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Test of a quick-don wet immersion suit for off-shore use
International Journal o f Industrial Ergonomics, 11 (1993) 321-330
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Elsevier
Test of a quick-don wet immersion suit for off-shore use N o e l J. Dawson
a
P a u l M c N . Hill
a
a n d S t e p h e n J. Legg b.
a Department of Physiology, School of Medicine, University of Auckland, Private Bag 92019, Auckland, New Zealand b Aviation Medicine Unit, Clark House, Royal New Zealand Air Force Base Auckland, Private Bag, Whenuapai 1450, New Zealand (Received November 10, 1992; accepted in revised form April 5, 1993)
Abstract Tests of "Spacetime" Survival Coverall suits and the associated gloves and boots of the ensemble were carried out on six men. Four m e n maintained their rectal temperature above 36°C for six hours in a bath of circulating water at 12°C. Two m e n had to be withdrawn from the bath because they reached a rectal temperature of 35.5°C, the ethical withdrawal criterion, before the six-hour period had elapsed. Linear regression of rectal temperature versus time during the terminal part of the exposure was carried out for all men. O n e of the subjects who was withdrawn would almost certainly have had a rectal temperature above 34°C at the end of six hours had he remained in the bath. The other subject would have had a rectal temperature classifiable as "incipient death" (30°C). The relatively rapid fall in rectal temperatures in two subjects did not appear to relate directly to body size, but leakage may have been a contributing factor. Comparison with data in the literature indicates that the immersion suit is advantageous, but separate experimentation would have to be carried out to determine quantitatively how valuable it is in extending the survival time of m e n immersed in cold water in comparison with other suits on the market.
Relevance to industry Workers on off-shore oil platforms are an example of people involved in an occupation where there is a possibility of prolonged immersion in cold water, for example when a platform must be abandoned. The provision of appropriate quick-don survival suits can slow the rate of drop of deep body temperature during immersion and thereby prolong survival time and increase the chance of rescue.
Keywords Cold water immersion; exposure; hypothermia; immersion suit; quick-don; survival suit; testing; wet suit.
Introduction Whilst New Zealand's annual crude oil production is only small, it is important to the country itself. All platforms are off-shore and the industry attaches great importance to safety and
Correspondence to: N.J. Dawson, D e p a r t m e n t of Physiology, School of Medicine, University of Auckland, Private Bag 92019, Auckland, New Zealand. * Present address: Centre for Sport Performance-UniSports, Auckland UniServices Ltd, T h e University of Auckland, Private Bag 92019, Auckland, New Zealand.
to protection of workers from exposure should they be forced to evacuate a platform into sea water, typically at a temperature of 12°C. It is recognised that such a work environment is hazardous, and that one of the significant hazards is immersion hypothermia (Golden, 1976). There are various types of immersion suits on the international market. They are either wet or dry, quick-don or constant wear. Potential industrial purchasers are faced with the difficult task of deciding which type of suit to buy, balancing cost against the requirements of local conditions. We tested a new, locally produced, quick-don, "loose" wet immersion suit ensemble, the
0169-8141/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
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"Spacetime" Survival Coverall with associated boots and gloves. The suit was being evaluated by a local oil company for use on its off-shore platforms. Various considerations attach to whether immersion suits should be tested on human volunteers or whether their performance should be predicted using suitable mathematical models. Problems related to the use of human subjects are stated by Allen (1988) to be the use of calm water, the cost of compensating volunteers, subject selection in the face of variability of physiological responses, and ethical limitations. He favours mathematical prediction, whilst conceding that it also has limitations. These limitations relate to the measurement of immersed insulation, variation of insulation over different regions of the manikin, and shortcomings of the model
itself, namely "the validity of such a predictive technique is only as good as the mathematical descriptions of physical and physiological responses incorporated in the model" (Allen, 1988). In a review of mathematical models of human body temperature regulation, Wissler (1988) found that those concerned with immersion in cold water had difficulty levels of 4 and 3, that is, the "probability of obtaining a useful result" was poor to fair, respectively. In practical terms, a "fair" performance yields computed values for central temperature and skin temperatures that are within +0.05°C and +2°C, respectively, of the measured values only 50% of the time. The figure for " p o o r " performance is 30%. In view of this evaluation, and because thermal manikins are not widely available, we decided to carry out tests using human volunteers.
Fig. 1. The "Spacetime SPECI" wet immersion suit with its associated boots and gloves. Note the adhesive strips ("Velcro") for sealing the cuffs around the ankles and wrists, the adjustable belt around the waist, and the adjustable belts around the thighs. The clear splash-guard is folded back into the hood.
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Method
Table 1 Anthropometric characteristics of subjects
Background An oil company wished to determine whether a survival suit produced in New Zealand was suitable for off-shore use. Two of the authors (N.J.D. and P.McN.H.) were commissioned to carry out the required tests as a research contract. At the Company's request, the tests were performed in accordance with United Kingdom specifications for immersion suits (Merchant Shipping (Life Saving Appliances) Regulations 1986, Schedule II, Part I; and Civil Aviation Authority Specification No. 19, Issue 1 Helicopter Crew Members Immersion Suits).
Test requirements The requirements of the Merchant Shipping (Life Saving Appliances) Regulations 1986, Schedule II, Part I were adhered to. The relevant sections are 3.2.10, 3.1.1, and 3.1.3. The overall requirement (3.2.10) is: " T h e complete protection system - immersion suit, inflated life jacket and the clothes worn under the suit - should provide sufficient insulation to maintain the deep body temperature of the wearer above 34°C for a period of 6 hours in circulating water at a temperature of 12°C. ''
The test ensemble The immersion suit was the "Spacetime" Survival Coverall (Spacetime Industries Ltd, N.Z.). It resembled a modified boiler suit fabricated from non-flammable synthetic material ("Nomex", Du Pont) for the outside and inside layers, with a layer of insulation (3 mm closed cell foam) between them. It was provided with a hood sewn onto the neck, a clear splash-guard to protect the face, a tightly closable neck, and straps to enable the cuffs to be closed around the wrists and ankles. Belts were provided around the waist and thighs. The rest of the ensemble consisted of boots ("Treadlite Texan" 133191, Treadlite Footwear (NZ) Ltd.) and soft leather industrial gloves lined with stockinet. An inflatable life-jacket was worn. The ensemble is illustrated in figure 1.
Subject
Weight (kg)
Height (m)
Body mass index (kg. m 2)
1 2 3 4 5 6
90.0 63.5 75.0 58.0 87.0 82.0
1.83 1.69 1.82 1.76 1.88 1.72
26.87 22.23 22.64 18.72 24.62 27.72
Subjects The subjects for the thermal tests were healthy volunteers, six men 30 years and under, who conformed to the criteria in the specification (3.1.1 Test Subjects). Anthropometric data are given in table 1.
