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Carbon monoxide hazard in sub-Antarctic exploration
Journal of Wilderness Medicine 5,4-10 (1994)
ORIGINAL ARTICLE
Carbon monoxide hazard in sub-Antarctic exploration .... ROWLAND M.P. GILL, MRCGP AFOM RAM College, Mil/bank, London SWIP 4RJ, UK
During a sub-Antarctic expedition, petrol stoves were used for cooking inside tents and snow-holes (snow caves). The carbon monoxide hazard from the use of petrol stoves was assessed by measuring atmospheric levels with direct-reading indicator tubes (Draeger tubes). Levels up to 300 ppm were recorded, higher than previous field experiments have shown. The levels recorded were unlikely to have affected the exercise capacity of expedition members or to have had other serious effects. Direct-reading tubes were a simple and effective means of measurement. One episode of acute serious hazard occurred and is described, but the atmospheric carbon monoxide level was not recorded. It is concluded that the most important hazard from carbon monoxide under mountaineering and exploration conditions is that of acute fatal poisoning. Key words: carbon monoxide, mountaineering, cooking, tents, snow shelters, snow-hole, snow caves
Introduction
Carbon monoxide poisoning is a known hazard of mountaineering and exploration, indicated in books of instruction [1], in personal accounts [2,3] and more recently brought to attention in a paper describing the deaths of two climbers on Mount McKinley [4]. This study was an attempt to obtain an assessment of the hazard by measuring carbon monoxide levels in the field using direct-reading indicator tubes during an expedition to the sub-Antarctic island of South Georgia. Carbon monoxide is a product of the incomplete combustion of carbonaceous material. Combustion under conditions of oxygen deficiency or inefficient burning at low temperatures will generate it in increased quantities. These conditions may be found while cooking using fossil fuel stoves in a confined space such as a tent or other shelter. It is worth noting that the efficient and highly serviceable stoves used by our expedition had warnings printed on the fuel tanks not to use them inside tents. Although this corresponds with the suggestion of Foutch and Henrichs [4] and presumably discharges US legal responsibilities, such statements ignore the reality of climatic limitations. Method
The Royal Anglian Expedition to South Georgia was a British Armed Forces-sponsored expedition comprising military personnel and civilians to explore the southern end of the island of South Georgia. The sub-Antarctic climate is harsh so conditions of high winds 0953-9859 © 1994 Chapman & Hall
Carbon monoxide hazard in sub-Antarctic exploration
5
and snow necessitated cooking inside tents and snow-holes for the major part of the expedition. The carbon monoxide content of the atmosphere inside these shelters during various activities, mainly involving the use of petrol stoves, was determined using shortand long-term (diffusion) direct reading indicator tubes. Purpose-designed pressurized petrol mountaineering stoves (Coleman Peak 1®) were used. Twenty-six successful measurements were carried out using direct-reading detector tubes (Draeger Ltd), both of the short-term variety (type lO/b, 10-300 ppm and 1003000 ppm) with a hand-operated bellows pump, and longer-term diffusion tubes (type 50/a-D, 1-8 h, 6.3-63 ppm up to 50-500 ppm). Short-term detector tubes are made of glass and contain a reagent which changes color on reacting with carbon monoxide. A measured volume of air is drawn through the tube using a small hand-held bellows pump and the concentration of carbon monoxide in the air is shown by the length of the stain on the reagent. These tubes give a reading reflecting the air concentration at the time of sampling. Long-term tubes are of similar design but are not connected to a pump and operate by diffusion over a period of time, which may be varied within limits. They give a time-weighted average reading over the whole period of sampling, without indicating the variation. The tubes were used during cooking and other activities in a variety of shelters in an attempt to identify circumstances particularly prone to excessive levels of carbon monoxide. Control readings were taken during the expedition. The relative standard deviation of both methods is 10-15%, and at temperatures below O°C, the standard deviation may increase. Temperature affecting readings was a potential problem in the snow-holes (not in the tents, where air temperature while stoves were lit was up to 20°C), and short-term tubes were pre-warmed by body heat before use. Calibration of the hand pump is infrequently required and was not undertaken on the expedition, but testing for leaks was carried out. Two of the expedition members were smokers, but supplies of tobacco were limited to a few cigarettes and one or two pipes a day. Smoking was unusual during periods of stove use. The weight of the hand pump was 240 g and of 20 tubes was 180 g. The size and weight of the equipment did not prohibit transport by man-hauled pulk to a forward camp on the glacier. The shelters used were: (1) four-man military arctic tent at sea-level, (2) two-man dome lightweight tent at sea-level, used by two men complete and by three men using the tent flysheet alone, (3) snow-hole at 2000 ft, used by two men and (when enlarged) by five men. Ventilation varied throughout the study. When the doors and air holes of the fourman tent were shut, it allowed little ventilation. The two-man tent was well-ventilated, and in high and moderate winds allowed considerable air to be blown in even when the doors were fully shut. The snow-hole was generally ventilated by a vent by the door formed by the basket of a ski-pole, which provided good ventilation when open, but frequently became blocked with falling snow and spindrift, requiring regular clearing. Sufficient diffusion of air through the walls remained for blockage not to be apparent under normal conditions unless the vent was checked. In high winds, the vent was kept as small as possible using the handle of the ski-pole only, to reduce the amount of spindrift blowing into the snow-hole.
