ORIGINAL ARTICLES: Author Commentaries
Editors’ Note: We are taking the unusual step of publishing two papers from Sweden that explore the same question in overlapping populations. The two studies examine the association between occupational exposures to electromagnetic fields and neurodegenerative diseases. We have asked the authors to clarify the key distinctions between their two studies, and to address the consistencies and inconsistencies in their two sets of results.
The 2 studies on extremely low frequency magnetic fields and neurodegenerative diseases presented in this issue provide partially consistent results. 1,2 There are many similarities in the study designs, and there is considerable overlap in the study populations. The studies use the same means to identify occupations, the same job-exposure matrix to assess exposure, and the same method to identify cases. The main difference is that Håkansson et al 2 focus on some specific industrial branches to increase the proportion of subjects with extremely high exposures in the highest exposure category, whereas we 1 include the entire working population. The subjects in their study are primarily a subset (10–15%) of our subjects, ie, individuals who at any time during their study period (1985–1994) were working in industries where resistance welding occurs. The only subjects who were not overlapping were people who were not economically active in 1980 (the year in which we identified our subjects) but who worked during the period of 1985 to 1994. These are most likely young people entering the workforce, but they may also include some immigrants. Welders comprised 7% of their cohort and 70% of their highest exposure group.
The studies found consistent results for Alzheimer’s disease, although Håkansson et al 2 found a considerably higher risk estimate among subjects in the highest exposure category. This difference may be explained by the high proportion of persons with extremely high exposure in their study, or by random variation, because the number of highly exposed cases was small.
For amyotrophic lateral sclerosis (ALS), however, the results regarding the effect of magnetic field exposure are not in agreement. We 1 found no increased risks in any of our analyses, whereas Håkansson et al 2 found increasing risks with increasing exposures. The difference is difficult to explain. If risk increases in the Håkansson study had been limited to the highest exposure category, greater exposure misclassification in our study could have been an explanation. However, they found increased risks in the intermediate exposure categories as well. It is also not likely that such exposure misclassification would differentially affect results for ALS and Alzheimer’s disease. In our study, the highest exposure category also included train drivers, who are exposed to magnetic fields with a frequency of 16.67 Hz, instead of the 50 Hz used for power frequency fields. Excluding train drivers did not change our results, and therefore this difference cannot explain the lack of consistency.
Confounding from some other risk factors within the occupations in the selected branches is a possible explanation for the inconsistent results. Restriction of the Håkansson 2 study to certain industrial branches is likely to have limited the number of occupations included in their cohort. In our study, 1 we probably have a broader range of occupations within the exposed categories. Although magnetic field exposure was not associated with ALS in our study, we did find increased risks of ALS among welders, radio and television assemblers and repairmen, and telephone and telegraph installers and repairmen. The 2 former occupational groups are mentioned in the Håkansson 2 article as being included in their cohort and thus have contributed to the increased risks they report. Further studies to clarify the associations are warranted.
1. Feychting M, Jonsson F, Pederson NL, et al.. Occupational magnetic field exposure and neurodegenerative disease. Epidemiology. 2003; 14: 413–419.
2. Håkansson N, Gustavsson P, Johansen C, et al. Neurodegenerative diseases in welders and other workers exposed to high levels of magnetic fields. Epidemiology. 2003; 14: 420–426.