Ann. occvp. Hxx Vol. 18, pp. 21-28. Pcrgamon Press 1975. Printed in Oreat Britain
THE APPLICATION OF THRESHOLD LIMIT VALUES FOR CARBON MONOXIDE UNDER CONDITIONS OF CONTINUOUS EXPOSURE* M.
DAVIES
Institute of Naval Medicine, Alverstoke, Gosport, Hants Abstract—In a totally closed environment such as that of the nuclear submarine, exposure to raised ambient levels of carbon monoxide is continuous. The principles on which the maximum permissible concentration (MPC) for such exposure should be based are explored with particular reference to non-smokers and smokers, and to the effects on individual and overall system performance and on health. Reference to ambient carbon monoxide levels in submarines; induced carboxyhaemoglobin levels in smoking and non-smoking crew members; the financial, material and operational cost constraints on the MPC; and to current evidence regarding toxic levels, leads to an assessment of the MPC based primarily on safeguarding the non-smokers. The recommendation is made that the current submarine MPC of 25 ppm by volume should be reduced to 15 ppm, bringing it into line with recommendations for spacecraft, and providing an MPC which is applicable in any closed-environment situation. INTRODUCTION
permissible levels for atmospheric carbon monoxide (CO), based on a combination of theoretical, empirical and epidemiological studies, are already available for occupational exposure (normally 8 hr/day, 5 days/week, with considerable variation of CO concentration), and for environmental exposure of the general population, including children, the sick and the old. For occupational exposures, the Threshold Limit Value (TLV) is widely used and, for general environmental exposure, maximum permissible concentrations for civilian populations (MPC civ. pop.) have been put forward by several countries. In each case exposure may be both intermittent and variable. For totally-closed environments yet another maximum permissible exposure level must be specified. It is needed for nuclear submarines and underground or closed air-circulation shelters where normal atmospheric pressure exists; for underwater habitats and spacecraft, where abnormal pressures exist; and for other environments with total or partial air-recirculation such as sealed cabin aircraft, surface warships, and some air-conditioned buildings. In these cases, continuous exposure to relatively constant raised CO levels occurs, sometimes for long periods, and effects may be amplified by the intermittent imposition of raised personal alveolar air CO levels due to smoking. For the nuclear submarine, the Royal Navy has decided that a maximum permissible CO concentration for continuous exposure for 90 days, the MPC90, is the requirement MAXIMUM
•Crown Copyright. 21
Downloaded from http://annhyg.oxfordjournals.org/ at University of California, Santa Barbara on July 13, 2015
D.
22
D. M. DAVIES
Current maximum permissible concentrations for CO in all environments are summarised in Table 1. Although certain operational, material and engineering constraints are placed upon the submarine MPC90, because the nuclear submariner is generally a medically-fit male between the ages of 18 and 45, it is apparent that the MPC90 will be between the two extremes of 1 and 50 ppm. CO IN THE NUCLEAR SUBMARINE
TABLE 1.
CURRENT MAXIMUM RECOMMENDED EXPOSURE LEVELS FOR CARBON MONOXIDE
Designation TLV (USA, UK) TLV (USSR) TLV (Czech) MPQ,o (USN, RN; submarines) MPC90 (USA; spacecraft) MPCtooo (USA; spacecraft) MPC CIY. pop. (USA) MPC
clv.
p O p. (FDR)
MPC c l v . p O p . (USSR)
TABLE 2.
Level (ppm by volume) 50 18 30 25 15 15 9 (8 hr average) 8 (24 hr average) 1
Reference ACGIH (1973) ACGIH (1971) KusTOv and TIUNOV (1971) ACGrH (1971) NASA (1973) NASA (1973) NASA (1973) KNELSON (1972a) KNELSON (1972b) KUSTOV and TIUNOV (197T
MAIN SOURCES OF CARBON MONOXIDE IN RN NUCLEAR SUBMARINES
Tobacco smoking 75-90% of total CO load Cooking particularly baking, roasting, toasting Oxidation of hydrocarbons lagging, oils, paints, solvents Overheating of electrical and mechanical equipment Endogenous production in crew Burning of oxygen-producing chlorate candles Fires
In a Polaris submarine, about two-thirds of the crew of 150 men will be smokers consuming a daily average of 20 cigarettes each, and about 2001. of CO is produced per day in a breathable air volume of 4000 m3. Thus, ambient CO levels would exceed the TLV within 24 hr if no form of CO-removal machinery was installed on board, and even if smoking was prohibited, toxic levels would be reached within only a few days of diving. In the absence of all other sources, endogenous production alone of about 0-5 ml/man hr would cause the current MPCo0 to be exceeded well before the end of patrol. Catalytic CO burners are installed, but they are expensive in terms of capital cost, running costs, and space within the submarine, and their weight limits operational performance. The MPC90 must therefore be considered in relation to the material and operational cost of achieving it, and account must also be taken of other environmental factors. These may affect the uptake or transport of CO within
Downloaded from http://annhyg.oxfordjournals.org/ at University of California, Santa Barbara on July 13, 2015
The main sources of CO in the nuclear submarine are shown in Table 2. The most important is tobacco smoking which is normally not restricted and produces up to 90 per cent of the total atmoshere load.