Gill
6 Results
The results are shown in Tables 1 and 2. Levels of carbon monoxide in parts per million (ppm) are given for the various activities and degrees of ventilation. Most of the readings indicate low levels of carbon monoxide. The highest single reading was recorded in the snow-hole, but there is little difference between the levels recorded in the snow-hole and those in the double-skinned tents. Melting snow was the activity associated with the highest readings in both the poorly-ventilated four-man tent and the snow-hole. Readings with long-term (diffusion) tubes covering shorter spells of high stove usage showed moderately high levels, but the longer measurements over the length of the day and night revealed that the average exposure was low. Again, the highest readings were obtained in the snow-hole.
Anecdote A noteworthy incident occurred early one morning in the snow-hole, following a night of high winds and heavy drifting. During the heating of water for breakfast with two stoves burning, it was observed that the customary inflow of spindrift through the vent by the snow-hole door had ceased. Shortly afterwards, an attempt to relight an extinguished candle demonstrated that matches and cigarette lighters were failing to light, and the Table 1. Short term levels of carbon monoxide for various activities and degrees of ventilation CO (ppm)
Activity 4-Man tent Melting snow Heating water Space heating Melting snow 2-Man tent complete Melting snow 2-Man tent - outer only Melting snow Heating water Resting 2 h latcr Snow hole Melting snow Heating water Melting snow Cooking food Cooking food Cooking food Melting snow 20 min later 60 min later - stoves out
*
170 100 10-20 10
Ventilation
doors elosed, high winds doors closed, high winds doors closed, calm door open, calm
10-20
door elosed, moderately windy
50 50
o
door open, little wind doors closed, moderately windy doors closed, high winds
10 <10 <10 10 20 20-50 300 100 10
door vent, moderately windy door vent, moderately windy door vent, calm door blocked door blocked minimal door vent, high winds door blocked* minimal door vent, high winds* door vent, high winds*
*See period marked Table 2 for average reading.
Carbon monoxide hazard in sub-Antarctic exploration
7
Table 2. Long term levels of carbon monoxide for various activities and degrees of ventilation
Activity 4-Man tent 8 h overnight 8 h daytime 8 h daytime 4 h evening Snow hole 1 h cooking food 8 h daytime 3 h cooking and heating water 8 h overnight 2 h cooking
CO (ppm)
<6 <6 6
12.5
100 12.5
70 <6 50
Ventilation
variable variable variable doors closed, light wind door blocked variable *variable over period marked * in Table 1 variable minimal door vent, moderately windy
occupants noticed feeling dizzy, were nauseated and short of breath. The stoves were extinguished and all persons applied themselves to digging out the entrance of the snowhole. On reaching the outside air, three out of five reported headache and all suffered excessive fatiguability. It was subsequently discovered that the exceptionally heavy drift had blocked the ventilation hole. No indicator tube readings were taken as the author was as preoccupied as the others with rapid egress from the snow-hole! Evidently, all were suffering from the combined effects of carbon monoxide toxicity and oxygen deficiency. Discussion
Carbon monoxide binds reversibly to heme proteins with a much higher affinity than oxygen, to form carboxyhemoglobin. This reduces the amount of hemoglobin available to carry oxygen. Moreover, there is a left shift of the hemoglobin-oxygen dissociation curve, which further reduces oxygen delivery to the tissues [5]. Elimination of carbon monoxide at low levels of carboxyhemoglobin has not been examined, but the half-life is estimated at an average of 5.4 h [6]. The toxic effects produced by carbon monoxide poisoning are greater than would be accounted for by these mechanisms alone, and various suggestions have been put forward to explain this [7]. In this study, it was not practical to measure carboxyhemoglobin for logistic reasons, but blood levels can be predicted from carbon monoxide exposure, duration of exposure and ventilation rate [8,9]. Using these calculations, it can be reckoned that a level of 5% carboxyhemoglobin would be attained after 2 h exposure to an atmosphere of 70 ppm CO, after 90 min at 100 ppm, and after 30 min at 300 ppm. These values are for an average sedentary man at sea-level (any cause of raised respiratory rate, such as exercise or oxygen deficiency would shorten the exposure time to reach the 5% level). These conditions were encountered in the snow-hole (Table 2). Various studies have examined the decrements in exercise performance associated with carboxyhemoglobin levels at the lower end of the range. Reduced exercise capacity in the form of reduction in maximal work time has been shown at concentrations of 4.3% [10], at 7% [11], and at 2.5% in
