BELDING, Ig G i ,Department of OccuJtational Health, Graduate Sc/tool of Public Health,Llniu er sity of Pitt s b ur g h, Pi t t s b ur g h, P ennsy I u ani a

Honrrc, Bnucr, A., Me.nvrN L. Rreonsnr-, eNo H,qpwoooS. Brr-orNc. Sueating in hot baths. J Appl Physiol. r6(4):647-65r. 196r.-Sweating of human volunteers immersedto the neck in hot water declined markedly after reaching apeak in the Ist hr of exposure. This decline always occurred infresh water regardless of level of thermal stress. Sweating inthe 3rd hr of exposure was about the same whether the waterr'vas hot in the rst z hr (sweat glands active) or cool (sweatglands inactive). Thus "fatigue" was not responsible for thedecline. It is suggested that observations of decline of sweatingin rvarm-humid air environments, attributed to "sweat glandfatigue," in reality may have been owing to soaking of theskin with sweat. The mechanism of suppression appears morecomplex than blockage of the sweat ducts by swelling of thecorneum. Rather, there is an association between the amountof decline and conditions favoring diffusion of water to deeperstrata of thc skin. For example, adding salt tc the r.,'ater reducedthe decline; no decline occurred in 1516 NaCl.

T r,ttru quANTirATrvE TNFoRMATToN is available onsu'eat production in hot baths. Apparentlv sweatinq wasmeasured in onlv two studies where full body immersionsIasted an hour or longer. Whitehouse, Hancock, andHaldane (r) reported weight losses of about r,ooo g after

5 hr in fresh w'ater and about z,Boo g in rB% salt water(NaCl). Bazett (:) stated that loss ol water bv sweating"occurred as freelv in the bath as in a hot room," withrates reported as high as t.67" body wt/hr. The useless-ness of sweat produced under water for cooling the bodypresumablv accounts for the lack of attention to thisparameter in the numerous studies of thermal effects ofbaths.

We undertook bath studies because investigation of therelationship between sweating and skin temperature de-

I{eceived for publication 6 March r96I.1 This study was partially supported by Research Grant RG-4347

from the National Institutes of Health, and partially by a contractbetween the U. S. Army Medical Research and DevelopmentCommand and the University of Pittsburgh.

2 This paper is an abridgment of the thesis submitted by Bruce A.Hertig to the Faculty of the Graduate School of Public Health,Unir.ersity of Pittsburgh, in partial fuifillment of the requirementsfor the degree of Doctor of Science in Hygiene.

3 Present address: Dept. of Biology, University of New Mexico,Albuquerque, N. M.

Reprinted Jrorn Jounlrl on t\pplrun PslsroroclVol. 16, No. 1, ,luly, 19ii1

Printt:rtinL..S.A. Hefti-g, BfuCe 4., MafVln L.Sweating in hot baths.

Riedesel and llarwoodJ. App. Phrrsjol_. 16:

S" Belding. 1961.647*657

manded assessment of mean skin temperatures withgreater accuracy than is possible in an air environmentwith currently available techniques. Such accuracy canbe achieved in the bath; the surface temperature of animmersed bodv differs from that of the . water by anegligiblv smali amount, owing to the high thermalconductivitv of r'vell-stirred water (3).

It soon became apparent that thermal stress was notthe onlv factor influencing sweat rate. Further explora-tion of this finding led to the discoverv that immersion,per se, progressivelv inhibits sweating. 'Ihis paper de-scribes our observations and discusses mechanisms pos-siblv responsible for the decline.

METHODS

Healthv voung men were immersed to the neck in alarge tank located within a constant temperaturechamber. Recirculation of the water through a heat ex-changer provided regulation of the water temperature towithin o.o2 C. The water was stirred with a batterv-powered outboard motor. Several leveis of activit\' \vereachieved bv varying the "stroke" rate on a rowingmachine. Ambient air temperatures in the test chamberwere maintained at q5C D.B. and z4 C W.B., cor-responding to a water vapor pressure of r 7 mm Hg.

Body and water temperatures were measured withthermistor devices. Heart rate was follor,r'ed by palpationat the radial arterv. Metabolic heat production was esti-mated from oxygen consumprion.

Subjects were encouraged to drink r'vater equal to theamount lost b,v sweating. Although complete rvaterbalance couid not be maintained under ali conditions,dehvdration cannot account for the decreased sweatingobserved (see orscussroN).

