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INVESTIGATION OF THE EFFECT OF COOLINGTHE FEET AS A MIEANS OF REDUCWNG

THERMAL STRZESS (U)

by

SMD. Livingstone and R.W. Nolan

41;IELECT.-~V'• JAN 16 19S22'•

9J2- 13 IS2

92-01139q

DEFENCE RESEARCH ESTABL SH,1NT OTTA; k%TECHNICAL NOTE 91-15

CaTc:,August 1,,n eni'

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INVESTIGATiON OF THE EFFECT OF COOLINGTHE FEET AS A M-EANS OF REDUCING

THERMAL STRESS (U)

by

S.D. Livingstone and R.W. NolanEnvironmental Proteczion Section

Protecive Sciences Divisio ,ir•1r-'-:,I MTiS ¢'~ fr,'\hUTKC ';,;9 [

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DEFENCE RESEARCH ESTABLISHMENT OTTAWATECHNICAL NOTE 91-15

PCN August 1991051LC1l Ottawa

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ABSTRACT

The purpose of this investigation was to determine if coolingthe feet is as effective as cooling the hands in the alleviation ofheat stress. Each of six volunteers dressed in chemical warfare(CW) protective clothing, sat for 120 min in an environment of 350Cand then immersed his feet in a water calorimeter for 20 min. Heatlost to the calorimeter was measured at water temperatures of 10,15, 20, 25 and 300 C. The use of water-cooled socks was alsoexamined under these environmental conditions. It was found thatheat loss from the feet was comparable to that from the hands,indicating that the two methods are equally efficient. Preliminaryresults indicate that the use of water-cooled socks may also bepractical.

Le but de cette enquate dtait de d~terminer si lerefroidissement des pieds est aussi efficace que le refroidissementdes mains pour alldger le stress thermique. Chacun des sixvolontaires habillds de l'ensemble de protection chimique,s'assirent pour 120 minutes dans un environnement de 35"C et puisimmergbrent leurs pieds dans un calorimbtre A eau pour 20 minutes.La chaleur perdue au calorimbtre fut mesurde & 10, 15, 20, 25 et30"C. L'utilisation de bas refroidis par eau fut egalement examindsous ces m~mes conditions environnementales. On a trouve que lachaleur perdue par les pieds etait comparable A celle par perduepar les mains, indiquant que les deux m~thodes sont aussiefficaces. Les resultats prdldminaires indiquent que l'utilisationde has refroidis par eau peut aussi 6tre pratique.

iii

EXECUJTIVB SUMVAAY

Hyperthermia (increase in body temperature due to heat stress)has long been recognized as a problem in various trades in theCanadian Forces. It has been found that immersing the hands incool water will increase the loss of body heat by as much as 100watts. This method of cooling, however, is impractical becausewhen the hands are being cooled they cannot be used to performtheir various tasks.

In this study the effect of cooling the feet on body heatcontent was examined as another means of cooling the body. Each ofsix volunteers dressed in winter underwear, woollen socks, combatboots, CW coveralls and impermeable gloves and then entered anenvironmental chamber at 350C. After sitting for 120 minutes, hissocks and boots were removed and his feet immersed in a water bathfor 20 minutes. This procedure was repeated at bath temperaturesof 10, 15, 20, 25 and 30'C. Additional experiments utilizingwater-cooled socks instead of the water calorimeter were alsoconducted.

As would be expected, more heat was lost from the feet as thewater temperature decreased. More than 100 watts can be removedfrom the body using this technique. This indicated that footimmersion was as effective as, and perhaps more effective than,hand immersion as a method of reducing body heat. Water-cooledsocks were found to have potential use as a means of coolingpersonnel in static situations.

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INTRODUCTION

Hyperthermia may occur in circumstances where individuals areobliged to wear clothing which provides excessive thermalinsulation or must operate in hot environments. Its prevention hasbeen a concern of the Canadian Forces (CF) for many years. Recentexperiments (Livingstone et al., 1989) have shown that theimmersion of the hands in cool water is an effective means ofremoving heat from the body when one is working in situations whichmight induce hyperthermia.

Since cooling the hands by immersion in water or by any othermeans effectively inhibits their use, cooling the body via thehands is not always practical. However, there are certain staticor semi-static situations in the CF (members of tank crews orhelicopter pilots on stand-by, for example) where restricting thefeet does not affect performance. In such cases it might bepossible to reduce heat stress by cooling the feet withouthindering performance. The current study was performed to determinewhether cooling the feet would be as effective as cooling the handsin alleviating heat stress.

METHODS

Six members of the DREO test team volunteered to act assubjects for our investigation. Their anthropometriccharacteristics are given in Table 1.

