recent developments in understanding of effects of exposi.]re to heat

7/29/2019 RECENT DEVELOPMENTS IN UNDERSTANDING OF EFFECTS OF EXPOSI.]RE TO HEAT

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This paper will be resr.ricted to a brief preliminaryof certain findings of our heat research groupUniversity of Pittsburgh. \!'e shall first presenrchart which we have developed for teaching purposesshow the strains and overstrains o[ exposure to heat.we shall deal with two activities rrhich arerelated to industrial heat exposures: one anwhich relates accidents in a steel mill with thethe other a study of events of recoverv fromin which body tenperature has been ele-And, finally, rve shall look at two findingsare of more fundarnental interest, includingwhich has hclped us better to understand theof acclirnatization to hear.This uork has Ind lon{-terrn support und.er GrantftG 13-17 fron the National hrlittltrs of Hcalthunder Coiltract I'r'. D.,l--/q-l,tr;--\lI).q{S titrr t/rcU.S. Arnty trIedical Ilistarch and Du,clofnttn! Colt-and.

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Flow Chart Indicating Strains and Orerelrains ofExposure to Heat.{ chart rshich we have developed to show t}reinterrclationships and interdependence of ph,vsiologiccYcnts resulting from exposure to heat appears asFigure l. \fe hare found this a useful device fordescribing the courscs of breakdorsn o[ the mech-anisms of adjustmenr to heat and the tiistinction be-t$een heat erhaustion anti hcat stroke for studentsanrl others not fullv acquainted with the phvsiologv ofhcat regulation. I he solid lines depict the adjusr.mcnts b! rrhich adcquate thcrmal hommsrasis is main-tainerl. Broken linis show the paths of phrsiologicbreakdown. A surlnrarr of the events leading to theseinjtrries. acconpanietl the figure and appean as an-{ppendir to this paper.S eather and Aecidents.{ studr of accitltrrts in relation to $'eather resulted

BELDING et al.

RECENTT DEVBLOPMENTS IN TJNDBRSTAI|IDING OFEFFECTS OF EXPOSI.]RE TO HEAT*H. S. BBr-uNG, T. F. H.lrcrr, B. A. Hsnrrc, and M. L. Rrrnnspr,Deparf,menr of Occuparional Health, Graduare School of public Eeal*LUniversity of Piruburgh, pinebugh, penncylvania


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840 woRK pnysror.ocy AND psycHor.ocyHEAT STRSSEATS 8EAT LOSSES 8Y RADIATIONGRATER HEAT LOSS MO CONVECTION ARE LESS THAil(OR LESS GAINI 8Y P8ODUCTION OF METASOLISII8A!j-{ICN AftD CONVECTToN

IFAILUR OF OEilTNAtcostRoL Hcruxi3x(gwgaTrlra 9gasEslttNAFD RISE IfCORE TEHPERAIUREllg sr8-or:frcunE I

from the observation of orrr colleague, Dr. A. G. Kam-mtr, tirat his medical dispensary in a large steel plantnear Chicago was considerably busier in the hotsummer than in the winter. He was able to obtaingood records of acciderits iir this plant for the fourimmediate post rvar years- This was a period of ful!cmployment. ,\n accident, as defned at this Plant,ws. any incident for which the plant might be liablethat involved a visit io the plant dispensary,Analysis indiated, as predicted . by Dr. Kammer,that the accident rate was intleed higher i:, the sum'rner, about twice as high as in winter, and that therates for fall antl spring were intermediate (Figure 2).The difference tretween seasons persisted despite asafety campaign which progressively reduced the over'all accident rate during the four years. l{e were in-terested in the thermal relationship, if any, and car-ried out a further analysis to re whether the hotterdepartments of the plant had a disproportionateshare of lhe accidents in the summer. It was notpossible to show any such relaiicnship" This did notrule out the possibility thal an analysis by hotness ofindividual jobs would rcveal a significant correlationby seasons (such an analysis was not feasible), hut itdid suggest that some factor beyond the plant itseltmight be oprrative. It has been suggestetl on the basisof studies l>y Ellsworth Huntington (i) and of com-rnon expcricncc tliat the general tonus of bodily acti-vity and alertness is related to environmen{al

.*,n rr*r*orr", *'a.aI {NeRvous nEcEPToRs FoRI .,-"'.roo'n *':-'^:."t"'| / \ -1:suilEvaPos^rED swEAr,IrilcREAsEc HEA' FL.H tliY t:::.T3\ / - usLrss FoR cooLrNGFnou sooY coRE F otrEils7 DE'E{'\ -/rr:r Rrsg tN sxtil '\- I \ -/'RPERATUaE \AUGMENTE0 sKrN | //,t".i13H,itli3,}, -llEArr{\q-salr,nraK-*-,3f;J,T,t",RoLoxGEo ExPosuRedaa I ,f | \ \\* \ ilausEArilcReAsEsBLo00voLUilE:gV\\-\B\

