Occupational and

Environmental MedicineAdopted as the Journal of the Faculty of

Occupational Medicine of the Royal College ofPhysicians of London

Editor: Anne Cockcroft (United Kingdom)Technical Editor: Judith HaynesEdiwrial Assistant: Rachel HarveyEditrial BoardT C Aw (United Kingdom) R McNamee (United Kingdom) M Singal (United States)

F J H Van Dijk (Holland) J Myers (South Africa) D C Snashall (United Kingdom)

J S Evans (United States) B Nemery (Belgium) 0 Svane (Denmark)RM Harrison (United Kingdom) T Okubo (Japan) G Theriault (Canada)

J Jeyaratnam (Singapore) P Paoletti (Italy) KM Venables (United Kingdom)F Kauffmann (France) L Rosenstock (United States) Editor, Briish MedicalJournalR L Maynard (United Kingdom) M Sim (Australia)

NOTICE TO CONTRIBUTORS Occupational and EnvironmentalMedicine is intended for the publication of original contribu-tions relevant to occupational and environmental medicine,including toxicological studies of chemicals of industrial, agri-cultural, and environmental importance, and epidemiologicalstudies. As well as full papers, short papers dealing with briefor preliminary observations relevant to occupational and envi-ronmental medicine will also be considered. Case reportsshould cover substantial new ground to merit publication.Other articles, including review or position papers, will be con-sidered but should not be submitted without first approachingthe Editor to discuss their suitability for the Journal. Letters tothe Editor are always welcome.INSTRUCTIONS TO AUTHORS Three copies of all submissionsshould be sent to: The Editor, Occupational and EnvironmentalMedicine, BMJ Publishing Group, BMA House, TavistockSquare, London WC1H 9JR, UK. All authors should sign thecovering letter as evidence of consent to publication. Papersreporting results of studies on human subjects must be accom-panied by a statement that the subjects gave written, informedconsent and by evidence of approval from the appropriateethics committee. These papers should conform to theprinciples outlined in the Declaration of Helsinki (BMJ1964;ii:177).

If requested, authors shall produce the data on which themanuscript is based, for examination by the Editor.Authors are asked to submit with their manuscript the

names and addresses of three people who they considerwould be suitable independent reviewers. They will notnecessarily be approached to review the paper.

Papers are considered on the understanding that they aresubmitted solely to this Journal and do not duplicate materialalready published elsewhere. In cases of doubt, where part ofthe material has been published elsewhere, the publishedmaterial should be included with the submitted manuscript toallow the Editor to assess the degree of duplication. TheEditor cannot enter into correspondence about papers rejectedas being unsuitable for publication, and the Editor's decisionin these matters is final.Papers should include a structured abstract of not

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lished by the first identification in the text of a particular tableor illustration. Include only references essential to the argu-ment being developed in the paper or to the discussion ofresults, or to describe methods which are being used when theoriginal description is too long for inclusion. Information frommanuscripts not yet in press or personal communicationsshould be cited in the text, not as formal references.Use the Vancouver style, as in this issue for instance, for a

standard journal article: authors (list all authors when seven orfewer, when eight or more, list only six and add et al), title,abbreviated title of journal as given in Index Medicus (if notin Index Medicus give in full), year of publication, volumenumber, and first and last page numbers.Proofs Contributors will receive one proof. Only minor cor-rections can be made at this stage; corrections other thanprinter's errors may be charged to the author.

Reprints Reprints will be charged for. The number ofreprints required should be stated on the form provided withthe proofs.Copyright c 1995 Occupational and Environmental Medicine.This publication is copyright under the Berne Convention andthe International Copyright Convention. All rights reserved.Apart from any relaxations permitted under national copyrightlaws, no part of this publication may be reproduced, stored ina retrieval system, or transmitted in any form or by any meanswithout the prior permission of the copyright owners.Permission is not, however, required to copy abstracts ofpapers or of articles on condition that a full reference to thesource is shown. Multiple copying of the contents of the publi-cation without permission is always illegal.

NOTICE TO ADVERTISERS Applications for advertisementspace and for rates should be addressed to the AdvertisementManager, Occupational and Environmental Medicine, BMJPublishing Group, BMA House, Tavistock Square, LondonWC1H 9JR.NOTICE TO SUBSCRIBERS Occupational and EnvironmentalMedicine is published monthly. The annual subscription rate(for 1995) is £139 (US $240). Orders should be sent to theSubscription Manager, Occupational and EnvironmentalMedicine, BMJ Publishing Group, BMA House, TavistockSquare, London WC1H 9JR. Orders may also be placed withany leading subscription agent or bookseller. (For the conve-nience of readers in the USA subscription orders with or with-out payment may also be sent to British Medical Journal, POBox 408, Franklin, MA 02038, USA. All inquiries, however,must be addressed to the publisher in London). All inquiriesregarding air mail rates and single copies already publishedshould be addressed to the publisher in London.Subscribers may pay for their subscriptions by Access, Visa, orAmerican Express by quoting on their order the credit orcharge card preferred together with the appropriate personalaccount number and the expiry date of the card.Second class postage paid Rahway NJ. Postmaster: sendaddress changes to: Occupational and Environmental Medicine,c/o Mercury Airfreight International Ltd Inc, 2323 RandolphAvenue, Avenel, NJ 07001, USA.FACULTY OF OCCUPATIONAL MEDICINE The Faculty ofOccupational Medicine of the Royal College of Physicians ofLondon is a registered charity founded to promote, for thepublic benefit, the advancement of knowledge in the field ofoccupational medicine. The Faculty has offices at 6 StAndrew's Place, Regent's Park, London NW.1 4LB.