TABLE I

Anthropometric characteristics of test subjects

Subject Age Height Weight(yr) (cm) (kg)

1 24 167 782 25 185 743 26 170 754 29 168 545 24 178 676 23 182 86

A small, commercially-available, 30 L insulated picnic coolerwas used as a water calorimeter to measure the amount of heatremoved from the body via the feet. After being filled with 24 Lof water, a Lauda Model T-l circulating pump was used to stir thewater constantly to ensure equal temperature distribution in thecalorimeter. In order to measure the water temperature,thermistors were placed near the top, middle and bottom of thecooler. Preliminary experiments were done to determine thetemperature characteristics of the calorimeter.

1

Each subject donned winter underwear, CW protective clothing,woolen socks and combat boots after which he entered anenvironmental chamber set at 35°C. He then sat in an upright chairfor a period of 120 minutes after which his boots and socks wereremoved, his underwear and pant legs rolled up to his knees andboth feet were immersed in the water calorimeter for a period of 20minutes. During each experiment, the temperature of the water inthe calorimeter was measured and recorded every 20 seconds. Thesubjects repeated the procedure on different days at watertemperatures of 10, 15, 20, 25 and 30"C.

The amount of heat transferred from the feet to thecalorimeter was determined for all cases using the followingequation:

Q = mct-' (T, - T, - k) W

where

m = mass of water (g)c = specific heat of water (J/g 0 C)t = time (s)T, = water temperature when feet immersed (°C)T, = water temperature when feet removed (°C)k = change in water temperature when feet not immersed ('C)

Since m = 2.4 x 10'g, c = 4.2 J, and t = 1.2 x 101s, the totalamount of heat transferred from the feet to the calorimeter duringeach experiment was:

83.7 (T, - T, - k) W

Changes in the surface temperatures of the legs below theknees and above the water level were examined during the immersionusing an infrared thermography system (AGA 680).

Analysis of variance was performed on the changes in body heatloss caused by foot immersion in the calorimeter using the methoddescribed in Snedecor and Cochran (1980).

Since immersing the bare feet in water in a military situationis not realistic, the manufacturer of whole-body cooling suits(Exotemp Ltd, Pembroke, Ontario) was contracted to design andconstruct prototype socks and insoles which could be used to coolthe feet. These can be worn inside military footwear and consistof two parallel circuits of approximately 10 m of 1.5 mm innerdiameter tygon tubing, spaced about 10 mm apart and sewn to theinside of calf-length Lycra socks. This ensemble was connected toan insulated reservoir containing about 20 L of cold water whichwas circulated through the tubing at approximately 100 ml/minute bya variable-speed electric pump.

2

.........

Further experiments using this footwear were performed. Asabove, the subjects dressed in their protective clothing plus thecooling socks and insoles and entered the environmental chamber at35"C. Instead of removing the footwear and immersing bare feet inthe water calorimeter at the end of 120 minutes in the chamber, thecooling unit's pump was turned on and cold water was circulatedthrough the socks on each foot for a period of 20 minutes. Notemperatures were measured.

RESULTS

The results of the experiments are shown in Figs 1 to 3 andTable 2. Only five values were obtained for the lowest watertemperature because one of the subjects could not complete theexperiment. The amount of heat transferred from the feet to thecalorimeter at various water temperatures during the experimentswas comparable to results obtained during our investigation of heatloss via the hands (Livingstone et al., 1989). As before, thequantity of heat lost increased as the water temperature decreased.

TABLE 2

Heat lost to water bath via the feet.

Temperature Heat LossCC) (watts ± S.E.)

10 151 ± 15*15 91 ± 12 #20 84 ±1325 82± 830 55± 5

• significantly different from other values at 1% level# significantly different from 300C value at 5% level

Straight line equation: y = -3.86x + 169.24

where: y is heat loss (watts)x is water temperature ('C)

During the thermographic studies of these experiments, thefeet were seen to be uniformly warm initially (Fig 2a). However,several minutes after immersing the feet in the water theperipheral blood vessels became clearly delineated, evidenced bythe dark lines (Fig 2b, 2c) which indicate a cooler surface areacaused by cool blood flowing up the superficial veins of the leg.Fig 3 is a thermograph of the right leg of one of the subjectstaken several minutes after his feet were placed in the water. Theareas adjacent to the veins appear darker, indicating an apparentlywell-established blood flow cooling the surrounding tissue as wellas cooling the body core.

