American Journal of Epidemiology Copyright © 1992 by The Johns Hopkins University School of Hygiene and Public Health All rights reserved

Vol. 135, No 12 Printed in U.S.A.

LETTERS TO THE EDITOR

Demers et al. (1) considered a number of potential limitations of their study of occupational electromagnetic field exposure and breast cancer. Low participation rates, selection bias, recall bias, confounding due to several factors, and occupation misclassification were each considered to be unlikely to contribute to a spurious association. Some other limitations (relating to assumptions about exposure in occupations and confounding due to other factors), however, should be mentioned. The idea that " . . . studies have largely confirmed that persons in jobs with presumed exposure are indeed exposed . . . " (1, p. 346) could be examined more thoroughly. Savitz and Calle (2) and Knave and Floderus (3), for example, have stressed the lack of evidence that presumed exposure of workers has actually occurred. It was assumed that electricians are " . . . more likely to be exposed to electromagnetic fields than are men in the other occupational groups . . . " (1, p. 345). According to Johnson and Spitz, however, construction electricians " . . . work largely with unenergized wiring, implying that they have a limited exposure to EMFs "[electromagnetic fields]" (4, p. 760). In addition, Wilkins and Hundley concluded that electricians and electrical and electronics workers have a " . . . low potential for exposure to non-ionizing radiation" (5, p. 996). Demers et al. stated that " . . . confounding due to some other factor related to both breast cancer and employment in an EMF-exposed job may be present" (1, p. 346). This idea was then dismissed by simply stating that"... EMF-exposed jobs are a rather heterogeneous group with respect to most other conceivable exposures" (1, p. 346). Potential confounders in certain occupations must still be considered. In an earlier study (6), for example, a significantly increased risk for death from brain tumors was observed in occupations supposedly involving exposure to electromagnetic fields. The risk disappeared, however, after subjects with exposure to lead, soldering fumes, and organic solvents were excluded from the analysis. In addition, regarding telephone linemen, potential effects of chlordane and herbicides used along rights-of-way of power lines must be considered. Although the etiology of male breast cancer is obscure, investigators have noted high incidences in workers who are in contact with many different compounds (7). The suggested link between melatonin levels

and human breast cancer is not based on substantial evidence. Results from different studies are conflicting (8). As Poole and Trichopoulos have noted, " . . . the notion that extremely lowfrequency electric and magnetic fields can cause cancer is not impossible or absurd but is hardly plausible" (9, p. 272). Considering the inherent difficulties in any study of this nature, the authors must be commended for their effort. Use of the Surveillance, Epidemiology, and End Results (SEER) program was a major strength of the study and was an excellent way of obtaining a large number of cases—an element that was lacking in earlier studies of male breast cancer. Because of the difficulties just mentioned, however, the conclusion that the results " . . . lend support to the theory that EMFs may be related to breast cancer in men" (1, p. 340) should be challenged. Uncertain measurements of exposure levels are to be expected in all studies of electromagnetic field effects in humans. Investigators will continue to encounter these uncertainties until estimates of exposure are more accurate.

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REFERENCES

1. Demers PA, Thomas DB, Rosenblatt KA, et al. Occupational exposure to electromagnetic fields and breast cancer in men. Am J Epidemiol 1991; 134:340-7. 2. Savitz DA, Calle EE. Leukemia and occupational exposure to electromagnetic fields: review of epidemiologic surveys. J Occup Med 1987;29:47-51. 3. Knave B, Floderus B. Exposure to low-frequency electromagnetic fields—a health hazard? Scand J Work Environ Health 1988; 14 (suppl l):46-8. 4. Johnson CC, Spitz MR. Childhood nervous system tumours: an assessment of risk associated with paternal occupations involving use, repair or manufacture of electrical and electronic equipment. Int J Epidemiol 1989; 18:756-62. 5. Wilkins JR III, Hundley VD. Paternal occupational exposure to electromagnetic fields and neuroblastoma in offspring. Am J Epidemiol 1990;131: 995-1008. 6. Thomas TL, Stolley PD, Stemhagen A, et al. Brain tumor mortality risk among men with electrical and electronics jobs: a case-control study. J Natl Cancer Inst 1987;79:233-8. 7. McLaughlin JK, Malker HSR, Blot WJ, et al. Occupational risks for male breast cancer in Sweden. Br J Ind Med 1988,45:275-6. 8. Dogliotti L, Berruti A, Buniva T, et al. Melatonin and human cancer. J Steroid Biochem Mol Biol 199O;37:983-7.

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RE: "OCCUPATIONAL EXPOSURE TO ELECTROMAGNETIC FIELDS AND BREAST CANCER IN MEN"

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Letters to the Editor

9. Poole C, Trichopoulos D. Extremely low-frequency electric and magnetic fields and cancer. Cancer Causes Control l99l;2:267-76.

