An International Unit for the BAC with Educational A dvantages

Transcription

1 An International Unit for the BAC with Educational A dvantages Trevor C. Beard3 In various parts of the world the blood alcohol concentration (BAC) may be stated in millligrams per 100 millilitres, grams per litre ( parts per thousand ) or grams per 100 millilitres ( parts per cent ). Some authors who would normally use other systems convert their figures into per cent to conform with what they regard as international usage. Yet none of the systems in current use is entirely free from objection, and the absolute percentage is in many ways the least satisfactory. It is technically a loose term open to misinterpretation, it has educational disadvantages and it is now contrary to the recommendations of the General Conference o f the International Bureau o f Weights and Measures (CGPM). MISINTERPRETATION OF PERCENTAGES The percentage of blood alcohol could be calculated by weight or by volume. When the alternative is not stated, misinterpretation can introduce an error of about per cent, as 4 grams o f alcohol occupy about 5 millilitres. Traditionally a percentage can also be calculated by weight/volum e or by volume/weight; the Breathalyser is calibrated for the first of these. But although it reflects common usage, it is strictly incorrect to form a ratio out of two different physical quantities (mass and volume) and to call it a percentage; such a relationship is not recognised in either the International System or the legislation of nations which have adopted it. For example a percentage is not an Australian legal unit of measurement under the Commonwealth Weights and Measures (National Standards) Act EDUCATIONAL DISADVANTAGES OF PERCENTAGES If one purpose of legislation is to alter human behaviour, we should be aware of the importance of communication to the quality of the outcome. The small values obtained on the per cent scale are an obstacle to communication with the general public. Many people are hard to reach with decimal fractions, while those who understand them may conclude that the am ount o f alcohol under discussion is not very im portant. It is inconceivable that a commercial advertiser who set out to persuade people with figures, would choose to present them on a scale where every entry began with a decimal a Commonwealth Department o f Health, Canberra, ACT, Australia. 332

2 A n International Unit fo r the B A C with Educational Advantages 333 point, sometimes followed by a zero. When the BAC is expressed on such a scale there is a danger that everyone in court, including the bench and the press, can form the impression that the evidence is based on a very small amount of alcohol, with the verdict hanging on a variation of less than one tenth part of one per cent. Yet it is only the choice o f scale which gives this impression. When a highly significant concentration of alcohol is measured, it would be better to express it in figures which convey the real meaning of the result. The alternative methods already mentioned are not entirely free from objection. Although the Report of the National Health & Medical Research Council s Standing Committee on the Health Problems of Alcohol2 recommended the adoption of milligrams per 100 millilitres for the BAC units in Australia, the third digit may imply a degree of accuracy not present in all BAC measurements. More importantly, the 100 ml unit no longer conforms with preferred standard international practice and, if it were adopted, there would inevitably be pressure to change again at some future date. Grams per litre are more acceptable under the International System, but for educational purposes these units have the disadvantage that the decimal point is retained in the legal limit at the level imposed in several countries, and is retained in every 2-digit result. For reasons given below, there is also a preference under the International System for a substance of known m olecular weight to be m easured in m olar units. THE INTERNATIONAL SYSTEM OF UNITS The Systeme International (SI) d Unites was adopted by the G eneral Conference o f the International Bureau of Weights & Measures (CG PM ) in The Australian Government joined the International Metric Convention in 1947 and adopted the International System (SI) of Units in Implementation has been gradual, as in other countries. The SI units differ from some of the original metric units in definition, and in the reduction of the approved units to a manageable num ber of terms and symbols which can be used throughout the world in science and technology without risk of ambiguity. The unit of volume is the cubic metre, but the CGPM has accepted the litre (redefined as a cubic decimetre) for use with SI units. The mole is the am ount of substance with a mass in grams equal to its molecular weight3 and is now a standard SI unit. The concentration of a dissolved substance can be expressed in moles per cubic metre, moles per litre, or in molar derivatives such as millimoles per litre. Since molar units are directly proportional to the number of molecules present (one of the determinants of a chemical reaction), they are much more significant to the chemist and biochemist than units of mass such as milligrams, which bear no direct relationship to chemical effect. They are always preferred for a substance of known molecular weight, and are now widely used in clinical biochemistry in Australia, and are expected to be used exclusively in this country within a few years.1 Molar units allow the amounts of other substances present to be directly compared, and osmotic effects calculated. A person charged with a traffic offence involving the consum p tion of alcohol often has a BAC comparable with the blood sugar level in diabetic coma (20-40 millimoles per litre). This particular observation may be interesting rather than clinically relevant, but it illustrates the way a comparison is possible in molar units. It has been well established that a BAC between and per cent by weight/volume represents a lot of alcohol, with serious personal and social implications, but the conversion of these figures to millimoles per litre brings out some relationships in body chemistry not revealed by units of mass.