Application of threshold limit values for carbon monoxide
23
the body, or may act additively or in synergism with the toxic effects produced by the CO. They include the concurrent presence of ambient carbon dioxide levels up to 1 %, many atmosphere contaminants at sub-toxic levels, and the unusual physical, occupational, social and domestic conditions (LAMBERT et al. 1972; DA VIES 1973). During the past 10 yr, ambient CO levels in RN nuclear submarines have been diminishing in parallel with the gradual reduction in both industrial TLVs and submarine MPCs. In current nuclear submarine operations, the mean CO level for more than 90 per cent of dived time varies between 5 and 15 ppm.
Carboxyhaemoglobin levels in nuclear submariners IOr|
|=Non smokers
| = Smokers (>l5ags/day)
2 -
Ashore
At sea CO = 9 r :
Al sea CO = 2 9 Dpn-
FIG. 1. Carboxyhaemoglobin levels in nuclear submariners (from LAWTHER, 1969 and LIGHTFOOT, 1972).
Downloaded from http://annhyg.oxfordjournals.org/ at University of California, Santa Barbara on July 13, 2015
CARBOXYHAEMOGLOBIN LEVELS AND SMOKING PATTERNS The ambient level of CO in a submarine depends largely on the balance between the smoking habits of the crew, the efficiency of the CO-burners in routine operation, and the CO-removal capacity available. It becomes fairly constant during the first days of the patrol and remains, under normal circumstances, steady throughout. Non-smokers come readily into equilibrium with the ambient CO and bear a practically constant carboxyhaemoglobin (COHb) load, day and night, throughout the patrol. LAWTHER (1969) and LIGHTFOOT (1972) investigated COHb levels in nuclear submariners ashore and on patrol, and Fig. 1 is based on their findings. They showed that mean COHb levels in the non-smokers, on two patrols with mean ambient CO levels of 29 and 9 ppm, were 4-7 and 1-4% respectively, compared with 0-7% ashore in fresh air, fitting the equilibrium equation of COHb% = 016 X CO (ppm). They also showed that the mean COHb level in the groups of submariners who smoked regularly more than 15 cigarettes per day was 8-5 % in both patrols, though some individuals might achieve 18 % saturation. The same groups ashore in fresh air showed a mean COHb level of 3 -8 per cent. On theoretical grounds, the mean COHb levels on the two patrols should vary by some 3 per cent, but experimental conditions, including the method of COHb measurement, differed in the two studies. However Fig. 1 demonstrates clearly the most important point that at a mean CO level of
24
D. M. DAVIES
29 ppm, well below the current TLV of 50 ppm and close to the MPC90 of 25 ppm, the non-smokers are carrying COHb burdens greater than those of the smokers ashore. From the foregoing it can be seen that at the current MPC90 the non-smokers' mean COHb level will be about 4 % and the smokers' about 8 %. The setting of the MPC90 then becomes a matter of assessing the significance of such levels to the submariner himself or to the submarine as an operational system. DEFINITION OF MPC
"The MPC for continuous exposure in a closed environmental system is that atmospheric concentration, exerting its effects on the occupants throughout their sojurn in the system, which causes or contributes to neither ill-health in the short or long term, nor lasting detectable functional shifts that would lower efficiency of performance to a level hindering fulfilment of the prescribed activities of the system or its occupants ". This implies that a level of CO must be selected so that the induced COHb, in association with all other adverse environmental factors present, does not cause acute illness or chronic disease, or play a part in the development of degenerative ailments such as cardiovascular disease. It also implies that while a minor degree of decrement in performance in an individual may be acceptable, the collective decrement in the crew must not affect the fighting efficiency of the submarine as a whole. Thus in an individual whose task on board is entirely based on auditory acuity a decrement in visual acuity, for example, might be acceptable, and vice-versa, but a decrement in individual vigilance, or a fall in collective crew sensibility and vigilance below a given minimum, will be unacceptable. In defining an acceptable CO level it seems logical to start with those levels at which induced COHb has been shown, by the best available experimental methods, to cause no decrement in performance at tasks relevant to submarine operations. Taking a similar simple approach to the development of disease associated with increased COHb levels is greatly hindered by the fact that the relationship between COHb levels and, for example, cardiovascular disease, can only be conclusively demonstrated epidemiologically. There now seems little doubt that smokers incur quite severe