Sweat rate was determined br'' observation of bodvrveight loss at intervals. Transfer of the subject from thewater to a scale placed beside the tank was controlled toallow for uniform run-off prior to weighing. A towel onthe scale caught the $.ater dripping from the subject.Weighings bv this procedure were reproducible to withinzo g and required less than a minute to complete.

To determine whether loss of weight bv sweating wasconfounded with uptake ol water from the bath, observa-tions r'r'ere made in neutral baths of 30 33 C. At these

6q'i

SUBJECT TMBATH: 36.7 "C

6+B

or23lrc. r. Time course of sweating in bath. Representative data for

3 levels of activity at intermediate bath temperature (:6.2 C).

temperatures the subjects were "comfortable" or "cool"and no active sr,r'eating u'as observed on the head. Wenoted a gain of about 30 g within the first 3o min, prob-ablr' representing storage in the stratural corneum. Nosubsequent uptake was detectable thereafter within theerror of measurement. These figures are in reasonableagreement with other fuii-bocir- immersion stuciies.Bazett's subjects gained 5o-roo g, all in the first 3o min(r). Whitehouse et al. (r) observed no more than 16 ggain, and up to 27 g loss. Data of Mali (4) and Buettner(5), when extrapolated to rvhole man, indicate that in-'nvard transfer bt'diffusion may occur at the rate of z-ro

HERTIG, RIEDESEL AND BELDING

g/hr. Inasmuch as estimated gains from the bath aboutequaled estimated losses from the lungs, observed weightIoss, corrected only for urine passed and water ingested,was accepted as sweating.

Time course of sweating was followed at three levelseach of bath temperature (36.o, 36.7, and 37.2 C) andactivitv (sitting at rest, and rowing at 6 and at 716strokes/min). Metabolic rates for these activities wereapproximately 5o, 95, and ro5 kcal/hr-m2, respectiveiY.The effects of acclimatization, hours of "presoakins" inthermally neutral water and salinity of the water werealso investigated.

Most of the data included in this report were collectedin 5i 3-hr exposures on 3 subjects. The same kinds of re-sponses were seen in work with five other volunteers inexposures at temperatures as high as 38 C and durationsup to 4 hr.

RESULTS

In fresh water, we found that s"veating reached a peakduring the r st hr of exposure, then declined markedlv(Fig. r). Peak rates were much less than we have seen atthese skin temperatures in air environments.

Temperature and actiaiQ. Sweating in response to heatstress is shown on semilog coordinates in Fig. s. Plotted inthis fashion, parallel straight lines mav be fitted to de-scribe the decline regardless of the level or modalitv ofthe stress. Total sweat output at 36.o C rvas substantiallvles-" before acclimatization, but it cieciineci simiiariv.

There were no consistent relationships between thetime course of sweating and the time course of otherphvsiologic parameters (Fig. :). Heart rate adjustedrapidly to a value dependent on the demands of heat andwork and then remained essentially constant throughout

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levels of bath temperature and activity.

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gland activitv, the subjects were immersed in thermallvneutral water for r or z hr before exposure to warm water.During presoaking thev rested; after presoaking thevexercised at a rate of 6 strokes/min in 36.7 C water.Sweat rates were about the same at the same point intime, regardless of the quantity of sweat which had beensecreted in the earlv part of the exposure (Fig. 4). Asdecline was independent of prior activity ol the sweatglands, fatigue could not have been a major factor in thedecline.

Salt water. Concentrations of 5 and roTo salt (NaCl)reduced the extent to which sweating declined; in t5Tathere was no decline at all. These curves extrapolate backto about the same value of sweat rate at "time zero,"as does the curve for ordinary water (Fig. 5). This ex-trapolated level of r,6oo g/[p corresponds with sweatrates observed in this and other laboratories for similarskin temperatures and levels of activitv, suggesting thatthe salt did not serve as a stimulus for sweating, butrather served to remove an inhibitorv influence.

DISCUSSION

This study has demonstrated that immersion in freshwater exerts an important influence on sweating in man.Leads suggested by the literature (e.g., sweat glandfatigue) were pursued, but none adequately explained thephenomenon. Those that seemed to offer the mostpromise are presented here in the form of hvpotheses:

r. Receptors for sweating adapt to the uniform ther-mal stimulus of the water. Temperature control in saltwater exposures was just as precise as in fresh, yet sweat-ing did not decline as much.

-2. Decline of sweating results from progressive loss ofbody water (6). The largest water deficit in this study(z% of body wt.) was recorded for subject -IS in r5 % saltwater. Sweating did not decline at all in this exposure.