Temperature measurements Temperatures were measured using thermocouples (Type K, RC Components Ltd., U.K.). Those for the measurement of skin temperature were attached medially in the lumbar region, on the dorsum of the hand, and on the dorsum of the foot with a strip of adhesive plaster 20 mm from the junction. The seeking junction was attached to a copper disk 6 mm in diameter. The measurement area was covered by a tightly applied piece of thin rubber sheet 30 x 30 mm, held in place around the edges by waterproof plaster. Core temperature was measured by a rectal thermocouple inserted 120 mm from the anal sphincter and retained by means of a large nylon bead (13 mm diameter). Junction potentials were amplified by conventional electronic apparatus, interfaced to a Macintosh computer via a 4-channel A-to-D converter (MacLab ®, Analog Digital Instruments Pty. Ltd., Australia), and logged using commercial software (Chart ©, Analog Digital Instruments Pty. Ltd., Australia).
Electrocardiogram The E C G was monitored using electrodes attached to the chest just below the nipples.
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Test tank
spa pool pump. Water temperature was maintained between 11 and 12°C by placing the tank and pump in a climatic chamber, ambient air temperature 11°C.
Tests were carried out by placing the subject in a 1500-1itre aluminium tank. External water circulation at the rate of 500 1.h-~ was provided by a
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Procedure The subject arrived at the laboratory dressed in the required test clothing. He signed a consent form after reading an information sheet and having any questions answered. The rectal probe was inserted by the subject under medical supervision. Skin temperature probes and ECG electrodes were attached. The subject then donned the immersion suit and life jacket, and entered the tank. The preliminary procedure took up to 1 hour and was carried out in an ante-room at the prevailing summer ambient temperature and humidity (typically 20-23°C and 60%). Rectal and skin temperatures were recorded continuously and the subject interrogated every 15 minutes after entering the tank. During the test the subject took nil by mouth, urinated in the suit when necessary, and maintained a semi-huddled posture. When the subject exited the tank, the ensemble and all leads were removed, he consumed a hot chocolate drink, and took a long hot shower.
Withdrawal A test could be terminated (1) if the subject's deep body temperature dropped to 35.5°C or the skin temperatures dropped to 10°C; (2) by the
supervising doctor for clinical reasons; (3) by the subject after discussion with the doctor. The subject had an absolute right of withdrawal from the experiment at any time without giving a reason.
Ethics approval The entire procedure was approved by the University of Auckland Human Subjects Ethics Committtee.
Results
Rectal temperatures Rectal temperature, as well as the skin temperatures, began to drop immediately on entry to the bath. Figure 2 depicts the rectal temperature responses of all subjects. Points are at 10-minute intervals to reduce crowding. Initial artefactual EMF readings, due to unavoidable differential heating of a thermocouple connector at the beginning of the test, have been eliminated. Two subjects (1 and 2) had to be withdrawn before the 6-hour period had expired (at 280 and 130 min, respectively) because their rectal tern-
Table 2 Regression data for the linear terminal part of the rectal t e m p e r a t u r e versus time curve and predicted rectal t e mpe ra t ure for each subject after 6 hours in the bath. The t e m p e r a t u r e predicted from our data is compared with the t e m p e r a t u r e expected in lightly clothed subjects in harbour conditions at the same temperature. This comparison m u s t be treated with the u t m o s t caution (see text). The Equation column gives the coefficients for the linear regression equation, where Tr is rectal temperature, b is the slope of the line, x is time in minutes, and a is the intercept; r 2 is correlation coefficient squared; p < 0.0001 in each case. Note: actual Tr for subjects 3 - 6 at the end of their 6-hour immersion were, respectively, 36.5, 36.3, 36.3, and 36.6°C. Subject
Equation T r = bx + a
r2
Predicted Tr at 6 hours in suit a (°C)
Excess of predicted Tr at 6 hours over predicted Tr without suit b (°C)
Estimated Tr advantage with suit (%)
1 2 3 4 5 6
-
0.902 0.843 0.281 0.197 0.334 0.798
34.7 30.7 36.4 36.3 36.3 36.4
11.6 7.6 13.3 13.2 13.2 13.3
50.2 32.9 57.6 57.1 57.1 57.6
0.008x + 0.019x + 0.001x + 0.001x + 0.001x + 0.003x +
37.537 37.546 36.759 36.698 36.660 37.493
a From data in this report. b Predicted Tr of 23.1°C at 6 hours without a suit in the sea at 12°C under the conditions reported by Hayward et al. (1975a).
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N.J. Dawson et al. / Immersion suit test
peratures reached the 35.5°C withdrawal criterion (criterion 1). The other four subjects maintained a rectal temperature above this limit for the dura-
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tion of the 6-hour test. No withdrawals under criteria (2) or (3) were necessary. For each subject, a regression line was fitted to
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Fig.3, Skintemperatureresponsesfor all subjects.Symbols:• foot;O lumbar; [] hand.
N.J. Dawson et al. / Immersion suit test
the linear terminal part of the rectal temperature versus time curve (from 100 minutes, except for subject 2) and the rectal temperature at 6 hours was calculated from the equation for the fitted line. Although the slope of the regression for most subjects is very slight, each is very highly significant ( p = 0.0001). Such high significance would derive from the small variance combined with, in most cases, the large number of degrees of freedom consequent on the frequent data acquisition rate. Note that for clarity data points are only shown in the figures at 10-minute intervals, but mathematical calculations were performed using all data points acquired. The regression equations allow us to predict what the rectal temperature would be at 6 hours for the two subjects who had to be withdrawn. The predictions for all subjects are given in table 2.
Skin temperatures Figure 3 depicts the skin temperature responses for all subjects. Points are given at 10minute intervals to reduce crowding. Initial artefactual E M F readings, due to unavoidable differential heating of a thermocouple connector at the beginning of the test, have been eliminated. The skin temperatures dropped immediately on entering the bath and then tended to level out after varying periods of time. It is notable that lumbar temperature dropped much more rapidly than hand or foot temperatures, but stabilised earlier and at a higher temperature. Hand and foot temperatures dropped rapidly during the first hour of exposure, after which they remained relatively stable at a temperature about I°C higher than water temperature.
Shivering Time to noting of overtly observable shivering after entering the bath was variable, 10-96 minutes (43 + 32 minutes, mean + SD); it then continued in an intermittent manner for the entire period of immersion. Subjects showed little variation in the rectal temperatures at which such shivering was noted: 37.7 __+0.8°C. The corresponding values for lumbar, hand and foot temperatures were, respectively: 25.8 ___2.2°C; 24.4 +
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2.6°C, and 20.7 + 2.0°C. All subjects ceased to shiver after a long, hot shower, but felt the need for extra clothing, despite the warm ambient temperature conditions prevailing at the time of the tests.