ISSN 1351-0711.

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Schins, Borm

and tumor necrosis factor-a by alveolar macrophages incoal workers' pneumoconiosis: comparison betweensimple pneumoconiosis and progressive massive fibrosis.Exp Lung Res 1990;16:73-80.

17 International Labour Organisation. Guidelines for the use ofILO international classification of radiographs ofpneumoco-nioses. Geneva: International Labour Office, 1980(Occupational Safety and Health Series No 22 revised).

18 Debets JMH, van der Linden CJ, Spronken IEM,Buurman WA. T-cells mediated production of TNF-alpha by monocytes. Scand3 Immunol 1988;27:601-9.

19 Lamers RJS, Schins RPF, Wouters EFM, vanEngelshoven JMA. Computed tomography of the lungsin coal workers with a normal chest radiograph. ExpLung Res 1994;21:411-9.

20 Remi-Jardin M, Degreef JM, Beuscart R, Voisin C, RimiJ. Coal worker's pneumoconiosis: CT assessment inexposed workers and correlation with radiographic find-ings. Radiology 1990;177:363-71.

21 Hanley JA, McNeil BJ. The meaning and use of the areaunder a receiver operating characteristic (ROC) curve.Radiology 1982;143:29-36.

22 van der Schouw YT, Segers MFG, Smits L, ThomasCMG, Verbeek ALM, Wobbes Th. Towards a morestandardized assessment of diagnostic tumour markers.IntJ Oncol 1993;3:979-85.

23 Gosset P, Lasalle P. Vanhie D, Waillaert B, Aerts C,Voisin C, Tonnel AB. Production of tumour necrosisfactor-a and interleukin-6 by human alveolar macro-phages exposed in vitro to coalmine dust. Am J RespirCellMolBiol 1991;5:431-6.

24 Vanhae D, Gosset P, Marquette CH, Wallaert B, LafitteUJ, Gosselin B, et al. Secretion and mRNA expression of

TNF-a and IL-6 in the lung of pneumoconiotic patients.Amy Respir Dis 1993;147:A906.

25 Attfield M, Wagner G. Respiratory disease in coal miners.In: Rom WN, ed. Environmental and occupational medi-cine, 2nd ed Boston: Little, Brown 1992:325-44.

26 Soutar C, MacLaren W, Annis R, Melville A. Quantitativerelations between exposure to respirable coal mine dustand coalworkers' simple pneumoconiosis in men whohave worked as miners but have left the coal industry.Br Ind Med 1986;43:29-36.

27 Mahieu B. L'ivoluton de la pneumoconiose en France aucours des 20 dernims annies (Corngendum expose). Hygieneindustrial dans les mines. Brussels: Commission desCommunautes Europeennes Direction GeniraleEmploi, Relations Industrielles et Affaires Sociales;Direction Sante et Securiti V/E/4, 1990.

28 Pantelidis P, Southcott AM, Du Bois RM. A comparisonof single cell TNF-a secretion in blood and lungmononuclear cells in interstitial lung disease. Am 7RespirDis 1993;147:A756.

29 Gordon Tanner W, Burress Welborn M, SheperdVL. Tumor necrosis factors and interleukin-la syner-gistically enhance phorbol myristate acetate-inducedsuperoxide production by rat bone marrow-derivedmacrophages. Am J Respir Cell Mol Biol 1992; 7:379-84.

30 Pfeffer KD, Huecksteadt TP, Hoidal JR Expression andregulation of tumor necrosis factor in macrophages fromcystic fibrosis patients. Am Y Respir Cell Mol Biol1993;9:51 1-19.

31 Dubar V, Gosset P. Aerts C, Voisin C, Wallaert B, TonnelAB. In vitro acute effects of tobacco smoke on tumournecrosis factor a and interleukin-6 production by alveo-lar macrophages. Exp Lung Res 1993;19:345-59.

Vancouver styleAll manuscripts submitted to Occup EnvironMed should conform to the uniformrequirements for manuscripts submitted tobiomedical journals (known as theVancouver style.)

Occup Environ Med, together with manyother international biomedical journals, hasagreed to accept articles prepared in accor-dance with the Vancouver style. The style(described in full in the BMJ, 24 February1979, p 532) is intended to standardiserequirements for authors.