8
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non-smokers [12]. No change was observed in non-smokers, and at a lower level of 2.3% no change in work time was demonstrated by the same study group [13]. In a middle-aged group, work time was reduced at a concentration of 4% [14]. It is difficult to predict the likely consequences of such exposure without some knowledge of the excretion of carbon monoxide. Comparatively little work has been done in this area, and the estimate of half-life as 5.4 h [6] is based on inactive subjects. None of these calculations take into account carboxyhemoglobin levels originating from other sources such as smoking.
Previous studies Previous investigations under laboratory conditions [15] using primus stoves demonstrated that the carbon monoxide concentration produced while burning freely (i.e. without a cooking pot) was in the order of 20-50 ppm, but when melting a pot of ice chips rose to 230-600 ppm. Other experimenters using a modified primus stove used in polar exploration, the Nansen stove [16], showed a similar effect, with a higher atmospheric concentration of 800-1000 ppm. They commented that it was the contact of the flame with the cold surface of the pot, lowering the temperature of the flame and hence reducing the efficiency of combustion, which was responsible for the increased production of carbon monoxide. A similar effect has been described with natural gas burners [17]. This corresponds with the results of the present study. One study under field conditions [18] also used short-term direct reading indicator tubes to examine atmospheric carbon monoxide levels (and other pollutants) in ten-man tents, using petrol stoves and lamps during the Norwegian winter. They reported levels predominantly in the region of 20-50 ppm, rising up to 100 ppm. Similar tubes were used again in a cold chamber experiment to investigate the carbon monoxide emission from petrol lanterns in a tent; levels of 50 ppm were recorded during normal operation [19]. Turner and colleagues [20] have measured carbon monoxide levels at altitude in tents, igloos and snow-holes under mountaineering conditions. In most respects other than altitude, their study was similar to this one, using a portable monitor rather than detector tubes. They also found the highest levels (up to 190 ppm) in snow caves, and by observing the decay of atmospheric carbon monoxide levels over time they were able to draw conclusions about effective ventilation. Other studies in the field have involved blood carboxyhemoglobin estimations. Pugh [15] also investigated the blood carbon monoxide levels of members of the 1956-1957 Commonwealth Trans-Antarctic Expedition, while cooking and heating in a tent. This demonstrated that melting ice produced a more rapid rise in levels than using the stove for heating alone. Maximum saturation was 10% carboxyhemoglobin after 2 h. Carbon monoxide levels were not measured in the field. A subsequent experiment in the laboratory measuring the carbon monoxide content inside a lightweight tent while melting ice produced a maximum concentration of only 30 ppm. Irving and colleagues [21] investigated blood carbon monoxide saturation while using varieties of primus stoves inside tents and snow-holes in winter conditions. The stoves were used for heating the air only. This failed to elevate blood carbon monoxide levels inside a conventional (porous-walled) tent, despite tightly sealing all apertures, although when using a laboratory-made impervious tent, levels were elevated. Levels were elevated (maximum 18% CO saturation) inside the snow-hole when the door was fully
Carbon monoxide hazard in sub-Antarctic exploration
9