Other obseruations Subjects had difficulty in closing the cuffs of the legs of the suit whilst floating in the water. Bending the knee pulled up the trouser-leg making it difficult to get a tight seal. All subjects complained of the cold and at least two complained of pain or discomfort in arms or legs. Even subjects who had a rectal temperature of around 36°C at 6 hours felt that they would not have been able to continue for much longer, essentially because of the discomfort. Some minor irregularities were noticed in the ECG of two subjects. One subject had difficulty urinating, but was able to do so when instructed consciously to relax between bouts of shivering. One subject complained of sore testicles. When the ankle cuffs were released after immersion, we estimated that several litres of water drained out of the suit.
Discussion
Rectal temperature response Deep body temperature was maintained above 36°C for 6 hours in four subjects. The two subjects who were withdrawn at a rectal temperature of 35.5°C (subjects 1 and 2) had the lowest predicted temperatures at 6 hours; one of them would have had a rectal temperature indicative of "incipient death", although the other would have had a temperature above 34°C. These two subjects were the first tested. They were allowed to do up the cuffs of their suits themselves. All other subjects had their cuffs tightly closed by one of the supervising physiologists after entering the bath. Because the linear extrapolation to estimate rectal temperature at 6 h is either for a relatively short period (a maximum of 3.8 h for subject 2) or within the period of data collection (4 subjects), we are confident that it provides a simple and accurate indication for the present
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N.J. Dawson et al. / Immersion suit test
purpose. We strongly caution against using the method to estimate 'survival time', that is, time to reach a rectal temperature of 34°C. In some cases, such long periods are predicted that they must be inaccurate for wet suits (cf. Hayes, 1988) in view of the likely metabolic changes that would take place during immersion beyond 6 h in water at 12°C. Long survival times have been predicted for subjects in dry suits (Hayward, 1984), although factors such as compression and degree of immersion can greatly affect the effectiveness of such suits (Hall, 1972). We have no reason to suppose that the wet suit we tested offered a similar degree of protection to that of dry suits. Subjects 1 and 2 both had rectal temperatures about I°C lower than the other four at the beginning of their immersion. The fit of the cuffs was the only other difference we were able to detect between the two subjects who had to be withdrawn and the other four. It is our impression that this latter is significant. Both withdrawn subjects noted that they could feel streams of cold water under the suit, whereas no other subjects mentioned it. It is our impression that if the cuffs are not done up very tightly, a bellows effect is created and cold water is sucked into the suit, destroying much of its insulative value. The rectal temperature response of subject 2 is notably different from the others. Interestingly, he was the only subject who wore a woollen pullover under the immersion suit; we have no way of determining the effect of this. It is possible that wearing the pullover had a detrimental effect through its action in trapping a significant volume of cold water next to that part of the skin surface with the highest temperature (cf. Hayward et al., 1973). This cold water would rapidly drain heat from the body in the initial period after immersion. If the water in the pullover were subsequently exchanged with cold water from the tank through a bellows effect resulting from movement, then the problem would be compounded. It has been demonstrated by Wolff et al. (1985) that flushing can approximately halve the effective insulation of wet-suits. The effect of flushing is likely to be equally detrimental for immersion suits. Although subject 2 had a Body Mass Index in the lower range for the group, it was very close to that of subject 3 and higher than that of subject 4, neither of whose rectal temperatures fell precipi-
tately. Although it has been shown that the insulation of subcutaneous fat is an important determinant of ability to stabilise body temperature in water, it was also found that there were "reactive individuals" who were able to increase their metabolic heat production more rapidly than others and who had more effective insulation in relation to fat thickness (Hayward and Keatinge, 1981). In view of these observations, we do not believe that body size alone is an explanation for the rapid rate of fall of rectal temperature in subject 2. The rate of drop of rectal temperature in this subject was less than what would be expected when not wearing a suit, but it was still much faster than in subject 1. The discrepancy between these two subjects, even bearing in mind the difference in their body weights or their Body Mass Index, leads us to be cautious about adopting cuff fit as the complete explanation. Skin temperature responses
Skin temperatures rapidly fell to a level which was then maintained above water temperature for the duration of the immersion. Lumbar temperature remained some 10°C higher than hand or foot temperature, which were about 2°C higher than water temperature. The temporal pattern of skin temperature responses resembled that observed experimentally by others, for example Hayward and Eckerson (1984), or what would be predicted mathematically (Tikuisis et al., 1988). Relatively rapid onset of shivering would be expected as a consequence of the sudden fall in skin temperature on immersion, but shivering is significantly dependent on central temperature as well (cf. Jessen, 1985); hence, actual time to onset would be expected to be variable. Immersion suits and hypothermia
The term hypothermia refers to the condition where the deep body temperature falls below 35°C. Hypothermia ensues when the body's heat production cannot match its heat loss, despite maximum vasoconstriction; consequently, deep body temperature falls. Immersion in water at the relatively moderate temperature of 12°C will produce hypothermia in about 69 minutes. This prediction is based on the time to reach a deep body
N.J. Dawson et al. / Immersion suit test
temperature of 35°C in lightly clothed people under harbour conditions, as determined from the observations of Hayward et al. (1975a). "Incipient death" occurs at 30°C (Hayward et al., 1975a). Using a mathematical model, Hayes (1988) calculated survival times for a sample of North Sea off-shore workers whilst wearing an immersion suit at various water temperatures. Survival time was taken as the time for arterial temperature to fall below 34°C. The predicted survival times in water at 12°C, estimated from his published graph, range from 2.2 to 6.4 hours. The shorter period is for an "improperly worn" immersion suit and represents a worst case; the longer period is an optimum case for an immersion suit with no leaks in calm water. For a "realistic" situation, the survival time was predicted to be 3.9 hours. The present data suggest that in the case of 4, and possibly 5, of our subjects, survival time would be rather longer than the mathematical prediction for the suits studied by Hayes (1988), at least in still water. If such is the case, then the suit could be assessed favourably, but we would caution against doing so merely on the basis of theoretical predictions. A direct comparison with other suits on the international market is desirable. The tests reported in the present study were not designed to determine how much better the immersion suit would be than no suit or some alternative protection. In our view, the only accurate way to obtain such information is by direct experiment. However, an estimate can be made using the data of Hayward et al. (1975a). From those data it is possible to calculate that rectal temperature in water at 12°C would be 23.1°C after 6 hours for the subjects tested under the conditions that prevailed at the time. The increased convective heat loss under harbour conditions, and the possible differences in physique of subjects means that any comparison can only be rough. It is likely that our subjects under similar harbour conditions would have done worse than in the bath - the question that cannot be answered from the literature or from the current test design is how much worse they would have done. It is known, however, that loose-fitting wet survival suits are the least effective of all types in rough seas (Steinman et al., 1987). A tentative estimate of the advantage to maintaining rectal
329
temperature of wearing the suit under the conditions in our study is 33-58% (Table 2).