References should be numbered consec-utively in the order in which they are firstmentioned in the text by Arabic numeralsabove the line on each occasion the refer-ence is cited (Manson' confirmed otherreports2-5 . . .). In future references topapers submitted to Occup Environ Med

should include: the names of all authors ifthere are seven or less or, if there are more,the first six followed by et al; the title ofjournal articles or book chapters; the titlesof journals abbreviated according to thestyle ofIndex Medicus; and the first and finalpage numbers of the article or chapter.Titles not in Index Medicus should be givenin full.

Examples of common forms of refer-ences are:

1 International Steering Committee of Medical Editors,Uniform requirements for manuscripts submitted tobiomedical journals. BrMedJ 1979;1:532-5.

2 Soter NA, Wasserman SI, Austen KF. Cold urticaria:release into the circulation of histamine and eosino-phil chemotactic factor of anaphylaxis during coldchallenge. N EnglJ7Med 1976;294:687-90.

3 Weinstein L, Swartz MN. Pathogenic properties ofinvading micro-organisms. In: Sodeman WA Jr,Sodeman WA, eds. Pathologic physiology, mechanismsofdisease. Philadelphia: W B Saunders, 1974:457-72.

450

Electroencephalographic study ofdivers with histories of neurological decompression illness

somatosensory evoked responses to posteriortibial and median nerve stimulation werecompared with those of non-divers. No evi-dence was given for the presence of subclini-cal damage to the neuraxis."8

Notwithstanding the ability of EEGs todetect subclinical changes in a wide variety ofpathological processes, it is possible that thetechnique may be insufficiently sensitive todetect the more subtle of the cumulative cere-bral changes that might occur in divers.

The study was funded by the Department of Energy andwas supported by the Medical Director General (Naval).The assistance of Surgeon Commander J J W Sykes,Dr M R Cross, Dr M R Hamilton-Farrell, Dr J D Cole, andDr L Merton is gratefully acknowledged.

1 Palmer AC, Calder IM, Yates PO. Cerebral vasculopathyin divers. Neuropathol Appl Neurobiol 1992;I8:1 13-24.

2 Polkinghorne PJ, Cross MR, Sehmi K, Minassian D, BirdAC. Ocular fundus lesions in divers. Lancet 1988;ii:1381-3.

3 Peters BH, Levin HS, Kelly PJ. Neurological and psycho-logic manifestations of decompression illness in divers.Neurology 1977-27:125-7.

4 Leach J, King J, Morris PE, Rawlins J. Memory and per-sonality changes of caisson workers with a history ofdecompression illness. In: Engineering and health in com-pressed air workers. Proceedings ofa conference. Cambridge:CIRIA, 1992.

5 Seppalainen AM, Lindstrom K, Martelin T. Neuro-physiological and psychological picture of solventpoisoning. Am J Ind Med 1980;1:31-42.

6 Murrison AW. The contribution of neurophysiologicaltechniques to the investigation of diving-related illness.Undersea Hyperb Med 1993;20:347-3.

7 Rozsahegyi I, Roth B. Participation of the central nervoussystem in decompression. Industrial Medicine and Surgery1966;35:101-8.

8 Leitch DR, Hallenbeck JM. A model of spinal cord dys-barism to study delayed treatment: II effects of treat-ment. Aviat Space Environ Med 1985;55:679-84.

9 Kwiatkowski SR Analysis of EEG records among divers.Bulletin of the Institute ofMedicine (Morsk) 1979;2:131-5.

10 Dolmierski R, Kwiatkowski SR, Nitka J, Palubicki J, LabaL. Neurological, psychiatric and psychological examina-tions of divers in the light of their professional work. BullInst Mar Trop Med Gdynia 1981;32:141-52.

11 Todnem K, Skeidsvoll H, Svihus R, Rinck P, Riise T,Kambestad BK, Aarli JA. Electroencephalography,evoked potentials and MRI brain scans in saturationdivers. An epidemiological study. Electroencephalogr ClinNeurophysiol 1991;79:322-9.

12 Fritz H, Hossmann KA. Arterial air embolism in cat brain.Stroke 1979;10:581-9.

13 Ingvar D H, The pathophysiology of occlusive cerebrovas-cular disorders related to neuroradiological findings,EEG, and measurements of regional cerebral blood flow.Acta Neurol Scand 1967;43:93-107.

14 Malhotra MS, Kumar CM. Electroencephalography innaval divers. Aviat Space Environ Med 1975;46:1000-1.

15 Cobb WA. The normal adult EEG. In: Hill D, Parr G,eds. Electroencephalography. New York: MacMillan 1963;232-49.

16 Hernberg S. Epidemiology in occupational health. In:Zenz C ed. Occupational medicine. Principles and practicalapplications 2nd ed. Chicago: Year Book MedicalPublishers 1988;1 132-3.

17 Hjorth R, Edmonds C, Vignaendra V. Electro-encephalographic and evoked cortical potentials assess-ments in divers. In: Edmonds C, ed. Abalone Diver.National Safety Council of Australia 1986;115-30.

18 Murrison AW, Glasspool E, Pethybridge RJ, Francis TJR,Sedgwick EM. Neurophysiological assessment of diverswith histories of neurological decompression illness.Occup Environ Med 1994;51:730-4.

