Ann. oecup. Hyg. VoL 18, pp. 2£-35. Pergamon Press 1975. Printed in Great Britain

THE THRESHOLD LIMIT VALUE FOR CARBON MONOXIDE S. J. SILK

Abstract—A summary of gassing accidents from carbon monoxide reportable under the Factories Act 1961 in the United Kingdom is given. Threshold limit values (TLVs) are defined and that for carbon monoxide is discussed. Acceptable levels of industrial exposure to carbon monoxide in other countries are referred to, together with the possible future trend of TLVs particularly in Western Europe. Difficulties of measurement and assessment of exposures in relation to the TLV are summarized. SECTION 80 of the Factories Act 1971 requires the notification of any accident where (a) loss of life is caused to a person employed in the factory, or (b) a person is disabled for more than three days from earning full wages at the work at which he was employed. Between the years 1962 and 1972 the number of notified cases of gassing said to be due to carbon monoxide in the United Kingdom halved from about 100 to 50 per year. Much of this reduction was no doubt due in the main to the replacement of manufactured gas with a substantial carbon monoxide content, by natural gas which does not contain carbon monoxide. Between 2 per cent in 1966 and 15 per cent in 1967 of these cases were fatal. It may be said that approximately 50 reportable cases of gassing from carbon monoxide per year is not a large number for the big industrial complex of this country, but some of these are fatal and in addition, there is undoubtedly a much larger number of workpeople gassed by carbon monoxide who are not away from their job for sufficient time for the incident to be reportable under the Factories Act. Also, in many cases the severity of the gassing is purely fortuitous. The most hazardous plants associated with carbon monoxide gassing are blast furnaces and coke ovens, particularly in the steel industry, which in 1972 was responsible for about 60 per cent of the total reportable cases of carbon monoxide gassing in this country.

THRESHOLD LIMIT VALUES

The question then arises as to what is the safe level of industrial exposure to carbon monoxide. For many years now the Department of Employment, HM Factory Inspectorate, have adopted the threshold limit values published by the American Conference of Governmental Industrial Hygienists (ACGIH, 1973), and these are published in the Department's Technical Data Note No. 2 (D. of E., 1974). Threshold limit values refer to airborne concentrations of substances and represent conditions under which it is believed that nearly all workers may be repeatedly exposed day after 29

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H.M. Factory Inspectorate, Department of Employment, Watford, Herts.

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S. J. SILK

THRESHOLD LIMIT VALUE FOR CARBON MONOXIDE AND DOCUMENTATION

The present threshold limit value for carbon monoxide, listed by the American Conference of Governmental Industrial Hygienists (ACGIH, 1973), in Technical Data Note No. 2 (D. of E, 1974), is 50 ppm of air by volume. The studies concerning the subject are so numerous that a book could be written about them. The Documentation of Threshold Limit Values (ACGIH, 1971) alone has 29 references. To put it simply, carbon monoxide is a chemical asphyxiant forming carboxyhaemoglobin in the blood, which reduces the availability of oxygen for the cellular systems of the body. The difficulty in assigning a threshold limit value for carbon monoxide is demonstrated by the fact that the health effect on exposed persons is dependent on: (i) (ii) (iii) (iv) (v)

The carbon monoxide content of the blood; the partial pressure of the oxygen in the air breathed; the duration of exposure; the ambient temperature; the work effort (the rate of uptake of carbon monoxide by the blood increases from three- to six-fold between rest and heavy work); (vi) the metabolic efficiency of the worker; (vii) the worker's health status; and (viii) the genotype, which determines the degree of, and capacity for inurement to exposure.

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day without adverse effect. Because of wide variation in individual susceptibility, however, a small percentage of workers may, in some cases, experience discomfort below the threshold limit value and a smaller percentage may be seriously affected by aggravation of a pre-existing condition or by development of occupational illness. Suitable screening tests for workers therefore improve this "coverage" of threshold limit values. Threshold limit values refer to time-weighted concentrations in air for a 7 or 8 hr working day and 40 hr working week. They should be used as guides in the control of health hazards and not as fine lines between safe and dangerous concentrations. Time-weighted averages permit excursions above the limit, provided they are compensated by excursions below the limit during the workday. The relationship between the threshold limit value and the excursion above the threshold limit value is laid down, but is a rule-of-thumb exercise and in certain cases may not apply. Threshold limit values are based on the best available information from industrial experience, experimental human and animal studies, and when possible, from a combination of the three. The basis on which the values are established may differ from substance to substance. Protection against impairment to health may be the guiding factor for some, whereas reasonable freedom from irritation, narcosis, nuisance, and other forms of stress form the basis for others. Some substances are assigned a ceiling value which places a definite boundary which concentrations in air should not be permitted to exceed generally for periods of more than 15 min. Exposures to two or more substances need special consideration.