Suggestions for improuement in suit design and use In cold water, heat loss will be greatest from areas that have the highest temperatures, such as the chest, but swimming activity increases the amount of body surface having such higher temperatures (Hayward et al., 1973). Nevertheless, there may be significant advantage in protecting the extremities (Tipton and Golden, 1987). For example, a plastic bag device, drawn over the lower body and secured under the axillae, would have the advantage of inhibiting leg movement and would thereby help ensure that the wearer kept relatively still. The bag would be a disadvantage under choppy conditions, however, because it would prevent the wearer from making stabilising movements. Great care should be taken to ensure tight closure of the cuffs after the wearer enters the water; the deficiencies in the current arrangement were apparent in the bath and during mobility tests. A superior alternative to the pockets would be a pair of mittens incorporated in holders on the suit. Once in the water, the wearer could draw on the mittens. The mittens should be designed to cover the cuff and sleeve, with provision for a tight seal. The recommended placement of the arms in the survival " H e a t Escape Lessening Posture" (Hayward et al., 1975b) is with the arms crossed over the life-jacket. Such a placement allows the hands to be released readily in rough water to help maintain stability. It is well known to engineers that adding insulation to cylinders of less than 13 mm diameter can increase rather than decrease heat loss from them. For this reason, mittens are much more effective than gloves for insulating the hands (Burton and Edholm 1955; Newburgh, 1949). We noted that a gap often appeared between the cuff and the glove, exposing bare skin to the water. Longer "dispatch rider" style gloves or mittens would be preferable in order to prevent such exposure.
Suggestions for further work It would be advantageous if the immersion suit could be compared in some way with other suits
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on the international market. The Air Standardisation Agreement 61/40 (1984) provides a means of estimating calm water survival times in relation to water temperature. However, survival times cannot be estimated unless the immersed insulation value ("immersed clo") of the suit is known. Determination of the immersed clo value for a suit (Romet et al., 1991) is a relatively sophisticated and expensive procedure. Nevertheless, due caution should be exercised in view of the findings reviewed by Wissler (1988) indicating that even the best mathematical predictions are acceptable only 50% of the time. Further work should include tests in the open sea, and on women in the same weight categories as for men. Women tend to have a thicker layer of subcutaneous fat than do men of equivalent weight and may have thinner extremities. Both factors could substantially affect heat loss. For these reasons, we would not wish to extrapolate our current data to women.
Acknowledgements The work reported in this paper was financed as contract research by Shell Todd Oil Services Ltd. The authors thank the company for permitting publication of the results, and Spacetime Industries Ltd, N.Z., for providing technical information on the suit's construction. The expert technical assistance of Messrs S. Glasson, C. Kulma and Miss R. Carson is gratefully acknowledged. We thank Professor J.D. Sinclair for valuable assistance and Mr A.B. Fergusson (Auckland UniServices Ltd) for help in recruiting volunteers.
References Allen, J.R., 1988. A technical basis for the development of thermal performance standards for immersion protection. In: I.B. Mekjavic, E.W. Banister and J.B. Morrison (Eds.), Environmental Ergonomics. Taylor & Francis, Philadelphia, pp. 205-220. Burton, A.C. and Edholm, O.G., 1955. Man in a Cold Environment. Edward Arnold, London. Golden, F.St.C., 1976. Hypothermia: A problem for North
Sea industries. Journal of the Society for Occupational Medicine, 26: 85-88. Hall, J.F., 1972. Prediction of tolerance in cold water and life raft exposures. Aerospace Medicine, 43: 281-286. Hayward, J.S., 1984. Thermal protection performance of survival suits in ice-water. Aviation, Space and Environmental Medicine, 55: 212-215. Hayward, J.S. and Eckerson, J.D., 1984. Physiological responses and survival time prediction for humans in icewater. Aviation, Space and Environmental Medicine, 55: 206-212. Hayward, J.S., Collis, M. and Eckerson, J.D., 1973. Thermographic evaluation of relative heat loss areas of man during cold water immersion. Aerospace Medicine 44: 708-711. Hayward, J.S., Eckerson, J.D. and Collis, M.L., 1975a. Thermal balance and survival time prediction of man in cold water. Canadian Journal of Physiology and Pharmacology, 53: 21-32. Hayward, J.S., Eckerson, J.D. and Collis, M.L., 1975b. Effect of behavioral variables on cooling rate of man in cold water. Journal of Applied Physiology, 38: 1073-1077. Hayward, M.G. and Keatinge, W.R., 1981. Roles of subcutaneous fat and thermoregulatory reflexes in determining ability to stabilize body temperature in water. Journal of Physiology (London), 320: 229-251. Hayes, P., 1988. The physiological basis for the development of immersion protective clothing, In: I.B. Mekjavic, E.W. Banister and J.B. Morrison (Eds.), Environmental Ergonomics. Taylor & Francis, Philadelphia, pp. 221-239. Romet, T.T., Brooks, C.J., Fairburn, S.M. and Potter, P., 1991. Immersed clo insulation in marine work suits using human and thermal manikin data. Aviation, Space and Environmental Medicine, 62: 739-746. Jessen, C., 1985. Thermal afferents in the control of body temperature. Pharmacology and Therapeutics, 28: 107134. Newburgh, L.H., 1949. Physiology of Heat Regulation and the Science of Clothing. W.B. Saunders, Philadelphia. Steinman, A.M., Hayward, J.S., Nemiroff, M.J. and Kubilis, P.S., 1987. Immersion hypothermia: Comparative protection of anti-exposure garments in calm versus rough seas. Aviation, Space and Environmental Medicine, 58: 550-558. Tikuisis, P., Gonzalez, R.R. and Pandolf, K.B., 1988. Thermoregulatory model for immersion of humans in cold water. Journal of Applied Physiology, 64: 719-727. Tipton, M.J. and Golden, F.St.C., 1987. The influence of regional insulation on the initial responses to cold immersion. Aviation, Space and Environmental Medicine, 58: 1192-1196. Wissler, E.H,, 1988. A review of human thermal models. In: I.B. Mekjavic, E.W. Banister and J.B. Morrison (Eds.), Environmental Ergonomics. Tayler & Francis, Philadelphia, pp. 267-285. Wolff, A.H., Coleshaw, S.R.K., Newstead, C.G. and Keatinge, W.R., 1985. Heat exchange in wet suits. Journal of Applied Physiology, 59: 770-777.