self-induced penalties in this respect, and the aim then must be to protect the non-smokers in the crew from effects due to the increased ambient CO levels produced largely by the smokers. Thus one would like to know those CO or COHb levels at which the risk of development of cardiovascular disease has been shown to be no greater than that in the non-smoking population at large. Assuming therefore that smoking is to be permitted on patrol, and that the inevitable proportion of smokers in the crew will have compensated physiologically and mentally in some way for their COHb burdens, the acceptable MPC90 for CO should be the highest at which the induced COHb in non-smokers has been shown to produce neither performance decrement nor increased risk of cardiovascular disease, the two major toxic hazards of CO exposure. The penalties incurred in achieving this ideal level then have to be balanced against the benefits gained, to arrive at a practical MPC which is viable in terms offinancial,material and operational cost, and which is
Downloaded from http://annhyg.oxfordjournals.org/ at University of California, Santa Barbara on July 13, 2015
A statement of the requirements which the MPC90 must meet, adapted from a general expression of KUSTOV and TIUNOV (1971), is:
Application of threshold limit values for carbon monoxide
25
biologically acceptable in terms of the definition given earlier. A simpler answer would be to limit or even ban smoking in nuclear submarines, and there is much in favour of such a solution. However, such action is not currently acceptable, because smoking helps to relieve the personal constraints of long submarine patrols and performance in habitual smokers who have been prevented from smoking may, in fact, decrease (GIBSON and MORONEY, 1972). Moreover, CO-burners have to be installed to cope with other CO sources. Downloaded from http://annhyg.oxfordjournals.org/ at University of California, Santa Barbara on July 13, 2015
MENTAL PERFORMANCE A vast amount of conflicting data has been presented in recent years concerning the effect of various COHb levels on the efficiency of performance of real tasks, simulated tasks, and tests of specific mental processes. From the point of view of deriving submarine MPCs, it is unfortunate that most investigations have been conducted under conditions of short-term exposure where body saturation at the prevailing ambient CO level has not been achieved. In some cases, COHb levels have not been determined, but have been estimated from the CO level and the exposure time. It seems quite likely that performance with a COHb load induced by acute exposure to a high CO level will differ from that where the same COHb load is the result of equilibrium saturation at a much lower CO level. Reports have been made that repeated or prolonged exposure to raised COHb levels leads to partial physiological and performance compensation and adaptation, and that increased attention by the subject to the task in hand may overcome the effects due to his COHb load, particularly if he is able to monitor his performance in some way (RAMSEY, 1969; FODOR and WINNEKE, 1972; NASA, 1973). There seems to be little doubt that COHb levels of 5 per cent and below can be shown to produce a decrement in fine indicators of psychological function and psychomotor efficiency, and in performance in real-task situations and simulations. Recent evidence includes the demonstration by GROLL-KNAPP et al. (1972) of reduced acoustic vigilance and reduced-amplitude electroencephalographic "anticipation waves" at calculated levels of 3-5% COHb, and the work of HORVATH et al. (1971) showing reduced visual vigilance at a calculated level of 4-2% COHb. WRIGHT et al. (1973) concluded from their studies on simulated driving tasks, that reduced performance in terms of safety occurred at a measured mean COHb level of 3-4%, and recommended revision of the 50 ppm TLV for 8 hour exposure accordingly. They also expressed the opinion that COHb levels of 2-3 % might reduce effectiveness in dealing with unexpected events, as well as degrading performance in routine tasks. However, FODOR and WINNEKE (1972), although agreeing that acoustic vigilance was compromised at calculated levels of 2-5-3% COHb, noted that it improved with the duration of exposure. They also found neither reduction in flicker fusion frequency nor decrement in a visual-cognitive speed of perception test at calculated levels of COHb approaching 5 %, and no decrease in psychomotor performance at 5 % in tests involving speed or precision and visual/motor co-ordination. Similarly, in a wideranging series of performance tests, including time estimation, reaction time and manual dexterity, STEWART et al. (1970) found that at measured levels of 11-13% COHb after exposure for 8 hr to 100 ppm CO, no impairment of performance could be detected and MCFARLAND (1973), in a study of laboratory-simulated and real-life automobile driving situations, concluded that at measured COHb levels of 6 % no