19 Elliott DH, Moon RE. Manifestations of the decompres-sion disorders. In: Bennett PB, Elliott DH, eds. Thephysiology and medicine of diving, 4th ed London: W BSaunders, 1993.

Instructions to authorsThree copies of all submissions should besent to: The Editor, Occupational andEnvironmental Medicine, BMJ PublishingGroup, BMA House, Tavistock Square,London WC1H 9JR, UK. All authorsshould sign the covering letter as evidenceof consent to publication. Papers reportingresults of studies on human subjects mustbe accompanied by a statement that thesubjects gave written, informed consentand by evidence of approval from theappropriate ethics committee. These papersshould conform to the principles outlinedin the Declaration of Helsinki (BMJt1964;ii: 177).

If requested, authors shall produce thedata on which the manuscript is based, forexamination by the Editor.Authors are asked to submit with

their manuscript the names andaddresses of three people who theyconsider would be suitable independentreviewers. They will not necessarily beapproached to review the paper.Papers should include a struc-

tured abstract of not more than 300words, under headings of Objectives,Methods, Results, and Conclusions.Please include up to three keywords orkey terms to assist with indexing.

453

469Lorry drivers' work stress evaluated by catecholamines excreted in urine

Djordjevik M. Mechanisms of cardiac risk among profes-sional drivers. Scand _7 Work Environ Health 1994;20:73-86.

5 McDonald N. Fatigue, safety, and the truck driver. London:Taylor and Francis, 1984.

6 Ursin H. Personality, activation and somatic health: a newpsychosomatic theory. In: Levine S, Ursin H, eds.Coping and health. New York: Plenum Press,1980:259-79.

7 Knardahl S, Ursin H. Sustained activation and the patho-physiology of hypertension and coronary heart disease.In: Orlebeke JF, Mulder G, Van Doornen LJH, eds.Psychophysiology of cardiovascular control. Models, methodsand data. New York: Plenum Press, 1985:223-37.

8 Moleman P, Tulen JHM, Blankestijn PJ, Man in 't VeldAJ, Boomsma F. Urinary excretion of catecholaminesand their metabolites in relation to circulating cate-cholamines. Arch Gen Psychiat 1992;49:568-72.

9 Frankenhaeuser M. Psychoneuroendocrine approaches tothe study of emotion as related to stress and coping. In:Howe HE, Dienstbier RA, eds. Nebraska Symposium on

Motivation 1978. Lincoln: University of Nebraska Press,1979: 123-61.

10 Dimsdale JE, Moss J. Plasma catecholamines in stress andexercise. JAAA 1980;243:340-3.

11 Mackie RR, Miller JC. Effects of hours of service, regularity ofschedules and cargo loading on truck and bus driver fatigue.Goleta, California: Human Factors Research, 1978.(Technical report 1765-F.)

12 Jenner DA, Reynolds V, Harrison GA. Catecholamineexcretion rates and occupation. Ergonomics 1980;23:237-46.

13 Mulders HPG, Meijman TF, O'Hanlon JF, Mulder G.Differential psychophysiological reactivity of city bus dri-vers. Ergonomics 1982;25:1003-11.

14 Kompier MAJ, Mulders HPG, Broersen JPJ, Westerink B,Meijman TF. Psychosomatic complaints and absencebehaviour as indicators of future breakdown in adapta-tion. In: Oborne DJ, ed. Contemporary ergonomics 1986.London: Taylor and Francis, 1986:232-6.

15 Mulders HPG, Meijman TF, Kompier MAJ, Mulder G,Broersen JPJ, Westerink B, O'Hanlon JF. Occupationalstress in city bus drivers. In: Rothengather JA, De BruinRA, eds. Road user behaviour, theory and research. Assen:Van Gorcum, 1988:152-63.

16 Vivoli G, Bergomi M, Rovesti S, Carrozzi G, Vezzosi A.Biochemical and haemodynamic indicator of stress intruck drivers. Ergonomics 1993;36:1089-97.

17 Van der Beek AJ, Frings-Dresen MHW. Physical workloadof lorry drivers: a comparison of four methods of trans-

port. Ergonomics 1995;38:(in press).18 Boos K-S, Wilmers B, Sauerbrey R, Schlimme E.

Development and performance of an automated HPLC-analyzer for catecholamines. Chromatographia 1987;24:363-70.

19 Dirken JM. Arbeid en Stress. Her vaststellen van aanpassings-problemen in werksituaties (Work and stress. Assessment ofadaptation problems in working situations). Groningen: JBWolters, 1967. (English summary.)

20 Visser APh. De betekenis van de VOEG: enkele gegevensover de begripsvaliditeit. (The meaning of the VOEG:some data on the construct validity). Gezondheid en

Samenleving 1983;4:177-88.21 Van Sonsbeek JLA. De gezondheidsmeting in de continue

gezondheidsenquete van het CBS: aspecten van debetrouwbaarheid en validiteit. (The measurement ofhealth in the Dutch continuous Health Interview Surveyof the Central Bureau of Statistics: aspects of the relia-bility and validity). T Soc Gezondheidsz 1989;67:11-4.