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Elsevier
Test of a quick-don wet immersion suit for off-shore use N o e l J. Dawson
a
P a u l M c N . Hill
a
a n d S t e p h e n J. Legg b.
a Department of Physiology, School of Medicine, University of Auckland, Private Bag 92019, Auckland, New Zealand b Aviation Medicine Unit, Clark House, Royal New Zealand Air Force Base Auckland, Private Bag, Whenuapai 1450, New Zealand (Received November 10, 1992; accepted in revised form April 5, 1993)
Abstract Tests of "Spacetime" Survival Coverall suits and the associated gloves and boots of the ensemble were carried out on six men. Four m e n maintained their rectal temperature above 36°C for six hours in a bath of circulating water at 12°C. Two m e n had to be withdrawn from the bath because they reached a rectal temperature of 35.5°C, the ethical withdrawal criterion, before the six-hour period had elapsed. Linear regression of rectal temperature versus time during the terminal part of the exposure was carried out for all men. O n e of the subjects who was withdrawn would almost certainly have had a rectal temperature above 34°C at the end of six hours had he remained in the bath. The other subject would have had a rectal temperature classifiable as "incipient death" (30°C). The relatively rapid fall in rectal temperatures in two subjects did not appear to relate directly to body size, but leakage may have been a contributing factor. Comparison with data in the literature indicates that the immersion suit is advantageous, but separate experimentation would have to be carried out to determine quantitatively how valuable it is in extending the survival time of m e n immersed in cold water in comparison with other suits on the market.
Relevance to industry Workers on off-shore oil platforms are an example of people involved in an occupation where there is a possibility of prolonged immersion in cold water, for example when a platform must be abandoned. The provision of appropriate quick-don survival suits can slow the rate of drop of deep body temperature during immersion and thereby prolong survival time and increase the chance of rescue.
Keywords Cold water immersion; exposure; hypothermia; immersion suit; quick-don; survival suit; testing; wet suit.
Introduction Whilst New Zealand's annual crude oil production is only small, it is important to the country itself. All platforms are off-shore and the industry attaches great importance to safety and
Correspondence to: N.J. Dawson, D e p a r t m e n t of Physiology, School of Medicine, University of Auckland, Private Bag 92019, Auckland, New Zealand. * Present address: Centre for Sport Performance-UniSports, Auckland UniServices Ltd, T h e University of Auckland, Private Bag 92019, Auckland, New Zealand.
to protection of workers from exposure should they be forced to evacuate a platform into sea water, typically at a temperature of 12°C. It is recognised that such a work environment is hazardous, and that one of the significant hazards is immersion hypothermia (Golden, 1976). There are various types of immersion suits on the international market. They are either wet or dry, quick-don or constant wear. Potential industrial purchasers are faced with the difficult task of deciding which type of suit to buy, balancing cost against the requirements of local conditions. We tested a new, locally produced, quick-don, "loose" wet immersion suit ensemble, the
0169-8141/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
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"Spacetime" Survival Coverall with associated boots and gloves. The suit was being evaluated by a local oil company for use on its off-shore platforms. Various considerations attach to whether immersion suits should be tested on human volunteers or whether their performance should be predicted using suitable mathematical models. Problems related to the use of human subjects are stated by Allen (1988) to be the use of calm water, the cost of compensating volunteers, subject selection in the face of variability of physiological responses, and ethical limitations. He favours mathematical prediction, whilst conceding that it also has limitations. These limitations relate to the measurement of immersed insulation, variation of insulation over different regions of the manikin, and shortcomings of the model
itself, namely "the validity of such a predictive technique is only as good as the mathematical descriptions of physical and physiological responses incorporated in the model" (Allen, 1988). In a review of mathematical models of human body temperature regulation, Wissler (1988) found that those concerned with immersion in cold water had difficulty levels of 4 and 3, that is, the "probability of obtaining a useful result" was poor to fair, respectively. In practical terms, a "fair" performance yields computed values for central temperature and skin temperatures that are within +0.05°C and +2°C, respectively, of the measured values only 50% of the time. The figure for " p o o r " performance is 30%. In view of this evaluation, and because thermal manikins are not widely available, we decided to carry out tests using human volunteers.
Fig. 1. The "Spacetime SPECI" wet immersion suit with its associated boots and gloves. Note the adhesive strips ("Velcro") for sealing the cuffs around the ankles and wrists, the adjustable belt around the waist, and the adjustable belts around the thighs. The clear splash-guard is folded back into the hood.
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Method
Table 1 Anthropometric characteristics of subjects
Background An oil company wished to determine whether a survival suit produced in New Zealand was suitable for off-shore use. Two of the authors (N.J.D. and P.McN.H.) were commissioned to carry out the required tests as a research contract. At the Company's request, the tests were performed in accordance with United Kingdom specifications for immersion suits (Merchant Shipping (Life Saving Appliances) Regulations 1986, Schedule II, Part I; and Civil Aviation Authority Specification No. 19, Issue 1 Helicopter Crew Members Immersion Suits).
Test requirements The requirements of the Merchant Shipping (Life Saving Appliances) Regulations 1986, Schedule II, Part I were adhered to. The relevant sections are 3.2.10, 3.1.1, and 3.1.3. The overall requirement (3.2.10) is: " T h e complete protection system - immersion suit, inflated life jacket and the clothes worn under the suit - should provide sufficient insulation to maintain the deep body temperature of the wearer above 34°C for a period of 6 hours in circulating water at a temperature of 12°C. ''
The test ensemble The immersion suit was the "Spacetime" Survival Coverall (Spacetime Industries Ltd, N.Z.). It resembled a modified boiler suit fabricated from non-flammable synthetic material ("Nomex", Du Pont) for the outside and inside layers, with a layer of insulation (3 mm closed cell foam) between them. It was provided with a hood sewn onto the neck, a clear splash-guard to protect the face, a tightly closable neck, and straps to enable the cuffs to be closed around the wrists and ankles. Belts were provided around the waist and thighs. The rest of the ensemble consisted of boots ("Treadlite Texan" 133191, Treadlite Footwear (NZ) Ltd.) and soft leather industrial gloves lined with stockinet. An inflatable life-jacket was worn. The ensemble is illustrated in figure 1.
Subject
Weight (kg)
Height (m)
Body mass index (kg. m 2)
1 2 3 4 5 6
90.0 63.5 75.0 58.0 87.0 82.0
1.83 1.69 1.82 1.76 1.88 1.72
26.87 22.23 22.64 18.72 24.62 27.72
Subjects The subjects for the thermal tests were healthy volunteers, six men 30 years and under, who conformed to the criteria in the specification (3.1.1 Test Subjects). Anthropometric data are given in table 1.