The threshold Emit value for carbon monoxide

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The relationship between exposure to carbon monoxide and the carboxy-haemoglobin content of the blood is very well documented, and equilibrium is usually attained after about 6-8 hr. The recommended threshold limit value for carbon monoxide of 50 ppm is based on an air concentration that should not result in blood carboxyhaemoglobin levels above 10 per cent, a level which, according to the documentation is just below that at which signs of borderline effects develop. THRESHOLD LIMIT VALUES IN OTHER COUNTRIES

THE THRESHOLD LIMIT VALUE FOR CARBON MONOXIDE IN THE FUTURE

Both the World Health Organisation and the National Institute for Occupational Safety and Health in the USA (NIOSH, 1972) recommend that the maximum acceptable level for carboxy-haemoglobin in the blood should be 5 per cent. To comply with this level would mean the lowering of the threshold limit value of carbon monoxide to 35 ppm. The NIOSH document says that this will: 1. prevent acute carbon monoxide poisoning; 2. protect the employee from deleterious myocardial alterations associated with levels of carboxy-haemoglobin in excess of 5 per cent; and 3. provide the employee with protection from adverse behavioural manifestations resulting from exposure to low levels of carbon monoxide. Because of the well-defined relationship between smoking and the concomitant exposure to carbon monoxide in inhaled smoke the criteria standard may not, of course, provide the same protection for smokers as for non-smokers. Likewise, with work at high altitudes, e.g. 5000-8000 ft above sea level, the standard needs to be appropriately lowered to compensate for loss of oxygen-carrying capacity of the blood. Workers with physical impairments that interfere with normal oxygen delivery to the tissues are likewise affected. The ceiling value of 200 ppm suggested in the NIOSH document is probably a more realistically-attainable level in blast furnace areas than the "rule-of-thumb" excursion value of 75 ppm computed from the ACGIH formula of 1 -5 x TLV for substances with

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There appears to be no divergence between the American Conference of Governmental Industrial Hygienists' threshold limit value for carbon monoxide and the acceptable level of industrial exposure in any of the Western Countries. In the USSR however, the listed maximum allowable concentration for carbon monoxide is 18 ppm by volume in air. The stated concept of the maximum allowable concentration in the USSR is that it should be used to denote such concentrations of harmful chemical substances in the form of gases, vapours, and dusts in industrial atmospheres, which, by their constant effect on workers during normal daily work over an indefinite period of time, produce no pathological change or disease detectable by current methods of examination. Such maximum allowable concentrations, therefore, can produce neither acute nor chronic occupational poisoning. It is acknowledged however that special, higher concentrations may be established for various types of work which involve exposure to toxic gases for short periods only, and one example quoted is work in the presence of exhaust gases in garages, which will almost certainly involve carbon monoxide.

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TLVs between 10 and 100 ppm by volume in air. Compliance with the time-weighted TLV still applies. Although there appears to be movement towards a lower threshold limit value for carbon monoxide, there is no information to date on any notice of intended change in either Western European or ACGIH lists.

ASSESSMENT OF EXPOSURES IN RELATION TO THE THRESHOLD LIMIT VALUE This, without doubt, is a very difficult aspect of the whole problem. Biological monitoring of the carboxyhaemoglobin content of the blood will provide an accurate measurement of the immediate past exposure and is an important factor in the control problem. However, where smokers are concerned, the situation is more obscure. Every effort is needed to prevent exposure in the first place, and while it is easy enough to make an accurate assessment of the carbon monoxide content of the air, in the absence of a reliable personal monitor it is very difficult to relate this figure to personal exposure, particularly over a long period, as in most problem areas the carbon monoxide concentration is not homogeneous. The tendency in problem areas such as in the vicinity of blast furnaces is for continuous monitoring of the atmosphere with an alarm system set at an appropriate level to provide warning if there is an escape of gas. In this connection a considerable amount of work is being carried out in the steel industry which should eventually lead to a much better control of a difficult problem. The industrial aspects of exposure to carbon monoxide are discussed in booklets published by H.M. FACTORY INSPECTORATE (1971) and BRITISH STEEL CORPORATION (1971). REFERENCES AMERICAN CONFERENCE OF GOVERNMENTAL INDUSTRIAL HYOIENISTS (1971) Documentation of the