26
D. M. DAVIES
PHYSICAL DISEASE
The smokers in the crew are accumulating the precursors of disease by their selfinduced COHb burdens, and their contribution to the ambient CO may increase the COHb levels of the non-smokers as well. As the non-smokers do not have the freedom to reject the associated disease risk, ethical considerations alone make it important to set the MPC90 as low as possible. The relationship between smoking, COHb and cardiovascular disease is now well established and there is probably no above-endogenous COHb level at which there is no increase in the risk of disease. A level could perhaps be found, however, from experimental and epidemiological evidence, at which the increased risk is so low that it can be ignored. WALD et al. (1973) showed that smokers with COHb levels greater than 5 % were 21 times more likely to be affected by atherosclerotic disease than those whose level was less than 3 %, and ASTRUP (1972) quoted figures showing that nonatherosclerotic smokers generally had COHb levels less than 4-6%, while atherosclerotics had levels of more than 6 %. KNELSON (1972a) demonstrated acute cardiodynamic changes in healthy persons, and further electrocardiographic changes in persons with an already abnormal ECG, at COHb levels of 5 % and above, and ADAMS et al. (1973) also demonstrated cardiodynamic changes in dogs at 4% COHb. Nevertheless, ECKHARDT et al. (1972) found no abnormal cardiac or brain pathological changes in cynomolgus monkeys carrying COHb loads of 6-8 % continuously for 2 yr. In humans, however, the evidence points to a substantial increase in disease risk associated with continuous COHb loads above 3 %. Therefore, allowing for the concurrent presence of other health hazards within the submarine atmosphere, a maximum COHb burden of 2-5%, equivalent to an ambient CO level of approximately 15 ppm, is again recommended.
Downloaded from http://annhyg.oxfordjournals.org/ at University of California, Santa Barbara on July 13, 2015
effect on driving ability or in related visual efficiency will occur, and that only minor decrements occur at 11 % COHb. Short of examining, under varying ambient CO concentrations, critical aspects of real-life conditions in operational submarines, such as periscope watchkeeping, visual and auditory signal detection, and reaction to developing potentially dangerous situations, it appears that laboratory investigations of critical faculties must be used for determination of the MPC90. Close laboratory simulation of operational tasks is difficult to justify because the true danger and responsibility elements which will greatly effect performance in. the real-life situation will be absent. Vigilance, meaning the general sensitivity of the central nervous system to incoming sensory information (HORVATH et al, 1971), is the faculty which most workers agree is affected by COHb levels well below 5 %, and is the one which is probably of most significance to the submarine system as an operational entity. Thus the MPC90 should be selected so that the COHb level induced in the non-smokers in the crew is the highest at which no decrement in any aspect of vigilance has been demonstrated by test methods acceptable to the authority denning the MPC. Failing this, it should be set below the lowest level at which a decrement has been detected by such methods. On current evidence, the relevant COHb level is 2-5 %, corresponding at equilibrium to a CO level of approximately 15 ppm. Thus, in terms of performance alone, an MPC90 of 15 ppm is recommended.
Application of threshold limit values for carbon monoxide
27
CONCLUSIONS
REFERENCES ACGIH (1971) Documentation of the Threshold Limit Values for Substances in Workroom Air, 3rd Ed. American Conference of Governmental Industrial Hygienists, Cincinatti, Ohio, U.S.A. ACGIH (1973) TLVsfor Chemical Substances in Workroom Air Adopted by the ACGIH for 1973. American Conference of Governmental Industrial Hygienists, Cincinnati, Ohio, U.S.A. ADAMS, J. D. F ERICKSON, H. H. and STONE, H. L. (1973) / . appl. Physiol. 34, 238.
ASTRUP, P. (1972) Staub 32(4), 20. DAVIES, D. M. (1973) Jnl. R. Coll. Physicians Land. 7, 132. ECKHARDT, R. E., MACFARLAND, H. R , ALARIE, Y. C. E. and BUSEY, W. M. (1972) Archs Envir.
Hlth. 25, 381. FODOR, G. G. and WINNEKE, G. (1972) Staub 32(4), 46. GIBSON, R. S. and MORONEY, W. F. (1972) Special Report 72-2. Naval Aerospace Medical Institute (NAMI), US Naval Aerospace Medical Centre, Pensacola, Florida, U.S.A. GROLL-KNAPP, E., WAGNER, H., HAUCH, H. and HAIDER, M. (1972) Staub 32
THE APPLICATION OF THRESHOLD LIMIT VALUES FOR CARBON MONOXIDE UNDER CONDITIONS OF CONTINUOUS EXPOSURE* M.