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IqLoIGEO EXPOSUREE-,/ I / : \ \ \ \cReAsEsBLo00voLUilEigV\\-\B\(AGCLIHATIZAiIONI V "PRIGKLY FATIGUE oF 0FAIN OrI.\ DRAIN oN FRtghLr aattcuL uF unArN oil\ oRAtfl oNtilaDEeu4rE vENous Hear' swEAT cLAnDs eoov wltgn \eoov saLt \-RETURilToHEAnT I / \ \

FLO', tO VITAL AREAS oIRCULATIoN- gltslfllSHEO TNADEOUATEI o\ SWEATI$O sTlMuLUs)l\:/Y \+/clculltoRY sHocx RtsE tx 800Y(RAPlOr wEAx PULSE; reuesurune ] =ow BLooo PREssufie! I \. \eoRE TEIFERATUnE * 'q TNCREASEDiSAY 8E XORXAL METABOLISMHEAT Erllausrlor,r i#ii.n#kFRFORHAIIgEI

REruRilroHEAnr I / \ \ \HEAT,r' \- | F \ -x"o cRIMtS,ro' "\ I DEHYuRATToN ,J".rrl'nion.IIiA0SOUI?E BLooO IilADEoUATE SKIN P g {oFTEN Ail

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temperature and humidity and that these are at a]ower ebb in hot weather. Can this explain the results?The data are inadequate to permit any broad conclu-sion, but indicate directions for future investigation.These might involve a study of seasonal variationsin accidents in other plants, where process heat isor is not a factor, and comparison of the accidentexperiences of plants in localities where seasonalfluctuations in weather are less severe.Recovery from Exposure to Heal

J\Iany hot jobs in intlustry require intemittentexposure to hrgh levels of heat. In the past studiesof c{fects of exposurc have largely been confined toobservations of physiologic responses dzring exposure,Whcn exposures arc intermitlent the process of r-couery is of interest, for iwo reasons: (a) if recoveryIrerween exposurcs is not complete, re-exposute wiilrcsrilt in cumtrlative effccts; and (b) time spent in re-covery is usually iost tirne from the point of view of!ilanagemcnt, and for operating cfficiency it is desir-able to minimire such Iime.Wc have had the practical objective of learninglrow hot jobs may be arranged to achieve efficiencyof personnel without undue risk of injury to health.Obviously, the drlration of rccovery as well as ofexposure are irnportant factors. Other factors whichwill affect the physiologic responses of the exposure-

/:.5 UilEVAPOAITED SWEAT,- usLtss Fd cooltilG


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cycle are intensity of environmettal heat andduring exposure and themal environment anddrtring recovery. The numbcr of variablesmade it impossible to arrive at any general form..f9r .arrangemenr of these jobs, dlspite manyobservations under iaboratory- and fiekiNevertheless, we have learned quite a lotihe subject. Dr. I-ucien Brouha (Z) ted ttreimmediatcly after the war bv demonstraring tharconditioning of reccvery areas can reduce cum-fatigue from intennittent exposures. We haveand extended this work and studied theof some of the other variables.In these studles rve have been most impressed by thelength of time required to restore rectal tem-to pre-exposrlre levels after it has been ele-in many cases return to base line values hadoccurred at the end of an hour. We have alsosurprised by the promptness of return of heartand sweating nearly to pre-exposure levels. Weone example which illustrates some of these(Figure.3). Here rectal temperaiure rose lo Ca 30 minute period in the heat, 38.3" C. Atrerminutes of rest in a thermallv neutral environmenttemperature had fallen only 0.1., after 30

841

minuts only 0.3", and after 60 minutes it rvas still0.4" C above the inidal temperaiure. Skin tempen-ture, sweating, and heaft nte had retumed close topre-exposure levels within a few minutes. Ii appearsthat these thrft items reprffnt inadquate critedlof recovery in that bas line levels are reached beforetrody core tempemture has returned to pre-exposurelevel. In fact, when men rvere rc-exposed iromediatelyafter heart rate rtrovery, additional heat was addedto that airead)' stored, with resulting added srrainand, in certain cases, circulatory collapse.Body Core Temperature nnd Sweating

None of us in this field are fully *tisfied e'irhreclal temperature as a measute of core temperature.Benzinger has recentlv reconmended use of thetemperature at the external surface of the tympanicmembrane as an indicator of temperature of the hy-pothalamus 13). and in our most rcren( studies wehat'e adopted this addirional measure. 1\-e find eartemperature to be more responsive to change inenvironmental condition or activitv, more labile, thanrectal temperature. Results of an exposure iri whichear, rectal, and mouth temperatures were followed

BELDING ' 4'.