22 Karasek RA. Job demands, job decision latitude, and men-

tal strain: implications for job redesign. AdministrativeScience Quarterly 1979;24:285-305.

23 Karasek RA, Theorell T. Healthy work: stress, productivity,and the reconstruction of working life. New York: BasisBooks, 1990.

24 Landsberg L. Pathophysiology of obesity-related hyperten-sion: role of insulin and the sympathetic nervous system.Cardiovasc Pharmacol 1994;23(suppl 1):S1-S8.

25 Meijman TF, Mulders HPG, Kompier MAJ, VanDormolen M. Individual differences in adrenaline/nora-drenaline reactivity and self-perceived health status. ZGesamte Hyg 1990;36:413-4.

26 Johansson G, Aronsson G, Lindstrom BO. Social psycho-logical and neuroendocrine stress reactions in highlymechanised work. Ergonomics 1978;21:583-99.

27 Schaubroeck J, Ganster DC. Chronic demands andresponsivity to challenge. JAppl Psychol 1993;78:73-85.

28 Dienstbier RA. Arousal and physiological toughness:implications for mental and physical health. Psychol Rev1989;96:84-100.

29 Dienstbier RA. Behavioral correlates of sympathoadrenalreactivity: the toughness model. Med Sci Sports Exerc1991;23:846-52.

30 Ursin H, Olff M. The stress response. In: Stanford C,Salmon P, Gray J, eds. Stress: an integrated approach.New York: Academic Press, 1992.

31 Meijman TF. Coping with work demands: some remarkson the role of catecholamines. In: Henschel U, ed.Experimental psychosomatics. Leiden: Leiden UniversityPress, 1993:77-98.

Correspondence and editorialsOccupational and Environmental Medicine wel- minimum. Letters are accepted on thecomes correspondence relating to any of the understanding that they may be subject tomaterial appearing in the journal. Results editorial revision and shortening.from preliminary or small scale studies may The journal also publishes editorials whichalso be published in the correspondence are normally specially commissioned. Thecolumn if this seems appropriate. Letters Editor welcomes suggestions regardingshould be not more than 500 words in length suitable topics; those wishing to submit anand contain a minimum of references. Tables editorial, however, should do so only afterand figures should be kept to an absolute discussion with the Editor.

Survey of the respiratory health of the workers ofa tale producingfactory

19 Boundy MG, Gold K, Martin KP, Burgess WA, DementJM. Occupational exposures to non-asbestiform talc inVermont. Dusts and disease. In: Lemen R, Dement JM,eds. Proceedings of the conference on occupational exposuresto fibrous and particulate dust and their extension into theenvironment. Park Forest South, II: Pathotox 1979.

20 Wegman DH, Peters JM, Boundy MG, Smith TJ.Evaluation of respiratory effects in miners and millersexposed to talc free of asbestos and silica. Br Ind Med1982;39:233-8.

21 Leophonte P, Fernet P, Pincemin J, Mousset iL, Rellier R,Delaude A. La pathologic respiratoire chronique destravailleurs du talc. Revue Franfaise des MaladiesRespiratoires 1980;8:43-5.

22 Leophonte P, Didier A. French talc pneumoconiosis. In:Bignon J, ed. Health related effects ofphyllosilicates. Berlin:Springer-Verlag, 1990:203-9. (NATO ASI series, volG21).

23 Cara M, Hentz P. Aide-mimoire of spirographic practice forexamining ventilatory function. 2nd ed. Luxembourg:Commission of the European Communities, 1971.(Industrial Health and Medicine, Series No 11.)

24 Prefaut C, Peslin R. L'exploration du souffle ou mesuredes volumes pulmonaires et des debits bronchiques. RevMal Resp 1986;3:323-32.

25 Graham WGB, Ashikaga T, Hemenway D, Weaver S,O'Grady RV. Radiographic abnormalities in Vermontgranite workers exposed to low levels of granite dust.Chest 1991;100:1507-14.

26 Castellan RM, Sanderson WT, Petersen MR. Prevalenceof radiographic appearance of pneumoconiosis in anunexposed blue collar population. Am Rev Respir Dis1985;131:684-6.

27 Dick JA, Morgan 'WKC, Muir DFC, Reger RB, SargentN. The significance of irregular opacities on the chestroentgenogram. Chest 1992;60:102-251.

Rejected manuscriptsFrom February 1994, authors whose sub- be returned to them. The Joumnal will destroymitted articles are rejected will be advised of remaining copies of the article but corres-the decision and one copy of the article, pondence and reviewers' comments will betogether with any reviewers' comments, will kept.