snowed over, but unaffected when the door was incompletely closed (and a strong wind blowing). This apparently clear-cut distinction was lost in a third experiment when little elevation of blood levels occurred despite burning the stoves in another snow-hole with the door sealed. Some studies have reported neurobehavioral effects at 5% carboxyhemoglobin (and at atmospheric levels of 70 ppm for 4 h) [22], although others have found lesser or no effects at higher levels [23]. A review of the literature [24] concluded that the evidence for neurobehavioral effects was contradictory, but that any such effects were marginal. The risk of acute fatal poisoning for healthy adults is associated with blood carboxyhemoglobin levels of 60-70% [25], and with atmospheric exposures of 1000 ppm for 8 h [9]. It has been suggested that at high altitude the risk of toxicity may be greater [4], and that mild carbon monoxide poisoning at altitude may predispose to the development of altitude sickness [26] (also reported by Hackett [4]). Conclusions
There are four main sources of concern regarding carbon monoxide exposure under expedition circumstances: the risk of fatal poisoning associated with massive exposure; possible effects limiting exercise capacity; neurobehavioral effects leading to errors of judgement in the potentially hazardous mountain environment; and a putative effect on predisposition to altitude sickness. The first is unquestioned, and the anecdote above demonstrates that it is a risk even to the forewarned. The second, well-documented in laboratory conditions, might cause problems in the mountain environment at the levels of exposure described here. Nevertheless, the increased elimination which would be expected to occur during exercise in fresh air would diminish this effect. With regard to the third, neurobehavioral effects are the subject of conflicting reports in the laboratory, and their possible consequences in the field are a matter for conjecture. This study throws no further light on the altitude sickness issue; it will be noted that this study was carried out at sea-level and at 2000 ft. Long-term effects of carbon monoxide poisoning, with the exception of neurological effects following single near-fatal exposure, are primarily linked with prolonged exposure over an extended period and are not considered here. From this study it can be concluded that under expedition conditions, acute fatal poisoning is the most important danger of carbon monoxide toxicity, and is best prevented by careful attention to ventilation while using stoves. There is also a theoretical possibility of errors of judgement arising from exposure at lower levels. A future study could profitably examine the relationship between atmospheric carbon monoxide and blood carboxyhemoglobin levels under field conditions. Acknowledgement
This study was kindly supported by a donation of equipment by Draeger UK Ltd. References 1. Langmuir, E. Mountaincraft and Leadership. Edinburgh: Scottish Sports Council, 1984: 79. 2. Scott, J.M. Gino Watkins. London: Hodder & Stoughton, 1935: 85.
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3. Stefansson, V. Unsolved Mysteries of the Arctic. New York: Macmillan, 1939: 281-4. 4. Foutch, RG., Henrichs, W. Carbon monoxide poisoning at high altitudes. Am J Emerg Med 1988;6:596-8. 5. Haldane, J.RS. The dissociation of oxyhaemoglobin inhuman blood during partial carbon monoxide poisoning. J. PhysioI1912-1913; 45: xii-xxiv. _ 6. Petersen, J.E., Stewart, RD. Absorption and elimination of carbon monoxide by inactive young men. Arch Environ Health 1970; 21: 165-71. 7. Ginsberg, M.D. Carbon monoxide intoxication: clinical features, neuropathology and mechanisms of injury. Clin Toxicol1985; 23: 281-8. 8. Coburn, RF., Forster, RE., Kane, P.B. Considerations of the physiological variables that determine the blood carboxyhaemoglobin concentration in man. J. Clin Invest 1965; 44(1): 1899-1910. 9. Peterson, J.E., Stewart, R.D. Predicting the carboxyhaemoglobin levels resulting from carbon monoxide exposures. J Appl Physiol1975; 39: 633-8. 10. Horvath, S.M., Raven, P.B., Dahms, T.E., Gray, D.J. Maximal aerobic capacity at different levels of carboxyhaemoglobin. J Appl Physiol1975; 38: 300-3. 11. Ekblom, R, Huot, R. Response to submaximal and maximal exercise at different levels of carboxyhaemoglobin. Acta Physiol Scand 1972; 86: 474-82. 12. Drinkwater, RL., Raven, P.R, Horvath, S.M., Gliner, J.A., Ruhling, RO., Boludan, N.W., Taguchi, S. Air pollution, exercise, and heat stress. Arch Environ Health 1974; 28: 177-81. 13. Raven, P.B., Drinkwater, RL., Horvath, S.M., Ruhling, R.O., Gilner, J.A., Sutton, J.e., Bolduan, N.W. Age, smoking habits, heat stress, and their interactive effects with carbon monoxide and peroxyacetylnitrate on man's aerobic power. Int J Biometeor 1974; 18: 222-32. 