Weolher ond Accident Frequencyo SteelMc Mohon ond Belding,

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a947 p48FIGUBE 2

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84? WORK PHYSIOLOGY AND PSYCHOLOGY

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appear in Figure 4. These are typial in that mouthand ear temperatures rise faster during exposure andfall faster afterward than rectal. This suggests thatrectal temperature may not reflect the over-all thermalstate of the body core. In fact, postexposure rise inrectal temperature which we have often noted (cf.Figure 3) and its slow remission suggest that duringand following exposure to heat the rectal wall isessentially deprived of blood flow, and that changein its temperature is primarily dependent on therelatively slow process of conduction from or to sur-rounding tissues.Note that sweating, as measuted by change in bodyweight, falls almost to pre-exposure level within afew minutes after exposure, in coincidence with anabrupt fall in skin temperature. This occurs longbefore corc temperature has returned to base lineralucs. I)r. Benzinger will find this observation inronflict with his tr,vpothesis that the level of hy-poth-alamir-: tcnrpcrature dctcrmincs sweat rate.

AcclimatizationThe mechanism of acclimatization remains obscure,but the dramatic results of acclimatizaaion to heatare well known. These are decrease in circulatorystrain, as evidenced by reduced heart rate: lessenedelevation of body core temperature; lowered skintemperature; and increased tolerance. Some investi-gators have reported a slight increase in sweat ratewith acclimatization; others little change.One of us (Hatch) has recently dcmonstrated howmuch lower the threshold skin temperature for sweat-ing was among a group of young Europcan men ac-climatized by.living in Singapore than it was for asimilar group living in Oxford, Engiand (4); actually,the threshold skin temprature for onset of sweatingwas al;out 1.4' C lower for the acclimatized subiectsat Singapore (ligure 5). This is indicated by the zerointercepts of tlre trend lines which show sweat re-sponscs of unacclimatized and acclimatized subjects,

EXPOSU R E5r"C Vp25m

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BELDING E' d/,

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adjusted for activity level. There is considerable scat-ter of data on individuals, but little overlap, suggest-ing the validity of the difference. The regression linesdrawn to show the average responses of the twogroups are 500 or more grams of sweat apart forthe same skin temperature, which means that ifsufficiently more diffrcult conditions of exposure areset for the acclimatized.man so that he exhibits thesame skin temperature as the unacclimatized, he canbe expected to sweat at least 0.5 I per hour more thanthe unacclimatized.Actually, he sweats at about the same rate* as theunacclimatized man when exposure conditions are thesame and when evaporation occurs freely. His skintemperature is 1.4' C lower, because he is enabledrThe requirement lor sueat (E eq) b ntaintain heatbalance in a given situalion is represented by thelamiliar: E..q=,trI+R+CTlte obsened, IotL,er skin tenperature in the accli-matizetl nan u,oultl increase only R anel C of thisrelalionship, arul undtr tlrc usrual .;t(untstencts t(, theextent ol. about )0 Calories/ltortr, refresc'ntcrl }r a50-g per hour irrentse il su,rclirrc. T/ris rr',,uli rurrr.r,'heleuale E"t b1'oth I or -'I,r'r.('rrl.

to sa'eat the required amount at a lower skin lemper-ature.The gain with acclimatiation is a substantiallylarger iempemture gradient between core and skin,which permits transfer of the metabolic heat with lesscirculatory strain. Some idea of the magnitude of theefiective change ma,v be gained from a umple situa-tion in rvhich core temperature is assumed to be 38" Cand the caloric equivalenr of the swear (S-ll) about200 K Cal per hour- Under these circumstancs, skintemperature of the average acclimatized men.at Sing-apore was about 3{.4' C; at Oxford it H'as 36-7. C.In the first instance the gradient for transfer throughthe skin was 3.6" C; in the recond, 1.3. C. Therelative demand for circulation to the skin was obvi-ously verv much greater for the unacclimatized-atleast three times as great. Experience indicates thatsuch a demand cannot be met. The result is a rise incore temperature and/or collapse.The atrove statement is applicable only under cir-cuurstances in uhich evaporation is free. \l'hen evap-oraiion is limitcd bv aurbient rapor pressureJ tireskin cquilibrates at a lrigher tempcrature lerel. onervhich nrav tre largelr intlrpt'ntlcnt o{ state of accli-nrllizlrion. In this crre rre rsoultl r\pect the accli-

TIME-COURSE OF EOOY TEXPERATURES AI{D SYEATIXDURlilc A WARtlt{G al{D cooLtr{G cycLEErP. (-5. t96O SUSJECI ilUDE, SI?TflG