DAVIES
Institute of Naval Medicine, Alverstoke, Gosport, Hants Abstract—In a totally closed environment such as that of the nuclear submarine, exposure to raised ambient levels of carbon monoxide is continuous. The principles on which the maximum permissible concentration (MPC) for such exposure should be based are explored with particular reference to non-smokers and smokers, and to the effects on individual and overall system performance and on health. Reference to ambient carbon monoxide levels in submarines; induced carboxyhaemoglobin levels in smoking and non-smoking crew members; the financial, material and operational cost constraints on the MPC; and to current evidence regarding toxic levels, leads to an assessment of the MPC based primarily on safeguarding the non-smokers. The recommendation is made that the current submarine MPC of 25 ppm by volume should be reduced to 15 ppm, bringing it into line with recommendations for spacecraft, and providing an MPC which is applicable in any closed-environment situation. INTRODUCTION
permissible levels for atmospheric carbon monoxide (CO), based on a combination of theoretical, empirical and epidemiological studies, are already available for occupational exposure (normally 8 hr/day, 5 days/week, with considerable variation of CO concentration), and for environmental exposure of the general population, including children, the sick and the old. For occupational exposures, the Threshold Limit Value (TLV) is widely used and, for general environmental exposure, maximum permissible concentrations for civilian populations (MPC civ. pop.) have been put forward by several countries. In each case exposure may be both intermittent and variable. For totally-closed environments yet another maximum permissible exposure level must be specified. It is needed for nuclear submarines and underground or closed air-circulation shelters where normal atmospheric pressure exists; for underwater habitats and spacecraft, where abnormal pressures exist; and for other environments with total or partial air-recirculation such as sealed cabin aircraft, surface warships, and some air-conditioned buildings. In these cases, continuous exposure to relatively constant raised CO levels occurs, sometimes for long periods, and effects may be amplified by the intermittent imposition of raised personal alveolar air CO levels due to smoking. For the nuclear submarine, the Royal Navy has decided that a maximum permissible CO concentration for continuous exposure for 90 days, the MPC90, is the requirement MAXIMUM
•Crown Copyright. 21
Downloaded from http://annhyg.oxfordjournals.org/ at University of California, Santa Barbara on July 13, 2015
D.
22
D. M. DAVIES
Current maximum permissible concentrations for CO in all environments are summarised in Table 1. Although certain operational, material and engineering constraints are placed upon the submarine MPC90, because the nuclear submariner is generally a medically-fit male between the ages of 18 and 45, it is apparent that the MPC90 will be between the two extremes of 1 and 50 ppm. CO IN THE NUCLEAR SUBMARINE
TABLE 1.
CURRENT MAXIMUM RECOMMENDED EXPOSURE LEVELS FOR CARBON MONOXIDE
Designation TLV (USA, UK) TLV (USSR) TLV (Czech) MPQ,o (USN, RN; submarines) MPC90 (USA; spacecraft) MPCtooo (USA; spacecraft) MPC CIY. pop. (USA) MPC
clv.
p O p. (FDR)
MPC c l v . p O p . (USSR)
TABLE 2.
Level (ppm by volume) 50 18 30 25 15 15 9 (8 hr average) 8 (24 hr average) 1
Reference ACGIH (1973) ACGIH (1971) KusTOv and TIUNOV (1971) ACGrH (1971) NASA (1973) NASA (1973) NASA (1973) KNELSON (1972a) KNELSON (1972b) KUSTOV and TIUNOV (197T
MAIN SOURCES OF CARBON MONOXIDE IN RN NUCLEAR SUBMARINES
Tobacco smoking 75-90% of total CO load Cooking particularly baking, roasting, toasting Oxidation of hydrocarbons lagging, oils, paints, solvents Overheating of electrical and mechanical equipment Endogenous production in crew Burning of oxygen-producing chlorate candles Fires
In a Polaris submarine, about two-thirds of the crew of 150 men will be smokers consuming a daily average of 20 cigarettes each, and about 2001. of CO is produced per day in a breathable air volume of 4000 m3. Thus, ambient CO levels would exceed the TLV within 24 hr if no form of CO-removal machinery was installed on board, and even if smoking was prohibited, toxic levels would be reached within only a few days of diving. In the absence of all other sources, endogenous production alone of about 0-5 ml/man hr would cause the current MPCo0 to be exceeded well before the end of patrol. Catalytic CO burners are installed, but they are expensive in terms of capital cost, running costs, and space within the submarine, and their weight limits operational performance. The MPC90 must therefore be considered in relation to the material and operational cost of achieving it, and account must also be taken of other environmental factors. These may affect the uptake or transport of CO within
Downloaded from http://annhyg.oxfordjournals.org/ at University of California, Santa Barbara on July 13, 2015
The main sources of CO in the nuclear submarine are shown in Table 2. The most important is tobacco smoking which is normally not restricted and produces up to 90 per cent of the total atmoshere load.