Threshold Limit Values for Substances in Workroom Air. 3rd Ed. ACGIH, Cincinnati, Ohio. AMERICAN CONFERENCE OF GOVERNMENTAL INDUSTRIAL HYOIENISTS (1973) Threshold Limit Values

for Substances in Workroom Air. ACGIH, Cincinnati, Ohio. BRITISH STEEL CORPORATION (1971) Prevention of Gassing Accidents in Iron and Steel Works. Brit. Steel Corp., London. * Maximum allowable concentrations were originally conceived with the ceiling value concept in mind, but are now generally accepted in Western Europe as an 8 hr time-weighted concentration.

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EUROPEAN THRESHOLD LIMIT VALUES AND MAXIMUM ALLOWABLE CONCENTRATIONS The only major European list of values appears to come from Western Germany (MAX values*) but the Council of Europe are currently preparing two lists, one where there are no differences between European and USA values and the other, much shorter list, where differences occur. Many of these differences are probably due to the annual up-dating of the ACGIH list. Information and documentation is being sought concerning toxic substances where differences occur, so that a decision can be made on which levels are to be accepted. At the moment, there is no divergence of opinion concerning carbon monoxide and the threshold limit value of 50 ppm by volume in air.

The threshold limit value for carbon monoxide

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H.M. FACTORY INSPECTORATE (1971) Carbon monoxide poisoning: Causes and Prevention. Dept. of Employment Health and Safety at Work Series No. 29. H.M.S.O., London. H.M. FACTORY INSPECTORATE (1974) Threshold limit values for 1973. Dept. of Employment Technical Data Note No. 2, H.M.S.O., London. NATIONAL INSTITUTE OF OCCUPATIONAL SAFETY AND HEALTH (1972) Criteria for a recommended

standard—occupational exposure to carbon monoxide, Publ. HSM-11000. U.S. Dept. of Health, Education & Welfare, Cincinnati, Ohio.

n == 10 m == 70 r == 190

A. B. 2 0 % of 35 r / x r 1-81

= 35 = 7 x 190

10

A+ B+ C

— Id

=76

•NIOSH (1972) Publ. HSM-11000. U.S. Dept. of Health, Education and Welfare, Cincinnati, Ohio.

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DISCUSSION Dr J. G. JONES (British Steel Corporation): In our 1973 survey of blast furnace workers, we found that 43 per cent of smokers in the blast furnace group and 36 per cent of smokers in the control group came to work before 06.00 hr with COHb levels of 5 per cent or more. This seems to suggest that there is little point setting a standard at this level. On another point, NIOSH* have issued instructions for determining compliance with the TLV of 35 ppm, based on a small number of instantaneous (grab) samples collected at random intervals during the workday: "Given: the results of n samples with a mean m and a range (difference between least and greatest) r. If, for from 3 to 10 samples, m is greater than the total of: A. The standard B. The percentage of systematic instrument error multiplied times the standard C. ' - * ! n then the true average concentration exceeds the standard (P < 0-05). The value of / is taken from the following table: n (number of samples) t (student's "t" test value) 3 2-35 4 2-13 5 2-01 6 1-94 7 1-89 8 1-86 9 1-83 10 1-81" The following example of a test of compliance has been carried out according to the NIOSH instructions using the results of ten indicator tube samples taken at random intervals during one 8hr shift: Sample number CO concentration (ppm) 1 10 2 0 3 30 4 150 5 190 6 150 7 100 8 20 9 0 50 10