14. Aronow, W.S., Cassidy, J. Effect of carbon monoxide on maximal treadmill exercise: a study in normal persons. Ann Intern Med 1975; 83: 496-9. 15. Pugh, L.G.C.E. Carbon monoxide hazard in Antarctica. Br Med J 1959(1): 192-6. 16. Henderson, Y., McCulloch Turner, J. Carbon monoxide as a hazard of polar exploration. Nature 1940; 145: 92-95. . 17. Brumbaugh, LV., Jones, G. W. Carbon monoxide in the products of combustion from natural gas burners. Washington, DC: Department of Commerce, Technologic Papers of the Bureau of Standards, 1992; 16(212): 431-50. 18. Worsley, D., Amor, A, Hughes, W.P., Ince, N., Ramsay, D. Physiological trial of cold weather clothing and equipment. Farnborough, Hampshire, UK: Army Personnel Research Establishment; 1974 Report 16/74: 77-83. 19. Ramsay, D.A Development trials of the prototype lamp, petrol (Arctic) MDR 311. Farnborough, Hampshire, UK: Army Personnel Research Establishment; 1979 Report 5/79. 20. Turner, W.A, Cohen, M.A., Moore, S., Spengler, J.D., Hackett, P.H. Carbon monoxide exposure in mountaineers on Denali. Alaska Med 1988; 30: 85-90. 21. Irving, L., Scholander, P.F., Edwards, G.A. Experiments on carbon monoxide poisoning in tents and snowhouses, J Industr Hyg & Toxicol1942; 24: 213-17. 22. Putz, V.R, Johnson, B.L., Setzer, J.V. A comparative study of the effects of carbon monoxide and methylene chloride on human performance. J Environ Pathol Toxicol1979; 2: 97-112. 23. Benigus, V.A, Muller, K.E., Barton, C.N., Prah, J.D. Effect of low level carbon monoxide on compensatory tracking and event monitoring. Neurotoxicol Teratol1987; 9 227-34. 24. Laties, V.G., Merigan, W.H. Behavioural effects of carbon monoxide on animals and men. Ann Rev Pharmacol Toxicol1979; 19: 357-92. 25. Killick, E.M. Carbon monoxide anoxaemia. Physiol Rev 1940; 20(3): 313-44. 26. Houston, e.S. Altitude illness. Emerg Med Clin North Am 1984; 2: 503-12.
ORIGINAL ARTICLE
Carbon monoxide hazard in sub-Antarctic exploration .... ROWLAND M.P. GILL, MRCGP AFOM RAM College, Mil/bank, London SWIP 4RJ, UK
During a sub-Antarctic expedition, petrol stoves were used for cooking inside tents and snow-holes (snow caves). The carbon monoxide hazard from the use of petrol stoves was assessed by measuring atmospheric levels with direct-reading indicator tubes (Draeger tubes). Levels up to 300 ppm were recorded, higher than previous field experiments have shown. The levels recorded were unlikely to have affected the exercise capacity of expedition members or to have had other serious effects. Direct-reading tubes were a simple and effective means of measurement. One episode of acute serious hazard occurred and is described, but the atmospheric carbon monoxide level was not recorded. It is concluded that the most important hazard from carbon monoxide under mountaineering and exploration conditions is that of acute fatal poisoning. Key words: carbon monoxide, mountaineering, cooking, tents, snow shelters, snow-hole, snow caves
Introduction
Carbon monoxide poisoning is a known hazard of mountaineering and exploration, indicated in books of instruction [1], in personal accounts [2,3] and more recently brought to attention in a paper describing the deaths of two climbers on Mount McKinley [4]. This study was an attempt to obtain an assessment of the hazard by measuring carbon monoxide levels in the field using direct-reading indicator tubes during an expedition to the sub-Antarctic island of South Georgia. Carbon monoxide is a product of the incomplete combustion of carbonaceous material. Combustion under conditions of oxygen deficiency or inefficient burning at low temperatures will generate it in increased quantities. These conditions may be found while cooking using fossil fuel stoves in a confined space such as a tent or other shelter. It is worth noting that the efficient and highly serviceable stoves used by our expedition had warnings printed on the fuel tanks not to use them inside tents. Although this corresponds with the suggestion of Foutch and Henrichs [4] and presumably discharges US legal responsibilities, such statements ignore the reality of climatic limitations. Method
The Royal Anglian Expedition to South Georgia was a British Armed Forces-sponsored expedition comprising military personnel and civilians to explore the southern end of the island of South Georgia. The sub-Antarctic climate is harsh so conditions of high winds 0953-9859 © 1994 Chapman & Hall