Application of threshold limit values for carbon monoxide
23
the body, or may act additively or in synergism with the toxic effects produced by the CO. They include the concurrent presence of ambient carbon dioxide levels up to 1 %, many atmosphere contaminants at sub-toxic levels, and the unusual physical, occupational, social and domestic conditions (LAMBERT et al. 1972; DA VIES 1973). During the past 10 yr, ambient CO levels in RN nuclear submarines have been diminishing in parallel with the gradual reduction in both industrial TLVs and submarine MPCs. In current nuclear submarine operations, the mean CO level for more than 90 per cent of dived time varies between 5 and 15 ppm.
Carboxyhaemoglobin levels in nuclear submariners IOr|
|=Non smokers
| = Smokers (>l5ags/day)
2 -
Ashore
At sea CO = 9 r :
Al sea CO = 2 9 Dpn-
FIG. 1. Carboxyhaemoglobin levels in nuclear submariners (from LAWTHER, 1969 and LIGHTFOOT, 1972).
Downloaded from http://annhyg.oxfordjournals.org/ at University of California, Santa Barbara on July 13, 2015
CARBOXYHAEMOGLOBIN LEVELS AND SMOKING PATTERNS The ambient level of CO in a submarine depends largely on the balance between the smoking habits of the crew, the efficiency of the CO-burners in routine operation, and the CO-removal capacity available. It becomes fairly constant during the first days of the patrol and remains, under normal circumstances, steady throughout. Non-smokers come readily into equilibrium with the ambient CO and bear a practically constant carboxyhaemoglobin (COHb) load, day and night, throughout the patrol. LAWTHER (1969) and LIGHTFOOT (1972) investigated COHb levels in nuclear submariners ashore and on patrol, and Fig. 1 is based on their findings. They showed that mean COHb levels in the non-smokers, on two patrols with mean ambient CO levels of 29 and 9 ppm, were 4-7 and 1-4% respectively, compared with 0-7% ashore in fresh air, fitting the equilibrium equation of COHb% = 016 X CO (ppm). They also showed that the mean COHb level in the groups of submariners who smoked regularly more than 15 cigarettes per day was 8-5 % in both patrols, though some individuals might achieve 18 % saturation. The same groups ashore in fresh air showed a mean COHb level of 3 -8 per cent. On theoretical grounds, the mean COHb levels on the two patrols should vary by some 3 per cent, but experimental conditions, including the method of COHb measurement, differed in the two studies. However Fig. 1 demonstrates clearly the most important point that at a mean CO level of
24
D. M. DAVIES
29 ppm, well below the current TLV of 50 ppm and close to the MPC90 of 25 ppm, the non-smokers are carrying COHb burdens greater than those of the smokers ashore. From the foregoing it can be seen that at the current MPC90 the non-smokers' mean COHb level will be about 4 % and the smokers' about 8 %. The setting of the MPC90 then becomes a matter of assessing the significance of such levels to the submariner himself or to the submarine as an operational system. DEFINITION OF MPC
"The MPC for continuous exposure in a closed environmental system is that atmospheric concentration, exerting its effects on the occupants throughout their sojurn in the system, which causes or contributes to neither ill-health in the short or long term, nor lasting detectable functional shifts that would lower efficiency of performance to a level hindering fulfilment of the prescribed activities of the system or its occupants ". This implies that a level of CO must be selected so that the induced COHb, in association with all other adverse environmental factors present, does not cause acute illness or chronic disease, or play a part in the development of degenerative ailments such as cardiovascular disease. It also implies that while a minor degree of decrement in performance in an individual may be acceptable, the collective decrement in the crew must not affect the fighting efficiency of the submarine as a whole. Thus in an individual whose task on board is entirely based on auditory acuity a decrement in visual acuity, for example, might be acceptable, and vice-versa, but a decrement in individual vigilance, or a fall in collective crew sensibility and vigilance below a given minimum, will be unacceptable. In defining an acceptable CO level it seems logical to start with those levels at which induced COHb has been shown, by the best available experimental methods, to cause no decrement in performance at tasks relevant to submarine operations. Taking a similar simple approach to the development of disease associated with increased COHb levels is greatly hindered by the fact that the relationship between COHb levels and, for example, cardiovascular disease, can only be conclusively demonstrated epidemiologically. There now