Temperature measurements Temperatures were measured using thermocouples (Type K, RC Components Ltd., U.K.). Those for the measurement of skin temperature were attached medially in the lumbar region, on the dorsum of the hand, and on the dorsum of the foot with a strip of adhesive plaster 20 mm from the junction. The seeking junction was attached to a copper disk 6 mm in diameter. The measurement area was covered by a tightly applied piece of thin rubber sheet 30 x 30 mm, held in place around the edges by waterproof plaster. Core temperature was measured by a rectal thermocouple inserted 120 mm from the anal sphincter and retained by means of a large nylon bead (13 mm diameter). Junction potentials were amplified by conventional electronic apparatus, interfaced to a Macintosh computer via a 4-channel A-to-D converter (MacLab ®, Analog Digital Instruments Pty. Ltd., Australia), and logged using commercial software (Chart ©, Analog Digital Instruments Pty. Ltd., Australia).
Electrocardiogram The E C G was monitored using electrodes attached to the chest just below the nipples.
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Test tank
spa pool pump. Water temperature was maintained between 11 and 12°C by placing the tank and pump in a climatic chamber, ambient air temperature 11°C.
Tests were carried out by placing the subject in a 1500-1itre aluminium tank. External water circulation at the rate of 500 1.h-~ was provided by a
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Procedure The subject arrived at the laboratory dressed in the required test clothing. He signed a consent form after reading an information sheet and having any questions answered. The rectal probe was inserted by the subject under medical supervision. Skin temperature probes and ECG electrodes were attached. The subject then donned the immersion suit and life jacket, and entered the tank. The preliminary procedure took up to 1 hour and was carried out in an ante-room at the prevailing summer ambient temperature and humidity (typically 20-23°C and 60%). Rectal and skin temperatures were recorded continuously and the subject interrogated every 15 minutes after entering the tank. During the test the subject took nil by mouth, urinated in the suit when necessary, and maintained a semi-huddled posture. When the subject exited the tank, the ensemble and all leads were removed, he consumed a hot chocolate drink, and took a long hot shower.
Withdrawal A test could be terminated (1) if the subject's deep body temperature dropped to 35.5°C or the skin temperatures dropped to 10°C; (2) by the
supervising doctor for clinical reasons; (3) by the subject after discussion with the doctor. The subject had an absolute right of withdrawal from the experiment at any time without giving a reason.
Ethics approval The entire procedure was approved by the University of Auckland Human Subjects Ethics Committtee.
Results
Rectal temperatures Rectal temperature, as well as the skin temperatures, began to drop immediately on entry to the bath. Figure 2 depicts the rectal temperature responses of all subjects. Points are at 10-minute intervals to reduce crowding. Initial artefactual EMF readings, due to unavoidable differential heating of a thermocouple connector at the beginning of the test, have been eliminated. Two subjects (1 and 2) had to be withdrawn before the 6-hour period had expired (at 280 and 130 min, respectively) because their rectal tern-
Table 2 Regression data for the linear terminal part of the rectal t e m p e r a t u r e versus time curve and predicted rectal t e mpe ra t ure for each subject after 6 hours in the bath. The t e m p e r a t u r e predicted from our data is compared with the t e m p e r a t u r e expected in lightly clothed subjects in harbour conditions at the same temperature. This comparison m u s t be treated with the u t m o s t caution (see text). The Equation column gives the coefficients for the linear regression equation, where Tr is rectal temperature, b is the slope of the line, x is time in minutes, and a is the intercept; r 2 is correlation coefficient squared; p < 0.0001 in each case. Note: actual Tr for subjects 3 - 6 at the end of their 6-hour immersion were, respectively, 36.5, 36.3, 36.3, and 36.6°C. Subject
Equation T r = bx + a
r2
Predicted Tr at 6 hours in suit a (°C)
Excess of predicted Tr at 6 hours over predicted Tr without suit b (°C)
Estimated Tr advantage with suit (%)
1 2 3 4 5 6
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0.902 0.843 0.281 0.197 0.334 0.798
34.7 30.7 36.4 36.3 36.3 36.4
11.6 7.6 13.3 13.2 13.2 13.3
50.2 32.9 57.6 57.1 57.1 57.6
0.008x + 0.019x + 0.001x + 0.001x + 0.001x + 0.003x +
37.537 37.546 36.759 36.698 36.660 37.493
a From data in this report. b Predicted Tr of 23.1°C at 6 hours without a suit in the sea at 12°C under the conditions reported by Hayward et al. (1975a).
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N.J. Dawson et al. / Immersion suit test
peratures reached the 35.5°C withdrawal criterion (criterion 1). The other four subjects maintained a rectal temperature above this limit for the dura-
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tion of the 6-hour test. No withdrawals under criteria (2) or (3) were necessary. For each subject, a regression line was fitted to
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Fig.3, Skintemperatureresponsesfor all subjects.Symbols:• foot;O lumbar; [] hand.
N.J. Dawson et al. / Immersion suit test
the linear terminal part of the rectal temperature versus time curve (from 100 minutes, except for subject 2) and the rectal temperature at 6 hours was calculated from the equation for the fitted line. Although the slope of the regression for most subjects is very slight, each is very highly significant ( p = 0.0001). Such high significance would derive from the small variance combined with, in most cases, the large number of degrees of freedom consequent on the frequent data acquisition rate. Note that for clarity data points are only shown in the figures at 10-minute intervals, but mathematical calculations were performed using all data points acquired. The regression equations allow us to predict what the rectal temperature would be at 6 hours for the two subjects who had to be withdrawn. The predictions for all subjects are given in table 2.
Skin temperatures Figure 3 depicts the skin temperature responses for all subjects. Points are given at 10minute intervals to reduce crowding. Initial artefactual E M F readings, due to unavoidable differential heating of a thermocouple connector at the beginning of the test, have been eliminated. The skin temperatures dropped immediately on entering the bath and then tended to level out after varying periods of time. It is notable that lumbar temperature dropped much more rapidly than hand or foot temperatures, but stabilised earlier and at a higher temperature. Hand and foot temperatures dropped rapidly during the first hour of exposure, after which they remained relatively stable at a temperature about I°C higher than water temperature.
Shivering Time to noting of overtly observable shivering after entering the bath was variable, 10-96 minutes (43 + 32 minutes, mean + SD); it then continued in an intermittent manner for the entire period of immersion. Subjects showed little variation in the rectal temperatures at which such shivering was noted: 37.7 __+0.8°C. The corresponding values for lumbar, hand and foot temperatures were, respectively: 25.8 ___2.2°C; 24.4 +
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2.6°C, and 20.7 + 2.0°C. All subjects ceased to shiver after a long, hot shower, but felt the need for extra clothing, despite the warm ambient temperature conditions prevailing at the time of the tests.