Carbon monoxide hazard in sub-Antarctic exploration
5
and snow necessitated cooking inside tents and snow-holes for the major part of the expedition. The carbon monoxide content of the atmosphere inside these shelters during various activities, mainly involving the use of petrol stoves, was determined using shortand long-term (diffusion) direct reading indicator tubes. Purpose-designed pressurized petrol mountaineering stoves (Coleman Peak 1®) were used. Twenty-six successful measurements were carried out using direct-reading detector tubes (Draeger Ltd), both of the short-term variety (type lO/b, 10-300 ppm and 1003000 ppm) with a hand-operated bellows pump, and longer-term diffusion tubes (type 50/a-D, 1-8 h, 6.3-63 ppm up to 50-500 ppm). Short-term detector tubes are made of glass and contain a reagent which changes color on reacting with carbon monoxide. A measured volume of air is drawn through the tube using a small hand-held bellows pump and the concentration of carbon monoxide in the air is shown by the length of the stain on the reagent. These tubes give a reading reflecting the air concentration at the time of sampling. Long-term tubes are of similar design but are not connected to a pump and operate by diffusion over a period of time, which may be varied within limits. They give a time-weighted average reading over the whole period of sampling, without indicating the variation. The tubes were used during cooking and other activities in a variety of shelters in an attempt to identify circumstances particularly prone to excessive levels of carbon monoxide. Control readings were taken during the expedition. The relative standard deviation of both methods is 10-15%, and at temperatures below O°C, the standard deviation may increase. Temperature affecting readings was a potential problem in the snow-holes (not in the tents, where air temperature while stoves were lit was up to 20°C), and short-term tubes were pre-warmed by body heat before use. Calibration of the hand pump is infrequently required and was not undertaken on the expedition, but testing for leaks was carried out. Two of the expedition members were smokers, but supplies of tobacco were limited to a few cigarettes and one or two pipes a day. Smoking was unusual during periods of stove use. The weight of the hand pump was 240 g and of 20 tubes was 180 g. The size and weight of the equipment did not prohibit transport by man-hauled pulk to a forward camp on the glacier. The shelters used were: (1) four-man military arctic tent at sea-level, (2) two-man dome lightweight tent at sea-level, used by two men complete and by three men using the tent flysheet alone, (3) snow-hole at 2000 ft, used by two men and (when enlarged) by five men. Ventilation varied throughout the study. When the doors and air holes of the fourman tent were shut, it allowed little ventilation. The two-man tent was well-ventilated, and in high and moderate winds allowed considerable air to be blown in even when the doors were fully shut. The snow-hole was generally ventilated by a vent by the door formed by the basket of a ski-pole, which provided good ventilation when open, but frequently became blocked with falling snow and spindrift, requiring regular clearing. Sufficient diffusion of air through the walls remained for blockage not to be apparent under normal conditions unless the vent was checked. In high winds, the vent was kept as small as possible using the handle of the ski-pole only, to reduce the amount of spindrift blowing into the snow-hole.
Gill
6 Results
The results are shown in Tables 1 and 2. Levels of carbon monoxide in parts per million (ppm) are given for the various activities and degrees of ventilation. Most of the readings indicate low levels of carbon monoxide. The highest single reading was recorded in the snow-hole, but there is little difference between the levels recorded in the snow-hole and those in the double-skinned tents. Melting snow was the activity associated with the highest readings in both the poorly-ventilated four-man tent and the snow-hole. Readings with long-term (diffusion) tubes covering shorter spells of high stove usage showed moderately high levels, but the longer measurements over the length of the day and night revealed that the average exposure was low. Again, the highest readings were obtained in the snow-hole.