477

Occupational and Environmental Medicine 1995;52:494-496

CORRESPONDENCE NOTICES

Use of the prevalence ratio v theprevalence odds ratio in view ofconfounding in cross sectional studies

Editor-Since we first submitted our letteron the prevalence ratio v the prevalenceodds ratio in cross sectional studies ofcommon disease,' as first discussed by Leeand Chia,5 Stromberg has made twocomments on this matter.34 In his secondletter he is critical of our notion that adifferent pattern of confounding is presentwhen considering the prevalence ratio (PR)compared with the prevalence odds ratio(POR)4-that is, that the use of PORimplies confounding even when the studybase is not confounded in terms ofprevalence data (and the reverse is alsotrue). Then, by constructing a set of crosssectional data, where an extraneous factor Faffects the prevalence ratio but not theprevalence odds ratio, he suggests that "F

may modify the effect of exposure withoutbeing a confounder in the conventionalmeaning; moreover, F may modify the PORand not the PR, and vice versa".By calculation of the adjusted PR and

POR (in Stromberg's example 2-14 and3 00, table) and comparison of these withthe crude PR and POR (2-14 and 2-83,respectively), we find that there is nodifference for PR whereas the crude POR islower than the adjusted POR. This impliesnegative confounding when the PORanalysis is applied.' This shows what ourcomment was about,' but Stromberg seemsto suggest that as F is not confounding theprevalence (and the PR), it is not aconfounder for the POR either. Theproblem is, however, that with a PORanalysis, F is no longer only a modifyingfactor but behaves as a confounder becauseit is differently associated with theexposures among the healthy people. Ourpoint was that this phenomenon should benoted and avoided by proper analysis whena disease is common, but the difference isnegligible when the disease is rare.Our arguments for preferring the PR as a

measure of risk rather than the POR weregiven earlier. It might be added here thateven if the POR under certaincircumstances can be taken as an incidence(density) ratio6 it is not appropriate to use a

POR estimate as a basis for obtaining theaetiological fraction, or perhaps with betterterminology, the excess fraction.78 This isworth noting as Stromberg also brieflymentions aetiological aspects in hiscomment.

Stromberg is critical of the use of Cox'sproportional hazards model and concludesthat "there is no useful statistical model fordirectly estimating a PR with adjustmentsfor several covariates". An estimation ofPOR is hardly a solution in this context,either it involves logistic regression or it

does not, as it may lead to distortion ratherthan adjustment for confounding covariates.Nor has an overall POR analysis any role inthe evaluation of a modifying effect from acovariate on the PR (as in the hypotheticaldata provided by Stromberg), as this inprinciple is a matter for stratum specificevaluations.

OLAV AXELSONMATS FREDRIKSSON

KERSTIN EKBERGDepartment of Occupational and

Environmental Medicine,University Hospital,S-581 85 Linkoping,

Sweden

1 Axelson 0, Fredriksson M, Ekberg K. Use ofthe prevalence ratio v the prevalence oddsratio as a measure of risk in cross sectionalstudies. Occup Environ Med 1994;51:574.

2 Lee J, Chia KS. Estimation of prevalence rateratios for cross sectional data: an example inoccupational epidemiology. Bra Ind Med1993;50:861-2.

3 Str6mberg U. Prevalence odds ratio v preva-lence ratio. Occupy Environ Med 1994;51:143-4.

4 Stromberg U. Prevalence odds ratio v preva-lence ratio-some further comments. OccupEnviron Med 1995;52:143.

5 Miettinen OS. Components of the crude riskratio. Amj Epidemiol 1972;96: 168-72.

6 Axelson 0. Elucidation of some epidemiologicprinciples. Sand Work Entvron Health1983;9:231-40.

7 Greenland S, Robins JM. Conceptual prob-lems in the definition and interpretation ofattributable fractions. Am Y Epidemiol 1988;128:1185-97.

8 Axelson 0. Some recent developments inoccupational epidemiology. Scand Y WorkEnviron Health 1994;20(special issue):9-18.

Recast and expanded tablefrom Strrimberg,4 with prevalence ratios (PR) and prevalence odds ratios(POR) by stratum and overall, crude and adjusted: the stratification factor, F, modifies but exertsno confounding on the prevalence of disease, but F differendy influences the proportion ofhealthysubjects among exposed and non-exposed people, thereby exerting confounding on the POR

Exposed Non-exposed POR PR

F present Diseased 500 250Healthy 500 750 3 00Subtotal 1000 1000 2-0

F absent Diseased 250 100Healthy 750 900 3 00Subtotal 1000 1000 2-50

Total Diseased 750 350Healthy 2000 2000 Crude 2-83 2-14

Adjusted* 3-00 2-14

*Adjusted either by Mantel-Haenszel weighing or as standardised mortality ratio.The confounding POR = 2-83/3 00 < 1; similarly, the confounding PR = 2-14/2-14 = 1-that is, negativeand no confounding, respectively.5

Symposium on the Health Effects ofFibrous Materials (excluding asbestos)Used in Industry. 30-31 October 1995.Sydney, Australia.

The Australian Insulation Wools ResearchAdvisory Board (IWRAB) is organising asymposium to present the latest informationon the health effects of fibrous materials(excluding asbestos). There will be presen-tations by some of the world's leadingresearchers in sessions on: physical andchemical properties and industrial use;exposure levels in industry; experimental invitro results; experimental in vivo results;epidemiology (morbidity and mortality).The implications of research results on thehealth of those who work with the materialsis an important aspect of the symposium.