seems little doubt that smokers incur quite severe self-induced penalties in this respect, and the aim then must be to protect the non-smokers in the crew from effects due to the increased ambient CO levels produced largely by the smokers. Thus one would like to know those CO or COHb levels at which the risk of development of cardiovascular disease has been shown to be no greater than that in the non-smoking population at large. Assuming therefore that smoking is to be permitted on patrol, and that the inevitable proportion of smokers in the crew will have compensated physiologically and mentally in some way for their COHb burdens, the acceptable MPC90 for CO should be the highest at which the induced COHb in non-smokers has been shown to produce neither performance decrement nor increased risk of cardiovascular disease, the two major toxic hazards of CO exposure. The penalties incurred in achieving this ideal level then have to be balanced against the benefits gained, to arrive at a practical MPC which is viable in terms offinancial,material and operational cost, and which is
Downloaded from http://annhyg.oxfordjournals.org/ at University of California, Santa Barbara on July 13, 2015
A statement of the requirements which the MPC90 must meet, adapted from a general expression of KUSTOV and TIUNOV (1971), is:
Application of threshold limit values for carbon monoxide
25
biologically acceptable in terms of the definition given earlier. A simpler answer would be to limit or even ban smoking in nuclear submarines, and there is much in favour of such a solution. However, such action is not currently acceptable, because smoking helps to relieve the personal constraints of long submarine patrols and performance in habitual smokers who have been prevented from smoking may, in fact, decrease (GIBSON and MORONEY, 1972). Moreover, CO-burners have to be installed to cope with other CO sources. Downloaded from http://annhyg.oxfordjournals.org/ at University of California, Santa Barbara on July 13, 2015
MENTAL PERFORMANCE A vast amount of conflicting data has been presented in recent years concerning the effect of various COHb levels on the efficiency of performance of real tasks, simulated tasks, and tests of specific mental processes. From the point of view of deriving submarine MPCs, it is unfortunate that most investigations have been conducted under conditions of short-term exposure where body saturation at the prevailing ambient CO level has not been achieved. In some cases, COHb levels have not been determined, but have been estimated from the CO level and the exposure time. It seems quite likely that performance with a COHb load induced by acute exposure to a high CO level will differ from that where the same COHb load is the result of equilibrium saturation at a much lower CO level. Reports have been made that repeated or prolonged exposure to raised COHb levels leads to partial physiological and performance compensation and adaptation, and that increased attention by the subject to the task in hand may overcome the effects due to his COHb load, particularly if he is able to monitor his performance in some way (RAMSEY, 1969; FODOR and WINNEKE, 1972; NASA, 1973). There seems to be little doubt that COHb levels of 5 per cent and below can be shown to produce a decrement in fine indicators of psychological function and psychomotor efficiency, and in performance in real-task situations and simulations. Recent evidence includes the demonstration by GROLL-KNAPP et al. (1972) of reduced acoustic vigilance and reduced-amplitude electroencephalographic "anticipation waves" at calculated levels of 3-5% COHb, and the work of HORVATH et al. (1971) showing reduced visual vigilance at a calculated level of 4-2% COHb. WRIGHT et al. (1973) concluded from their studies on simulated driving tasks, that reduced performance in terms of safety occurred at a measured mean COHb level of 3-4%, and recommended revision of the 50 ppm TLV for 8 hour exposure accordingly. They also expressed the opinion that COHb levels of 2-3 % might reduce effectiveness in dealing with unexpected events, as well as degrading performance in routine tasks. However, FODOR and WINNEKE (1972), although agreeing that acoustic vigilance was compromised at calculated levels of 2-5-3% COHb, noted that it improved with the duration of exposure. They also found neither reduction in flicker fusion frequency nor decrement in a visual-cognitive speed of perception test at calculated levels of COHb approaching 5 %, and no decrease in psychomotor performance at 5 % in tests involving speed or precision and visual/motor co-ordination. Similarly, in a wideranging series of performance tests, including time estimation, reaction time and manual dexterity, STEWART et al. (1970) found that at measured levels of 11-13% COHb after exposure for 8 hr to 100 ppm CO, no impairment of performance could be detected and MCFARLAND (1973), in a study of laboratory-simulated and real-life automobile driving situations, concluded that at measured COHb levels of 6 % no