Other obseruations Subjects had difficulty in closing the cuffs of the legs of the suit whilst floating in the water. Bending the knee pulled up the trouser-leg making it difficult to get a tight seal. All subjects complained of the cold and at least two complained of pain or discomfort in arms or legs. Even subjects who had a rectal temperature of around 36°C at 6 hours felt that they would not have been able to continue for much longer, essentially because of the discomfort. Some minor irregularities were noticed in the ECG of two subjects. One subject had difficulty urinating, but was able to do so when instructed consciously to relax between bouts of shivering. One subject complained of sore testicles. When the ankle cuffs were released after immersion, we estimated that several litres of water drained out of the suit.
Discussion
Rectal temperature response Deep body temperature was maintained above 36°C for 6 hours in four subjects. The two subjects who were withdrawn at a rectal temperature of 35.5°C (subjects 1 and 2) had the lowest predicted temperatures at 6 hours; one of them would have had a rectal temperature indicative of "incipient death", although the other would have had a temperature above 34°C. These two subjects were the first tested. They were allowed to do up the cuffs of their suits themselves. All other subjects had their cuffs tightly closed by one of the supervising physiologists after entering the bath. Because the linear extrapolation to estimate rectal temperature at 6 h is either for a relatively short period (a maximum of 3.8 h for subject 2) or within the period of data collection (4 subjects), we are confident that it provides a simple and accurate indication for the present
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purpose. We strongly caution against using the method to estimate 'survival time', that is, time to reach a rectal temperature of 34°C. In some cases, such long periods are predicted that they must be inaccurate for wet suits (cf. Hayes, 1988) in view of the likely metabolic changes that would take place during immersion beyond 6 h in water at 12°C. Long survival times have been predicted for subjects in dry suits (Hayward, 1984), although factors such as compression and degree of immersion can greatly affect the effectiveness of such suits (Hall, 1972). We have no reason to suppose that the wet suit we tested offered a similar degree of protection to that of dry suits. Subjects 1 and 2 both had rectal temperatures about I°C lower than the other four at the beginning of their immersion. The fit of the cuffs was the only other difference we were able to detect between the two subjects who had to be withdrawn and the other four. It is our impression that this latter is significant. Both withdrawn subjects noted that they could feel streams of cold water under the suit, whereas no other subjects mentioned it. It is our impression that if the cuffs are not done up very tightly, a bellows effect is created and cold water is sucked into the suit, destroying much of its insulative value. The rectal temperature response of subject 2 is notably different from the others. Interestingly, he was the only subject who wore a woollen pullover under the immersion suit; we have no way of determining the effect of this. It is possible that wearing the pullover had a detrimental effect through its action in trapping a significant volume of cold water next to that part of the skin surface with the highest temperature (cf. Hayward et al., 1973). This cold water would rapidly drain heat from the body in the initial period after immersion. If the water in the pullover were subsequently exchanged with cold water from the tank through a bellows effect resulting from movement, then the problem would be compounded. It has been demonstrated by Wolff et al. (1985) that flushing can approximately halve the effective insulation of wet-suits. The effect of flushing is likely to be equally detrimental for immersion suits. Although subject 2 had a Body Mass Index in the lower range for the group, it was very close to that of subject 3 and higher than that of subject 4, neither of whose rectal temperatures fell precipi-
tately. Although it has been shown that the insulation of subcutaneous fat is an important determinant of ability to stabilise body temperature in water, it was also found that there were "reactive individuals" who were able to increase their metabolic heat production more rapidly than others and who had more effective insulation in relation to fat thickness (Hayward and Keatinge, 1981). In view of these observations, we do not believe that body size alone is an explanation for the rapid rate of fall of rectal temperature in subject 2. The rate of drop of rectal temperature in this subject was less than what would be expected when not wearing a suit, but it was still much faster than in subject 1. The discrepancy between these two subjects, even bearing in mind the difference in their body weights or their Body Mass Index, leads us to be cautious about adopting cuff fit as the complete explanation. Skin temperature responses
Skin temperatures rapidly fell to a level which was then maintained above water temperature for the duration of the immersion. Lumbar temperature remained some 10°C higher than hand or foot temperature, which were about 2°C higher than water temperature. The temporal pattern of skin temperature responses resembled that observed experimentally by others, for example Hayward and Eckerson (1984), or what would be predicted mathematically (Tikuisis et al., 1988). Relatively rapid onset of shivering would be expected as a consequence of the sudden fall in skin temperature on immersion, but shivering is significantly dependent on central temperature as well (cf. Jessen, 1985); hence, actual time to onset would be expected to be variable. Immersion suits and hypothermia
The term hypothermia refers to the condition where the deep body temperature falls below 35°C. Hypothermia ensues when the body's heat production cannot match its heat loss, despite maximum vasoconstriction; consequently, deep body temperature falls. Immersion in water at the relatively moderate temperature of 12°C will produce hypothermia in about 69 minutes. This prediction is based on the time to reach a deep body
N.J. Dawson et al. / Immersion suit test
temperature of 35°C in lightly clothed people under harbour conditions, as determined from the observations of Hayward et al. (1975a). "Incipient death" occurs at 30°C (Hayward et al., 1975a). Using a mathematical model, Hayes (1988) calculated survival times for a sample of North Sea off-shore workers whilst wearing an immersion suit at various water temperatures. Survival time was taken as the time for arterial temperature to fall below 34°C. The predicted survival times in water at 12°C, estimated from his published graph, range from 2.2 to 6.4 hours. The shorter period is for an "improperly worn" immersion suit and represents a worst case; the longer period is an optimum case for an immersion suit with no leaks in calm water. For a "realistic" situation, the survival time was predicted to be 3.9 hours. The present data suggest that in the case of 4, and possibly 5, of our subjects, survival time would be rather longer than the mathematical prediction for the suits studied by Hayes (1988), at least in still water. If such is the case, then the suit could be assessed favourably, but we would caution against doing so merely on the basis of theoretical predictions. A direct comparison with other suits on the international market is desirable. The tests reported in the present study were not designed to determine how much better the immersion suit would be than no suit or some alternative protection. In our view, the only accurate way to obtain such information is by direct experiment. However, an estimate can be made using the data of Hayward et al. (1975a). From those data it is possible to calculate that rectal temperature in water at 12°C would be 23.1°C after 6 hours for the subjects tested under the conditions that prevailed at the time. The increased convective heat loss under harbour conditions, and the possible differences in physique of subjects means that any comparison can only be rough. It is likely that our subjects under similar harbour conditions would have done worse than in the bath - the question that cannot be answered from the literature or from the current test design is how much worse they would have done. It is known, however, that loose-fitting wet survival suits are the least effective of all types in rough seas (Steinman et al., 1987). A tentative estimate of the advantage to maintaining rectal
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temperature of wearing the suit under the conditions in our study is 33-58% (Table 2).