Anecdote A noteworthy incident occurred early one morning in the snow-hole, following a night of high winds and heavy drifting. During the heating of water for breakfast with two stoves burning, it was observed that the customary inflow of spindrift through the vent by the snow-hole door had ceased. Shortly afterwards, an attempt to relight an extinguished candle demonstrated that matches and cigarette lighters were failing to light, and the Table 1. Short term levels of carbon monoxide for various activities and degrees of ventilation CO (ppm)
Activity 4-Man tent Melting snow Heating water Space heating Melting snow 2-Man tent complete Melting snow 2-Man tent - outer only Melting snow Heating water Resting 2 h latcr Snow hole Melting snow Heating water Melting snow Cooking food Cooking food Cooking food Melting snow 20 min later 60 min later - stoves out
*
170 100 10-20 10
Ventilation
doors elosed, high winds doors closed, high winds doors closed, calm door open, calm
10-20
door elosed, moderately windy
50 50
o
door open, little wind doors closed, moderately windy doors closed, high winds
10 <10 <10 10 20 20-50 300 100 10
door vent, moderately windy door vent, moderately windy door vent, calm door blocked door blocked minimal door vent, high winds door blocked* minimal door vent, high winds* door vent, high winds*
*See period marked Table 2 for average reading.
Carbon monoxide hazard in sub-Antarctic exploration
7
Table 2. Long term levels of carbon monoxide for various activities and degrees of ventilation
Activity 4-Man tent 8 h overnight 8 h daytime 8 h daytime 4 h evening Snow hole 1 h cooking food 8 h daytime 3 h cooking and heating water 8 h overnight 2 h cooking
CO (ppm)
<6 <6 6
12.5
100 12.5
70 <6 50
Ventilation
variable variable variable doors closed, light wind door blocked variable *variable over period marked * in Table 1 variable minimal door vent, moderately windy
occupants noticed feeling dizzy, were nauseated and short of breath. The stoves were extinguished and all persons applied themselves to digging out the entrance of the snowhole. On reaching the outside air, three out of five reported headache and all suffered excessive fatiguability. It was subsequently discovered that the exceptionally heavy drift had blocked the ventilation hole. No indicator tube readings were taken as the author was as preoccupied as the others with rapid egress from the snow-hole! Evidently, all were suffering from the combined effects of carbon monoxide toxicity and oxygen deficiency. Discussion
Carbon monoxide binds reversibly to heme proteins with a much higher affinity than oxygen, to form carboxyhemoglobin. This reduces the amount of hemoglobin available to carry oxygen. Moreover, there is a left shift of the hemoglobin-oxygen dissociation curve, which further reduces oxygen delivery to the tissues [5]. Elimination of carbon monoxide at low levels of carboxyhemoglobin has not been examined, but the half-life is estimated at an average of 5.4 h [6]. The toxic effects produced by carbon monoxide poisoning are greater than would be accounted for by these mechanisms alone, and various suggestions have been put forward to explain this [7]. In this study, it was not practical to measure carboxyhemoglobin for logistic reasons, but blood levels can be predicted from carbon monoxide exposure, duration of exposure and ventilation rate [8,9]. Using these calculations, it can be reckoned that a level of 5% carboxyhemoglobin would be attained after 2 h exposure to an atmosphere of 70 ppm CO, after 90 min at 100 ppm, and after 30 min at 300 ppm. These values are for an average sedentary man at sea-level (any cause of raised respiratory rate, such as exercise or oxygen deficiency would shorten the exposure time to reach the 5% level). These conditions were encountered in the snow-hole (Table 2). Various studies have examined the decrements in exercise performance associated with carboxyhemoglobin levels at the lower end of the range. Reduced exercise capacity in the form of reduction in maximal work time has been shown at concentrations of 4.3% [10], at 7% [11], and at 2.5% in
8
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non-smokers [12]. No change was observed in non-smokers, and at a lower level of 2.3% no change in work time was demonstrated by the same study group [13]. In a middle-aged group, work time was reduced at a concentration of 4% [14]. It is difficult to predict the likely consequences of such exposure without some knowledge of the excretion of carbon monoxide. Comparatively little work has been done in this area, and the estimate of half-life as 5.4 h [6] is based on inactive subjects. None of these calculations take into account carboxyhemoglobin levels originating from other sources such as smoking.