26
D. M. DAVIES
PHYSICAL DISEASE
The smokers in the crew are accumulating the precursors of disease by their selfinduced COHb burdens, and their contribution to the ambient CO may increase the COHb levels of the non-smokers as well. As the non-smokers do not have the freedom to reject the associated disease risk, ethical considerations alone make it important to set the MPC90 as low as possible. The relationship between smoking, COHb and cardiovascular disease is now well established and there is probably no above-endogenous COHb level at which there is no increase in the risk of disease. A level could perhaps be found, however, from experimental and epidemiological evidence, at which the increased risk is so low that it can be ignored. WALD et al. (1973) showed that smokers with COHb levels greater than 5 % were 21 times more likely to be affected by atherosclerotic disease than those whose level was less than 3 %, and ASTRUP (1972) quoted figures showing that nonatherosclerotic smokers generally had COHb levels less than 4-6%, while atherosclerotics had levels of more than 6 %. KNELSON (1972a) demonstrated acute cardiodynamic changes in healthy persons, and further electrocardiographic changes in persons with an already abnormal ECG, at COHb levels of 5 % and above, and ADAMS et al. (1973) also demonstrated cardiodynamic changes in dogs at 4% COHb. Nevertheless, ECKHARDT et al. (1972) found no abnormal cardiac or brain pathological changes in cynomolgus monkeys carrying COHb loads of 6-8 % continuously for 2 yr. In humans, however, the evidence points to a substantial increase in disease risk associated with continuous COHb loads above 3 %. Therefore, allowing for the concurrent presence of other health hazards within the submarine atmosphere, a maximum COHb burden of 2-5%, equivalent to an ambient CO level of approximately 15 ppm, is again recommended.
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effect on driving ability or in related visual efficiency will occur, and that only minor decrements occur at 11 % COHb. Short of examining, under varying ambient CO concentrations, critical aspects of real-life conditions in operational submarines, such as periscope watchkeeping, visual and auditory signal detection, and reaction to developing potentially dangerous situations, it appears that laboratory investigations of critical faculties must be used for determination of the MPC90. Close laboratory simulation of operational tasks is difficult to justify because the true danger and responsibility elements which will greatly effect performance in. the real-life situation will be absent. Vigilance, meaning the general sensitivity of the central nervous system to incoming sensory information (HORVATH et al, 1971), is the faculty which most workers agree is affected by COHb levels well below 5 %, and is the one which is probably of most significance to the submarine system as an operational entity. Thus the MPC90 should be selected so that the COHb level induced in the non-smokers in the crew is the highest at which no decrement in any aspect of vigilance has been demonstrated by test methods acceptable to the authority denning the MPC. Failing this, it should be set below the lowest level at which a decrement has been detected by such methods. On current evidence, the relevant COHb level is 2-5 %, corresponding at equilibrium to a CO level of approximately 15 ppm. Thus, in terms of performance alone, an MPC90 of 15 ppm is recommended.
Application of threshold limit values for carbon monoxide
27
CONCLUSIONS
REFERENCES ACGIH (1971) Documentation of the Threshold Limit Values for Substances in Workroom Air, 3rd Ed. American Conference of Governmental Industrial Hygienists, Cincinatti, Ohio, U.S.A. ACGIH (1973) TLVsfor Chemical Substances in Workroom Air Adopted by the ACGIH for 1973. American Conference of Governmental Industrial Hygienists, Cincinnati, Ohio, U.S.A. ADAMS, J. D. F ERICKSON, H. H. and STONE, H. L. (1973) / . appl. Physiol. 34, 238.
ASTRUP, P. (1972) Staub 32(4), 20. DAVIES, D. M. (1973) Jnl. R. Coll. Physicians Land. 7, 132. ECKHARDT, R. E., MACFARLAND, H. R , ALARIE, Y. C. E. and BUSEY, W. M. (1972) Archs Envir.
Hlth. 25, 381. FODOR, G. G. and WINNEKE, G. (1972) Staub 32(4), 46. GIBSON, R. S. and MORONEY, W. F. (1972) Special Report 72-2. Naval Aerospace Medical Institute (NAMI), US Naval Aerospace Medical Centre, Pensacola, Florida, U.S.A. GROLL-KNAPP, E., WAGNER, H., HAUCH, H. and HAIDER, M. (1972) Staub 32