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844 WORK PHYSIOLOGY AND PSYCHOLOGY

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natized man to sweat considerably more than theihe unacclimatized, perhaps 0.5 i per hour mote, aspredicted b,v the Oxford-Singapore analysis.This approach yields a better understanding ofthe gain to the human thermal eccrtomy resultingfrom rhe lowered thrshold skin temperature forsweating. The shift in thresholcl appears to explainthe advantage of acclimatization in dry heat, and ithas focused our attention on the mechanism by whichthe shift is achieved. More effective circuiation of bioodto the superficial layers of skin must be the funda-mental change. As a result of more effective circula-tion, the temperature of skin receptors involved intriggering the sweating mechanisms could be higherfor any given set of values Iot T''ote and I'rr", Thus,

III

34 36MEAT{ SKIN TE MPERATURE

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Frcunr 5--4ooling potential of sweat (with allowance for metabolism) in relation to skin te_mperature:Thirty-two uaacclimaiized men from Oxfcrd 1o-, ) and an equal number of acclimatized men_from Sin-ga-pore'(4 - *).Larye o and + designate data obtained under identical conditions. Small + and 'indicaiei another vaiue cn same individuai obtained with different vctiyity or environmenta-l condition.without any basic shift in tire threshold for responseof the receptors, we coukl account for more sweatingat a given skin temperature. The gain of acclimatiza-tion rn wet heat, v/hen evaporation is limiting, maybc largely ascribed to the increased circulatory efficien-cy per se in bringing the metabolic heat to the surfaceof the skin. The lowcred threshold for sweating canhave little efiect in this instance.Conclusions

These are a few highlights of our studies over theIast several years. Some are no more than stlggestive.Our goal is to continue at work, which may con'tribute to solutions of practical problems in industry.


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also wish to acquire betrer understanding ofof the more fundamental aspects of thermalFulfillment of the first objective will involveof a better index for predicting theof hot jobs. Fulfillment of the second obiectiverequirc a more careful asse$ment of circulationhe skin.REFERBNCES

untington, Ellsworth: Ciuilization and Clinatc.University Press, New Haven, 1913.rouha, L. A.: Protecting the wo.ker in hot en-Trans. 20th Annual Meeting Indust.H1,g. F ound.ation, 1955.enzinger, T. H.: On physicai heal regulation andsens of tenperature in man. Proc, Nat. Acad,.{J:645, 1959.on supplementary data providedHellon, R. F.: Jones. R. NI.: Mac Pherson, R. K.;Weiner. J. S.. auLhore of: Natural and arti-acclimatization to hor environments. J. Plty-132:559, 1956.Chart Indicaring Strains andof Erposure to Heat*physiolrgic meanings of the terms "heat ex-and "heat stroke" are frequently confused.(Figure l) has been prepared to show rheand cause of these overstrains together withmrmal bodily responses to heat. Solid lines in-nannal responses; brohen lines indicate theltltysiologic breahd.own,Exne,usrrol-Heat stress results in opening

Haruood S. Belding, Ph-D.

BER.DAN ANd PAFNOTE 845up of skin bioori vcs:els and augmented flow of blo+,ito the skin, bringing heat from the body core, whereit is produced, to ihe skin. For those who are r6tacclimatized, or who haye inadequate ardiac {unc-tion, the heart may not receive adequate return ofblood. This gives rise to increased heart rate andinadequate heart output, manifest by a rapid andfeeble pulse and lowered blood pressure. Inadequatecardiac output first deprives the brain of neededoxygen, becuse of the unfavorable position of thehead with respect ta gravity. The result is faintness,collapse, heat exhaustiorz. In heat exhaustion bodytemperature may be normal. Relief is obtained byassuming horizontal atdtude, by fanning, by a coolshower, or by a cooler air environment.HcAT STRoKE (on Sul Srnoxr)-The normal secondline of defense against heat is sweating, which requireswater and salt. Inadequate water intake or prolongedheavy activity of the sweat glands results in dimin-ished sweating. Diminished sweating meairs higherskin temperature and lesened removal of hat fromsite of.production to skin- Inadequate skin circulationmay also contribute to body temperature r ise. As bodytemperalure rises, the body tissues themselves produceheat faster and further.contribute to the ri- Sufficientrise in bra:'n temperature, to 4l' C (106" D or above,may bring complete failure of the sweating mechanismand curtailment of evaporative cooling. This is heatstroke. Drastic measures are required to break thevicious circle and promptly reduce body temperatureto a level which does not rcult in tissue damage.Hrer Currrs are painful muscular spasms whichresult directly from slt depletion and are remediedand may be prevented by ingestion of adequate slt,

recent developments in understanding of effects of exposi.]re to heat

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