Previous studies Previous investigations under laboratory conditions [15] using primus stoves demonstrated that the carbon monoxide concentration produced while burning freely (i.e. without a cooking pot) was in the order of 20-50 ppm, but when melting a pot of ice chips rose to 230-600 ppm. Other experimenters using a modified primus stove used in polar exploration, the Nansen stove [16], showed a similar effect, with a higher atmospheric concentration of 800-1000 ppm. They commented that it was the contact of the flame with the cold surface of the pot, lowering the temperature of the flame and hence reducing the efficiency of combustion, which was responsible for the increased production of carbon monoxide. A similar effect has been described with natural gas burners [17]. This corresponds with the results of the present study. One study under field conditions [18] also used short-term direct reading indicator tubes to examine atmospheric carbon monoxide levels (and other pollutants) in ten-man tents, using petrol stoves and lamps during the Norwegian winter. They reported levels predominantly in the region of 20-50 ppm, rising up to 100 ppm. Similar tubes were used again in a cold chamber experiment to investigate the carbon monoxide emission from petrol lanterns in a tent; levels of 50 ppm were recorded during normal operation [19]. Turner and colleagues [20] have measured carbon monoxide levels at altitude in tents, igloos and snow-holes under mountaineering conditions. In most respects other than altitude, their study was similar to this one, using a portable monitor rather than detector tubes. They also found the highest levels (up to 190 ppm) in snow caves, and by observing the decay of atmospheric carbon monoxide levels over time they were able to draw conclusions about effective ventilation. Other studies in the field have involved blood carboxyhemoglobin estimations. Pugh [15] also investigated the blood carbon monoxide levels of members of the 1956-1957 Commonwealth Trans-Antarctic Expedition, while cooking and heating in a tent. This demonstrated that melting ice produced a more rapid rise in levels than using the stove for heating alone. Maximum saturation was 10% carboxyhemoglobin after 2 h. Carbon monoxide levels were not measured in the field. A subsequent experiment in the laboratory measuring the carbon monoxide content inside a lightweight tent while melting ice produced a maximum concentration of only 30 ppm. Irving and colleagues [21] investigated blood carbon monoxide saturation while using varieties of primus stoves inside tents and snow-holes in winter conditions. The stoves were used for heating the air only. This failed to elevate blood carbon monoxide levels inside a conventional (porous-walled) tent, despite tightly sealing all apertures, although when using a laboratory-made impervious tent, levels were elevated. Levels were elevated (maximum 18% CO saturation) inside the snow-hole when the door was fully
Carbon monoxide hazard in sub-Antarctic exploration
9
snowed over, but unaffected when the door was incompletely closed (and a strong wind blowing). This apparently clear-cut distinction was lost in a third experiment when little elevation of blood levels occurred despite burning the stoves in another snow-hole with the door sealed. Some studies have reported neurobehavioral effects at 5% carboxyhemoglobin (and at atmospheric levels of 70 ppm for 4 h) [22], although others have found lesser or no effects at higher levels [23]. A review of the literature [24] concluded that the evidence for neurobehavioral effects was contradictory, but that any such effects were marginal. The risk of acute fatal poisoning for healthy adults is associated with blood carboxyhemoglobin levels of 60-70% [25], and with atmospheric exposures of 1000 ppm for 8 h [9]. It has been suggested that at high altitude the risk of toxicity may be greater [4], and that mild carbon monoxide poisoning at altitude may predispose to the development of altitude sickness [26] (also reported by Hackett [4]). Conclusions
There are four main sources of concern regarding carbon monoxide exposure under expedition circumstances: the risk of fatal poisoning associated with massive exposure; possible effects limiting exercise capacity; neurobehavioral effects leading to errors of judgement in the potentially hazardous mountain environment; and a putative effect on predisposition to altitude sickness. The first is unquestioned, and the anecdote above demonstrates that it is a risk even to the forewarned. The second, well-documented in laboratory conditions, might cause problems in the mountain environment at the levels of exposure described here. Nevertheless, the increased elimination which would be expected to occur during exercise in fresh air would diminish this effect. With regard to the third, neurobehavioral effects are the subject of conflicting reports in the laboratory, and their possible consequences in the field are a matter for conjecture. This study throws no further light on the altitude sickness issue; it will be noted that this study was carried out at sea-level and at 2000 ft. Long-term effects of carbon monoxide poisoning, with the exception of neurological effects following single near-fatal exposure, are primarily linked with prolonged exposure over an extended period and are not considered here. From this study it can be concluded that under expedition conditions, acute fatal poisoning is the most important danger of carbon monoxide toxicity, and is best prevented by careful attention to ventilation while using stoves. There is also a theoretical possibility of errors of judgement arising from exposure at lower levels. A future study could profitably examine the relationship between atmospheric carbon monoxide and blood carboxyhemoglobin levels under field conditions. Acknowledgement
This study was kindly supported by a donation of equipment by Draeger UK Ltd. References 1. Langmuir, E. Mountaincraft and Leadership. Edinburgh: Scottish Sports Council, 1984: 79. 2. Scott, J.M. Gino Watkins. London: Hodder & Stoughton, 1935: 85.
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