MINDER, CHRISTOPH

PFLUGER, DOMINIK

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International Journal of Epidemiology© International Epidemiotogical Association 1993Vol. 22, No. 6Printed in Great BritainLetters to the EditorCancer Mortality and Electric Transmission EquipmentFrom DOMINIK PFLUGER AND CHRISTOPH MINDERSir—We would like to comment on the paper bySchreiber et al.' on cancer mortality and electrictransmission equipment mostly because of the seriouslimitations of the investigation.Most studies of cancer and exposure to electro-magnetic fields have been of the case-control type (seee.g. ref. 2). The reason for this is that the relative risksof exposed to non-exposed ranges from 1 to 2, at leastin the frequently observed range of exposures. Incohort studies this would necessitate the observation ofa very large number of person-years of follow-up,unless the exposure is substantial. Thus, lack of powermay be a serious problem of cohort studies and needsto be assessed before the study is conducted.Power calculations 'post-festum' are shown in Table1 for the key outcomes of leukaemia, brain cancer andthe sum of those two, both for men and womenseparately and together.The expected numbers quoted in Table 1 are derivedfrom the data in the paper. For the determination ofTABLE 1 Power (Vt) of the Schreiber study to delect relative risks of1.5 and 2.0 based on person-years of follow-up and expected numbersof the study (ref. 3, p. 274)SexMenWomenBoth sexesCause of deathleukaemiabrain cancerboth combinedleukaemiabrain cancerboth combinedleukaemiabrain cancerboth combinedExpectedno. of casesamong exposed2.271.543.811.751.142.894.022.686.70Relative risk1.51781588171517262.0392141212139413964NB Combining both sexes and both causes of death is putting heavyrestrictions on the risk model considered.power, they were rounded up. As one can see, thepower of this study is low in all cases, and neverexceeds 41 % for any single cause of death. It is doubt-ful whether the study would have been conducted hadsuch a calculation been performed beforehand. Table1 also shows that the study really cannot provide anyinformation on the serious question of cancer and ex-posure to electromagnetic fields.The study has other shortcomings. A possible ad-vantage of the cohort approach, namely more accurateexposure assessment, was not realized in this study.There are several possible biases which were insuffi-ciently addressed in the paper. The general populationmay not be suitable as a reference, as the comparisonmay be biased by differences in socio-economic status,lifestyle and other exposures: transmission linesusually do not pass through the most affluent regions.The effect of indoor exposure was neglected so thatone can expect major misclassifications in the actualexposures associated with the two groups distinguishedin the study.REFERENCES1 Schreiber G H, Swaen G M H, Meijers J M M, Slangen J J M, Stur-mans F. Cancer mortality and residence near electricitytransmission equipment: A retrospective cohort study. Inl JEpidemiol 1993; 22: 9-15.2 Coleman M, Beral V. A review of epidemiological studies of thehealth effects of living near or working with electricity genera-tion and transmission equipment. Int J Epidemiol 1988; 17;1-13.3 Breslow N E, Day N E. Statistical Methods in Cancer Research.Volume II. The design and analysis of cohort studies. I ARCScientific Publication No. 82, 1987.Author's ResponseFrom G M H SWAENSir—Indeed most studies of cancer and exposure to ex-treme low frequency (ELF) electromagnetic fields haveDepartment of Social and Preventive Medicine, University of Berne,Finkenhubelweg 11, CH-3012 Berne.Department of Epidemiology, Rijksuniversileit Limburg, PO Box 616,6200 MD Maastricht, The Netherlands.1203

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