THE FIRST THREE A UTHORS REPL Y Dr. Jauchem raises a number of points regarding our classification of exposure to electromagnetic fields which he apparently feels may threaten the validity of the study (I). The assignment of exposure to electromagnetic fields in retrospective studies is problematic and, as we state in our paper, " . . . , there is undoubtedly a high degree of misclassification introduced by using job titles alone" (2, p. 346). However, the bias resulting from this nondifferential misclassification would likely be in the direction of obscuring any true difference in exposure between cases and controls (3, 4) and, thus, would be unlikely to cause a spurious positive association between occupational exposure to electromagnetic fields and breast cancer. Dr. Jauchem is particularly concerned with the probability of exposure to electromagnetic fields among electricians. In an analysis not included in the published paper, we assigned electromagnetic field exposure to cases and controls using the categories proposed by Lin et al. (5), an exposure classification scheme based on the assumed probability of exposure. Under this classification scheme, electricians in industry were assigned definite electromagnetic field exposure while other electricians were assigned probable exposure. The odds ratios for breast cancer among men with possible, probable, and definite exposure were 0.9 (95 percent confidence interval (CI) 0.6-1.4), 1.2 (95 percent CI 0.6-2.4), and 2.6 (95 percent CI 1.1-6.1), respectively. Dr. Jauchem also raises the issue of confounding due to other occupational factors as a possible threat to the validity of our study. As stated in our paper (2), we consider the heterogeneity of the potentially exposed jobs, with respect to most exposures other than electromagnetic fields, to be a strong argument against confounding due to some unknown occupational exposure. For example, broadcast equipment operators, radio announcers, and dispatchers (2, table 1, group 3, p. 342, odds ratio = 2.9) and electricians, telephone linemen, and electric power workers (2, table 1, group 1, p. 342, odds ratio = 6.0) have little in common with regard to occupational exposures other than electromagnetic fields.

REFERENCES

1. Jauchem JR. Re: "Occupational exposure to electromagnetic fields and breast cancer in men." (Letter). Am J Epidemiol 1992; 135:1423-4. 2. Demers PA, Thomas DB, Rosenblatt KA, et al. Occupational exposure to electromagnetic fields and breast cancer in men. Am J Epidemiol 1991; 134:340-7. 3. Savitz DA, Pearce NE, Poole C. Methodological issues in the epidemiology of electromagnetic fields and cancer. Epidemiol Rev 1989;11:59-78. 4. Poole C, Trichopoulos D. Extremely low-frequency electric and magnetic fields and cancer. Cancer Causes Control 1991;2:267-76. 5. Lin RS, Dischinger PC, Conde J, et al. Occupational exposure to electromagnetic fields and the occurrence of brain tumors. An analysis of possible associations. J Occup Med 1985^7:413-19. 6. Mabuchi K, Bross DS, Kessler II. Risk factors for male breast cancer. J Natl Cancer Inst 1985;74: 371-5. 7. Lenfant-PejovicMH, Mlika-CabanneN,Bouchardy C, et al. Risk factors for male breast cancer a

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James R. Jauchem Directed Energy Division Occupational and Environmental Health Directorate Armstrong Laboratory Brooks Air Force Base, TX 78235

We believe Dr. Jauchem's statement that "investigators have noted high incidences in workers who are in contact with many different compounds" (1, p. 1423) somewhat overstates the body of knowledge dealing with occupational exposures and breast cancer in men. The only occupational factors that have been noted by more than one study to be associated with an increased risk are employment in occupations with exposure to high temperatures (6-8) and in primary metal industries (6, 9). Neither of these appeared to be confounding our results. The quote in Dr. Jauchem's letter from Poole and Trichopolous (4) is taken somewhat out of context. The authors were discussing the biologic theories relating electromagnetic fields to cancer in general and concluding that they fell into the "conceivable" rather than the "plausible" category. We believe that the evidence for a relation between electromagnetic fields and melatonin (10), and between melatonin and breast cancer (11, 12), in rats is based on solid biomedical research, although existing evidence is insufficient to draw conclusions about such relations in humans (see ref. 13 for a review). The theory that exposure to electromagnetic fields may be related to the risk of breast cancer was proposed by Stevens in 1987 (14), and our analysis of this potential relation was initiated because of an excess of breast cancer observed among male utility workers (15). We found a relation between breast cancer and occupational exposure to electromagnetic fields. We could identify no potential biases whose likely effect would have been to create a spurious relation between employment in the occupations we classified as exposed to electromagnetic fields and breast cancer. To conclude that our results lend support to the theory seems perfectly reasonable.

Re: "Occupational exposure to electromagnetic fields and breast cancer in men".

American Journal of Epidemiology Copyright © 1992 by The Johns Hopkins University School of Hygiene and Public Health All rights reserved Vol. 135, N...
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