3

The subjective results from the experiments in which thewater-cooled socks were worn also indicated that this method ofcooling showed promise. During the two-hour period at 35"C, allsubjects felt uncomfortable and perspired profusely. When the coolwater began to circulate through the socks, the subjectsimmediately felt more comfortable and claimed to have stoppedsweating. One of the subjects, a helicopter pilot, felt thatcooling using the socks would be feasible while flying ahelicopter.

These results indicate that the removal of body heat via thefeet is comparable to that from the hands, indicating that the twomethods are equally efficient. Under the conditions of ourexperiments, it was seen that blood flowing from the feet to thebody does so via the superficial veins of the legs. This supportsthe hypothesis of Vanggaard (1975) who has postulated that bloodflows directly to the heart via the superficial veins rather thanreturning to the core via the central veins. Thus, cooling theblood in the extremities is effective in lowering the temperatureof the body core. This statement is in direct contrast with, butdoes not repudiate, the long held theory of countercurrent heatexchange in which it is believed that the body is able to controlheat loss or gain from the environment by means of heat exchangebetween the central arteries and veins in the leg. However, ifthis is valid we would see a gradual cooling from the foot to thebody when the feet are immersed in cool water and vice versa.Instead, our results indicate that at least a portion of the bloodis returning through the superficial veins.

We have shown that more than 100 watts of heat may be removedfrom the body simply by immersing the feet in cool water. Thisamount is slightly more than the amount of heat loss when the handswere immersed in cool water (Livingstone et al., 1989) indicatingthat this method of cooling is as effective as cooling the hands.Although the quantity of heat lost when water-cooled socks wereworn was not measured, it is felt that this is a feasible method ofcooling heat-stressed individuals.

Although this method of cooling may be difficult to apply tomilitary personel in mobile situations, it may be useful inincreasing heat loss in those in static or semi-static situationssuch as pilots, tank personnel and others whose hands must be freeto remain operative. Further investigations are being planned todetermine if the bulky and heavy water cooler and pump can berepllaced by a much smaller water source and a light portable powersupply so as to make the system more portable.

4

CONCLUSION

It wds found that immersing the feet in cool water was afeasible means of alleviating heat stress in individuals dressed inCW clothing sitting in an environment of 35'C.

REFERENCES

1. Livingstone S.D., Nolan R.W. and Cattroll S.W. Heat losscaused by immersing the hands in water. Aviat. Space Environ.Med. 1989: 60: 1166-71.

2. Snedecor G.W. and Cochran W.G. Statistical Methods, 7th Ed.(Chapter 12) Ames IA: Iowa State University Press, 1980.

3. Vanggaard L. Physiological reactions to wet-cold. Aviat.Space Environ. M(1. 1975: 46: 33-36.

200 -

175-

150

• 125 -

S* *

CO 100 -

"* * *

75-

50- **

25-

0I I I

0 10 20 30BATH TEMPERATURE (-C)

Figure 1: Heat loss from the feet immersed in water atvarious temperatures.

6

(a) (b)

! ,

(c)

Fig 2. Thermograms of the legs of a subject after feet were placedin calorimeter. The thermogram is set in the white-hot mode

i.e. the whiter the colour the warmer the temperature. Thethermograms show the initial temperature distribution of the legsimmediately after entering the water (2a), two minutes afterimmersion (2b) where .t can be seen that a cool area has appearedon the calf of the left leg just above the bath, and six minutesafter immersion (2c) where the cool area has progressed up theleg giving a wider and more definite dark pattern.

7

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Fig 3o Thermogram of the right leg approximately 15 min afterimmersion in calorimeter. The thermogram is in thewhite-hot mode. The dark areas on the leg can berecognized readily as a venous pattern. It can also beseen that the cool areas are larger than the veinsshowing that the tissue around the vein is beingcooled.

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y. Investigation of the Effect of Cooling the Feet as a Means of Reducing Thermal

Stress (U)

4- AUTH.ORS (Lan taione. tirr riwie middle innuto

LIVINGSTONE, Sydney D. and NOLAii Richard W.

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The purpose of this investigation was to determine if coolingthe feet is as effective as cooling the hands in the alleviation ofheat stress. Each of six volunteers dressed in chemical warfare(CW) protective clothing, sat fo 120 min in an environment of 35 0 Cand then immersed his feet in a water calorimeter for 20 min. Heatlost to the calorimeter was measured at water temperatures of 10,15, 20, 25 and 300C. The use of water-:ooled socks was also examinedunder these environmental1 conditiovs. It was found that heat lossfrom the feet was comparable to that from the hands, indicatingthat the two methods are equally efficient. Preliminary re~sultsindicate that the use of water-cooled socks m~ay also be practical.

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Hype rthermiaBody TemperaturesWater-Cooled SocksCalorime try

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