For further information contact: GeoffreyBerry, Health Effects of Fibrous MaterialsSymposium, PO Box 746, Turramurra,NSW 2074, Australia. Telephone: + 61-2-449-1525. Fax: + 61-2-488-7496.

Health Consequences of Chernobyl andOther Radiological Accidents. 20-23November 1995. WHO, Geneva,Switzerland.

The health and environmental conse-quences attributed to the accident at theChernobyl nuclear power plant in Ukraineon 26 April 1986 have been subject toextensive investigation. A clearer picture onthe health consequences of this accident isexpected to emanate from the pilot phaseof the International Programme on theHealth Effects of the Chernobyl Accident(IPHECA).

Concurrently with the publication of thereport on the results of the pilot phase ofIPHECA, the World Health Organization isplanning to convene a major internationalconference. This conference will not onlyput IPHECA results into focus but will alsoreview new findings from other radiologicalevents. Its broad objectives can be stated asfollows:* Highlight major findings from IPHECAphase I

* Compare IPHECA phase I results withthose of other studies on health effects ofChernobyl

* Obtain improved (updated) understandingof the type, magnitude, and severity ofpresently known and expected futurehealth effects from the Chernobyl accident

* Add new results from investigations ofhealth effects of other radiological eventsto complement the health effects picture

* Examine the effectiveness of remedialmeasures regarding health during andafter accidents and propose futureimprovements

* Advance or confirm current knowledge ofhealth effects related to radiation

* Provide information to the ongoing or newinvestigations ofUNSCEAR

* Point out interesting trends and develop-ments that need research attention in thefuture

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Book Reviews

Interested scientists in this field are invit-ed to present papers or posters on theirresearch at the conference and should con-tact the Office of Global and IntegratedEnvironmental Health, World HealthOrganization, 1211 Geneva 27,Switzerland, facsimile number (41 22) 4123for further details. The deadline for submit-ting abstracts is 3 July 1995.

For further information contact: DrT Kjellstrom, Director, EHG, WorldHealth Organization, 1211 Geneve 27,Switzerland. Tel: +41 22 791 3756;Facsimile: +41 22 791 4123; e-mail:JOHNSONJ@WHO.CH. Internet:gopher.who.ch. -p WHO's Major Pro-grammes -- PEH -- EHG -. Messagesfrom EHG.

BOOK REVIEWSBook review editor: R L Maynard

If you wish to order, or require furtherinformation regarding the titles reviewedhere, please write or telephone the BMJBookshop, PO Box 295, London WX1H9TE. Tel: 0171 383 6244. Fax: 0171 3836662. Books are supplied post free in theUK and for British Forces Posted Overseasaddresses. Overseas customers should add15% for postage and packing. Payment canbe made by cheque in sterling drawn on aUK bank, or by credit card (MasterCard,VISA, or American Express) stating cardnumber, expiratory date, and your fullname.(The price and availability are occasionallysubject to revision by the Publishers.)

Work and Aging; A EuropeanPerspective. Edited by: JAN SNEL, ROELCREMER. (Pp 417, price £24.50). 1994.London: Taylor and Francis. ISBN 0-7484-0165-2.

This indigestible book is packed with scien-tific papers covering topics as diverse asphysiology, experimental psychology,ergonomics, and many others. The unifyingtheme is that of aging. This is the proceed-ings of the 1993 European Symposium onWork and Aging, with a few additional con-tributions.The commentaries interspersed with the

papers provide interesting insight into thelunacy of international business practicethat "lays off' by early retirement or redun-dancy, increasing numbers of mature work-ers, thereby displacing costs on to the State.This is in spite of steadily falling numbers ofnew entrants into work. The demographicchanges say it all, as the average age of theEuropean population continues to rise.The papers taken individually are of

interest, but the book as a whole is confus-ing to the reader, and it has clearly been dif-ficult for the editors to maintain a coherentapproach. There is little of immediate rele-vance to the occupational physician,although the paper on health effects of shiftwork on the older workers is of some inter-est. I found greater fascination in the essayon motivational and cultural factors by M JSchabracq, who is an organizational psy-chologist.

This book has the stated intention ofstimulating discussion within a wide audi-ence, which includes occupational health

practitioners. It fails in its purpose by being The visual system (chapter 7) is well cov-too diffuse, but is nevertheless relevant ered, with a list of neurotoxins and theirreading for those who formulate and main- specific effects. Brain tumours associatedtain the personnel and political policies with occupational chemical exposure (chap-currently in place across Europe. It is of ter 8), and a literature review of substanceslittle comfort that the United Kingdom is with an excess risk are tabulated and sum-not alone in its current pattern of pension- marised. Chapter 9 provides an alphabeticaling off the older worker. list of neurotoxic compounds, human expo-

S B DIGGLE sure, and clinical manifestations.Chapter 11 by Buckle from the Robens