3. Thermal conductance of the stratum corneum isincreased by hydration, thereby permitting the tempera-ture to decline at a site critical for sr,r'eating. Regardless ofthe amount of water absorbed, the conductivity could

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rrc. 4. Time course of sweating in bath (26.2 C,6 strokes/min)after o, I, and s hr ofpresoaking at rest in thermally neutral water

(TM and AW, g+.+ C; J,S, 33.9 C)soaking time.

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Time of exposure includes pre-

TIIIE OF EXPOSURE- HOURS

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rrc. 3. Data of z representative exposures illustrating decline ofsweating whether initial sweat rate was high or low, and whetherrectai temperature was rising or remained steady.

the exposure. Deep bodv temperature variouslv rosemoderately to a steadv level, rose continuously over theentire exposure, or fell to a steadv level.

Presoaking. To dissociate time of immersion and sweat

65o HERTIG. RIEDESEL AND BELDING

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nrc. 5. Time course of sweating in salt baths (NaCl) of variousconcentrations.

rrc. 6. Time course of sweating in hot-dry and warm-humidenvironments. (From Gerking and Robinson (rI); used by per-

mission. )

nrc. 7. Relationship between per cent decrease of sweating and

an arbitrary index of skin wetness. "Index of Wetness" : total

not exceed that of water, a maximum possible increaseof about twofoid. Because of the thinness of the hornvIayer, its thermal conductance is normallv verv large;doubling it would reduce the temperature gradient bvless than o. r C, hardiv suflicient a change to influencesr,r'eating so profoundlv.

4. Swelling of the stratum corneum mechanicallv-blocks the outpouring of sweat (7). This is the mostreadiiv- acceptable hypothesis, -vet there are several in-consistencies. Uptake of water b)' the corneum appar-entlv was completed before sweating had reached a

maximum. Postulation of an accelerated failure of theglands because of increased resistance to secretion failsto account for the reduced peaks after presoaking. Hadthe glands continued to secrete uniformlv and hadsrveat been prevented from reaching the surface, rve

should have seen miliaria; we observed none. Alterna-

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heat load minus evaporatir.e capacity of environment, Kcal/hr.Decrease in sweating is expressed as:

initial sweat rate - 6th hr sweat rate

iritiul .*..1 *1. x too

Calculations are based on data of Robinson, Turrell, andGerking (r o), and Gerking and Robinson (r r ).

tively, reabsorption from the ducts would have had tooccur much more rapidlt' than is norv thought possible(B)

To recapitulate, the evidence suggests that sweatingdeclined because the glands became progressivelv less

active; the decreased activity was not related to func-tional impairment of the glands attributable to excessive

activitv, and adding salt to the water reduced the de-cline. This last point is considered next.

Buettner (s) and others found that 2 r,r of varioussolutes prevented water from diffusing inward throughthe skin of hands and feet. Diffusion through skin of otherareas was prevented bv raising the concentration to 3 r,r.

A 3-u soiution of NaCl is 15% by weight, the amountwhich we found eliminated the decline of srveating. Thehigh resistance to diffusion imposed by the skin's barrierzone would be expected to delay diffusive equiiibrium

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SWEATING IN HOT BATHS

betu'een tissue that was between the barrier zone and thewater, in keeping with the observed semilogarithmic timecourse of decline.

How a few grams of water might so profoundly affectsweating we can onlv guess. Could storage of water in theprickle cell la1'er (cited because of its location immedi-ately behind the barrier zone and its small total mass)have rendered local receptors for thermal regulation lessresponsive? Reduced ionic concentrations of perfusingmedia have been shown by Diamond, Gray, and Inman(g) to decrease the amplitude and rate of response ofPacinian corpuscles.

We see a parallei in the decline of sweating in the bathand the declines observed in severely stressful air environ-mcnts where high humiditv restricted evaporation (e.g.,Fig. 6). Calculations based on data of Robinson's group(Io, rr) show an association between the percentage of

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65t

decline and skin wetncss (FiS. Z). Thaysen and Schrvartz(rz) observed "fatigue of the sweat glands" in environ-ments permitting evaporation of onlv roo-3co g,/hr ofthe 5oo Boo g/hr of swear produced. The decreasedsweating rvhich Ladell (r.t) attributed to rise of rectaltemperature above a critical level was recorded in ncarlysaturatcd environments. What he referred to as "quasi-fatigue" (lessened sweat rate for a standard task when q

hr of rest in the heat preceded the task) developed underconditions analogous to those of presoaking in this study.

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r 955.13. Leonu., W. S. S. 7-rans. Roy. ,\oc. 7'rop. Med. Hyg.5r: r8g,

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