Suggestions for improuement in suit design and use In cold water, heat loss will be greatest from areas that have the highest temperatures, such as the chest, but swimming activity increases the amount of body surface having such higher temperatures (Hayward et al., 1973). Nevertheless, there may be significant advantage in protecting the extremities (Tipton and Golden, 1987). For example, a plastic bag device, drawn over the lower body and secured under the axillae, would have the advantage of inhibiting leg movement and would thereby help ensure that the wearer kept relatively still. The bag would be a disadvantage under choppy conditions, however, because it would prevent the wearer from making stabilising movements. Great care should be taken to ensure tight closure of the cuffs after the wearer enters the water; the deficiencies in the current arrangement were apparent in the bath and during mobility tests. A superior alternative to the pockets would be a pair of mittens incorporated in holders on the suit. Once in the water, the wearer could draw on the mittens. The mittens should be designed to cover the cuff and sleeve, with provision for a tight seal. The recommended placement of the arms in the survival " H e a t Escape Lessening Posture" (Hayward et al., 1975b) is with the arms crossed over the life-jacket. Such a placement allows the hands to be released readily in rough water to help maintain stability. It is well known to engineers that adding insulation to cylinders of less than 13 mm diameter can increase rather than decrease heat loss from them. For this reason, mittens are much more effective than gloves for insulating the hands (Burton and Edholm 1955; Newburgh, 1949). We noted that a gap often appeared between the cuff and the glove, exposing bare skin to the water. Longer "dispatch rider" style gloves or mittens would be preferable in order to prevent such exposure.
Suggestions for further work It would be advantageous if the immersion suit could be compared in some way with other suits
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on the international market. The Air Standardisation Agreement 61/40 (1984) provides a means of estimating calm water survival times in relation to water temperature. However, survival times cannot be estimated unless the immersed insulation value ("immersed clo") of the suit is known. Determination of the immersed clo value for a suit (Romet et al., 1991) is a relatively sophisticated and expensive procedure. Nevertheless, due caution should be exercised in view of the findings reviewed by Wissler (1988) indicating that even the best mathematical predictions are acceptable only 50% of the time. Further work should include tests in the open sea, and on women in the same weight categories as for men. Women tend to have a thicker layer of subcutaneous fat than do men of equivalent weight and may have thinner extremities. Both factors could substantially affect heat loss. For these reasons, we would not wish to extrapolate our current data to women.
Acknowledgements The work reported in this paper was financed as contract research by Shell Todd Oil Services Ltd. The authors thank the company for permitting publication of the results, and Spacetime Industries Ltd, N.Z., for providing technical information on the suit's construction. The expert technical assistance of Messrs S. Glasson, C. Kulma and Miss R. Carson is gratefully acknowledged. We thank Professor J.D. Sinclair for valuable assistance and Mr A.B. Fergusson (Auckland UniServices Ltd) for help in recruiting volunteers.
References Allen, J.R., 1988. A technical basis for the development of thermal performance standards for immersion protection. In: I.B. Mekjavic, E.W. Banister and J.B. Morrison (Eds.), Environmental Ergonomics. Taylor & Francis, Philadelphia, pp. 205-220. Burton, A.C. and Edholm, O.G., 1955. Man in a Cold Environment. Edward Arnold, London. Golden, F.St.C., 1976. Hypothermia: A problem for North
Sea industries. Journal of the Society for Occupational Medicine, 26: 85-88. Hall, J.F., 1972. Prediction of tolerance in cold water and life raft exposures. Aerospace Medicine, 43: 281-286. Hayward, J.S., 1984. Thermal protection performance of survival suits in ice-water. Aviation, Space and Environmental Medicine, 55: 212-215. Hayward, J.S. and Eckerson, J.D., 1984. Physiological responses and survival time prediction for humans in icewater. Aviation, Space and Environmental Medicine, 55: 206-212. Hayward, J.S., Collis, M. and Eckerson, J.D., 1973. Thermographic evaluation of relative heat loss areas of man during cold water immersion. Aerospace Medicine 44: 708-711. Hayward, J.S., Eckerson, J.D. and Collis, M.L., 1975a. Thermal balance and survival time prediction of man in cold water. Canadian Journal of Physiology and Pharmacology, 53: 21-32. Hayward, J.S., Eckerson, J.D. and Collis, M.L., 1975b. Effect of behavioral variables on cooling rate of man in cold water. Journal of Applied Physiology, 38: 1073-1077. Hayward, M.G. and Keatinge, W.R., 1981. Roles of subcutaneous fat and thermoregulatory reflexes in determining ability to stabilize body temperature in water. Journal of Physiology (London), 320: 229-251. Hayes, P., 1988. The physiological basis for the development of immersion protective clothing, In: I.B. Mekjavic, E.W. Banister and J.B. Morrison (Eds.), Environmental Ergonomics. Taylor & Francis, Philadelphia, pp. 221-239. Romet, T.T., Brooks, C.J., Fairburn, S.M. and Potter, P., 1991. Immersed clo insulation in marine work suits using human and thermal manikin data. Aviation, Space and Environmental Medicine, 62: 739-746. Jessen, C., 1985. Thermal afferents in the control of body temperature. Pharmacology and Therapeutics, 28: 107134. Newburgh, L.H., 1949. Physiology of Heat Regulation and the Science of Clothing. W.B. Saunders, Philadelphia. Steinman, A.M., Hayward, J.S., Nemiroff, M.J. and Kubilis, P.S., 1987. Immersion hypothermia: Comparative protection of anti-exposure garments in calm versus rough seas. Aviation, Space and Environmental Medicine, 58: 550-558. Tikuisis, P., Gonzalez, R.R. and Pandolf, K.B., 1988. Thermoregulatory model for immersion of humans in cold water. Journal of Applied Physiology, 64: 719-727. Tipton, M.J. and Golden, F.St.C., 1987. The influence of regional insulation on the initial responses to cold immersion. Aviation, Space and Environmental Medicine, 58: 1192-1196. Wissler, E.H,, 1988. A review of human thermal models. In: I.B. Mekjavic, E.W. Banister and J.B. Morrison (Eds.), Environmental Ergonomics. Tayler & Francis, Philadelphia, pp. 267-285. Wolff, A.H., Coleshaw, S.R.K., Newstead, C.G. and Keatinge, W.R., 1985. Heat exchange in wet suits. Journal of Applied Physiology, 59: 770-777.