3 334 T. C. Beard SOME PRACTICAL ASPECTS OF CONVERSION Alcohol has a molecular weight of 46, so that 1 mole weighs 46 grams, and a concentration of 1 millimole per litre is equivalent to grams per 100 ml. Table I shows the relationship between some m olar units o f alcohol and some units o f mass, to the nearest third digit. TABLE I Molar concentrations o f alcohol and their equivalent in some units o f mass/volume in common use, rounded to the nearest third digit. SI UNITS mmol/ L g 'L (preferred) (acceptable) OBSOLESCENT UNITS mg/ 100ml per cent w/ V In practice, when the BAC is derived from breath analysis, 2 digits are considered sufficient on the percentage scale. The second digit (.01 per cent) corresponds very closely with 2 millimoles per litre (.0092 per cent), and so the existing level of accuracy can be carried on in the new units by expressing the result in even numbers. The scale should be calibrated with this in m ind (Figure 1). Translating the BAC into the am ount of alcohol consumed introduces many variables but, if one 200 ml glass of beer raises the BAC by abou* 0.01 per cent, as commonly stated, this still comes to about one interval on the scale, or 2 millimoles per litre. The Australian legal limits can be converted to molar units and rounded without altering them. With the automatic Breathalyser, it would be immaterial for example whether 0.05 became 10.9 or 11, or whether 0.08 became 17.4 or 17, as all these values would lie between the even numbers on the print-out. On other equipm ent with the dial calibrated in 2-millimole intervals, the same applies if the operator reads the result to the nearest 2 mmol interval, MILLIMOLES PER LITRE gg PER CENT WEIGHT/VOLUME Figure 1 M olar units fo r B A C compared with percentage, over the Breathalyser range o f values.

4 A n International Unit fo r the B A C with Educational Advantages 335 as suggested. In those parts of the world where the legal limit is 0.1 per cent (21.8 mm ol/l) the figure might be rounded to 20 w ithout serious opposition if it is accepted that this represents less than a whole glass o f beer. The printed symbol recommended internationally for millimoles per litre is mmol/1. Possible confusion between the letter 1 and the figure 1, on most typewriters can be avoided by using m m ol/l, and within Australia the Metric Conversion Board recommends the universal use of L for litre in both typescript and letterpress. This is also recommended in the United States, but the Bulletin of the World Health Organisation requires authors to write litre in full in typewritten manuscripts submitted for publication. In the daily newspapers an editor might prefer to print millimoles per litre where the context seemed to call for accuracy, but otherwise the average reader could be well catered for with looser terms such as units, SI units or molar units. On the hospital wards unit made a satisfactory informal term for units o f 500 ml when blood transfusion were no longer given in pints. CONCLUSION One of the first rules of communication is to use language (and figures) that the audience can readily understand. It is not suggested that the average person would understand millimoles any better than percentages. W hat is suggested is that most people will understand 20 better than 0.09, and 50 better than The term millimole is for the scientist and the legislator, and could well be replaced by unit for general use. The conversion to molar units which is gradually taking place in science, technology and medicine will assist public education and law enforcement when applied to blood alcohol. It will be in strict accordance with an international reform of weights and measures to which member nations of the International Metric Convention are already committed, and its early im plem entation is desirable. REFERENCES 1. Carter, N., SI units in Clinical Pathology. Australian Family Physician, 5: passim, National Health and Medical Research Council. Report of the Standing Committee on Health Problems of Alcohol, National Health and Medical Research Council, Canberra, Owen, J. A., Edwards, R. G. & Coller, B. A. W., The Mole in Medicine and Biology, Edinburgh, E. & S. Livingstone, BIBLIOGRAPHY Australian Standard , The International System (SI) Units and Their Application,1Standards Association, of Australia, Baron, D. S. et al. The Use of SI units in Reporting Results Obtained in Hospital Laboratories, Journal of Clinical Pathology, 27, , ISO-lOOO-1973, International Standard 1000, SI Units and Recommendations for the Use of Their Multiples and Certain Other Units, International Organisation for Standardisation, National Physical Laboratory, Ministry of Technology SI: the International System of Units, London, Her Majesty s Stationery Office, Royal College of Pathologists of Australia and Australian Association of Clinical Biochemists, SI Units and You 1973 Young, D. S., Standardised Reporting of Laboratory Data, New England Medical Journal, 290(7): , 1974.

5