Occupational Neurology and Clinical Institute in the United Kingdom begins theNeurotoxicology. By MARGIT L BLEECKER. second main section (chapters 11-15). The(Pp 384; price £67) 1994. Baltimore: mechanical effects on the extremities, nerveWilliams and Wilkins. ISBN 0-683- entrapment, spinal biomechanics, and00848-X. industrial low back pain are examined, fol-

lowed by occupational rehabilitation. TheseThis multidisciplinary, multiauthor book of chapters are well illustrated with mathemat-approaching four hundred pages examines ical and mechanical formulae aimed essen-the nervous system as it is affected by expo- tially at prevention and advice for thesure to toxins and ergonomic stressors at occupational physician.work. It considers epidemiology, exposure Chapter 10 "Interaction of Medical andmeasurement, dysfunction evaluation, neu- Legal Systems" is entirely American based,ropsychiatric disorders, outcomes, rehabili- and examples of United States law aretation, and medico-legal consequences. The quoted from courts in various states. A"centre of gravity" is in the United States, number of general principles apply univer-with 13 of the 15 chapters having American sally, but American details have limitedauthors. value to the United Kingdom reader.The authors start from the disciplines of Inevitably with many writers, style varies,

neurology and neurotoxicology, and focus but the editor has done well in pulling it allonto the "rapidly developing specialty of together. The various authors themselvesoccupational neurology". Thus they bring have taken care to make what could betogether into one book, chemical and physi- turgid reading as interesting as possible, bycal stressors affecting the nervous system. the insertion of practical examples that livenThe occupational physician from the the text, and focus the mind on the typicalUnited Kingdom, used to chemical toxicol- everyday problems the reader will face.ogy and ergonomics as separate subjects In future editions, as research progresses,may find the juxtaposition unusual. and the evaluation and understanding ofThe book can be divided structurally into psychiatric disorders relating to toxins and

two main sections. The first (chapters 1 to occupation expands, then the section on9) looks at the effects of chemicals; the sec- ergonomics and mechanical effects couldond (chapters 11 to 15) at physical forces well be hived off into a separate publication.on the nervous system. There is an inter- For the occupational physician, particu-spersed third small section (chapter 10) on larly working in the chemical industry, thismedico-legal issues. will be a useful new publication that shouldAs should be expected, this essentially enhance knowledge, and act as a guide in

American book uses United States refer- tackling problems arising from actual andences, occupational health standards, and potential workplace exposure. The sectionexamples. OSHA, NIOSH, ACGIH, and on physical agents will have an additionalTLVs* are used throughout with a brief interest, and the book as a whole could bemention of German MAKt values. helpful for candidates for the Associate ofThe first main section begins with epi- the Faculty of Occupational Medicine exam.

demiology in the neurological context COLIN JUNIPER(chapter 1) with a description of the basicelements needed to relate exposure to ner- *Occupational Safety Health Administrationvous tissue injury. Methods are then devel- (OSHA), National Institute for Safety and Healthoped to analyse and interpret results and (NIOSH), American Conference ofGovernmental Industrial Hygienists (ACGIH),statistics. The difficulties in diagnosis of, for Threshold Limit Values (LVs).instance, a diffuse chemically induced tMaximal Arbeitsplatzkonzentrations (MAKs)encephalopathy are not shirked, and variousmethods of avoiding pitfalls are illustrated.Retrospective and prospective case-control Environment Health Risks and Publicstudies are encompassed so that the reader Policy: Decision Making- in Freemay acquire a firm understanding of mod- Societies. By DAVID V BATES. (Pp 117;em epidemiological practice. price $US30 (cloth) $US13 (paper)) 1995.

Assessment of exposure and dose (chap- Washington: University of Washingtonter 2) and the continuum from exposure to Press. ISBN 0-295-97336-6 (cloth) 0-295-disease, provide an overview of occupational 97337-4 (paper).hygiene practice. Next comes biological This is a fascinating short volume on how to

monitoring and clinical disease (chapter 3) present data to policy makers. The author iscovering solvents, liquids, metals, pesticides, a distinguished pulmonary physician andand gases, with useful summaries of named former Dean of a Medical School. In hischemicals and their biological indicators. later years he became interested in epidemi-

Neurological examination (chapter 4) ology and its use in the investigation of theevaluates ways of determining mental status health effects of air pollution. The bookas well as quantitative methods of neurolog- deals with five major topics, air pollution,ical testing, with brief descriptions of the cigarette smoking, asbestos, lead, and elec-various ancillary tests in current use. tromagnetic fields and their association withNeuropsychological tests used in assess- health effects. In the first part of the bookment are covered (chapter 5) and the neu- selected studies are chosen that have beenropsychiatric consequences of exposure to used to show the possible risks. The secondneurotoxins (chapter 6). The overview of part deals with some of the policy issues inpsychosis, dementia, depression, anxiety, relation to these risks. The third part pro-and post-traumatic stress disorder in rela- vides cautionary tales of the way in whichtion to occupation provides a useful synop- the public, media, courts, and governmentssis for the occupational doctor. approach the problems of risk assessment

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