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The average concentration, m, is therefore not greater than the total of A + B + C and the true average concentration docs not exceed the TLV. Thus it can be seen that the atmospheric concentration can fluctuate considerably within a ceiling level of 200 ppm without exceeding the TLV. Mr SILK: Air sampling to show compliance with TLV is difficult and the use of snap sampling can be misleading. For example, concerning lead, I have heard of a case reported in the U.S.A. where a firm has been fined on the basis of one short-term sample. While it will show whether there is compliance at that particular time, it is not very realistic in assessing the overall hazard. Mr VERDIN (Analysis Automation, Oxford): Does the error in the instrument (B) contribute appreciably to the sum of A + B + C? Had the error been only 1 per cent in your example, A + B + C would have been 69-65, i.e. lower than m, and the result would have been the other way round. Dr JONES: I hope to put that into perspective in my paper on warning levels and alarms (this issue pp. 79-82), but if the samples are taken with an indicator tube, the possible error is 20 percent. Mr VERDIN : In the USA some 200 NIOSH students of occupational hygiene have been equipped with Ecolysers with which these errors have, to a large extent, been met. Mr SILK: In order to prevent hazard to the worker it would be far better to have a switching monitor in suitable sampling positions which would at least give an alarm when the carbon monoxide content reaches a certain level. Snap sampling is obviously some help but does not give the worker sufficient protection over the whole working period. Dr J. A. BONNELL (Central Electricity Generating Board): When you speak of an excursion figure of 200 ppm, how long is the excursion? Would it be measured in minutes or seconds? Mr SILK: This is a question which is causing a lot of controversy at the moment. A ceiling value is the level which should not be exceeded, but as with excursion values, the ACGIH suggest this is the value which should produce no adverse effects for up to 15 min. At the moment there is a working party, set up by British Steel Corporation, considering, among other things, the time that should be allowed at the ceiling value or excursion value in relation to the threshold limit value for carbon monoxide. Dr J. R. GLOVER (Wesh National School of Medicine): Surely the time allowed for testing at a ceiling value should be less than it would take to kill the man. Mr SILK: In the present TLV lists the excursion value for carbon monoxide is 75 ppm. I think it would be generally agreed that this is a concentration which is unlikely to affect the average person within 30 min—double the suggested time length for an excursion value. The TLV concept for 8hr should still be observed. Protection against higher levels by continuous monitoring with alarms, as I mentioned before, is most important in relation to the acute effect. Dr WALD (University of Oxford): Dr Jones asked whether it was necessary to pay as much attention as has been done to TLV CO values, since smoking is a more important source of exposure to CO than occupational sources. Two points are relevant here: (1) People choose to smoke and expose themselves to CO as a result, whereas the purpose of TLV values for CO is to protect people from the effects of CO when they have not had the opportunity to make such a choice. These two aspects should be considered separately: (2) The effects of occupational and smoking CO exposure on COHb level is additive in so far as a smoker will tend to raise his COHb level above the "background" equilibrium level. Mr SILK: In this context it would be difficult to sort out the industrial exposure to CO. Dr WALD: I agree. An important point is that when a person smokes his COHb will rise above a higher "background" if he is exposed to carbon monoxide than if he is not—so, for example, his COHb might be 11 instead of 8 per cent. Mr SILK: It is obviously a problem. Lt-Cdr R. F. COLVTLL (British Steel Corporation): A CO dosemeter is under trial in the BSC. One of the difficulties may be that it could have a relatively slow response to an alarm level. It is very important, therefore, that the times during which peak values can be acceptable should be defined. Are there any such figures? Or have you any comments? Mr SILK: It is important to have an instrument which is very quick in response, to deal with emergency situations. Also to know what levels can be safely tolerated for short periods. No figures have been produced so far, but this is one of the objects of the working party which has been set up to consider all aspects of this matter; for example, the NIOSH recommendation is for a ceiling value of 200 ppm—if this is acceptable, for what period of time can it be tolerated, bearing in mind that compliance is also required with the TLV of 50 ppm? Dr J. M. CAMPBELL (British Steel Corporation, General Steels Division Research Organisation): The hazards of carbon monoxide fall into two distinct areas: acute toxicity—which we have been discussing—and chronic toxicity. It is easy to arrive at methods of measurement, working practices and systems of work for the avoidance of gassing incidents, but chronic toxicity is less well defined and difficult to measure because of the medical considerations and the behavioural factor.

The threshold limit value for carbon monoxide

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One other comment is that the NIOSH criteria document says that 500 ppm is an alarm level for carbon monoxide in an acute toxicity area; this figure should be included in the discussion also. Mr SILK: I do not know whether the NIOSH criteria are used for the ACGIH TLV for carbon monoxide and hence for Technical Data Note No. 2; I have not yet seen any notice of a proposed change. The Eastern European countries differ in that they suggest a level which they consider will be safe against both acute and chronic effects. Whether such levels are attainable in, for example, blast furnace areas, is problematical. There is not much information on the chronic aspects of exposure; the paper by JONES and SINCLAIR (this issue pp. 15-20) is one of the few which has investigated this aspect of the problem.

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The threshold limit value for carbon monoxide.

Ann. oecup. Hyg. VoL 18, pp. 2£-35. Pergamon Press 1975. Printed in Great Britain THE THRESHOLD LIMIT VALUE FOR CARBON MONOXIDE S. J. SILK Abstract—...
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