In this study, some occupations with probable exposure to magnetic fields were significantly associated with the diagnosis of dementia. We observed an increased risk for dementia among electrical and electronics workers as well as among construction workers; furthermore, we found an increased risk for Alzheimer's disease among metal workers. However, we could not find any association between cumulative or peak (maximum) exposure to magnetic fields and dementia.
The relationship between the above‐mentioned occupational groups and dementia might be alternatively explained by chemical exposures (eg, solvents), psychosocial factors (eg, scarcely challenging jobs17
) or by chance. When challenge at work—assigned to cases and controls by linking lifetime job titles with a Finnish job‐exposure matrix called FINJEM17,24
—was included as a confounder in an additional occupational group analysis, electrical and electronics workers were no longer significantly associated with Alzheimer's disease; also, metal workers were no longer significantly associated with dementia of any type. When adjusting for challenge at work, likewise, the association between dementia and blue‐collar work in general lost its statistical significance. The increased risk for dementia of subjects having worked in blue‐collar occupations potentially exposed to magnetic fields might therefore at least partly reflect the increased risk for dementia in less challenging jobs.
Next of kin might frequently not be familiar with detailed job tasks, potentially introducing differential misclassification of BAuA expert assessment. Indeed, agreement between subjects and next of kin was relatively weak for expert‐rated cumulative exposure to magnetic fields (see section Agreement between participants and next of kin). However, the comparison of magnetic fields according to expert assessment based on self‐interviews versus next of kin interviews did not provide evidence for a systematic misclassification of exposure to magnetic fields on the basis of the next of kin job history. We therefore regard a differential misclassification of the BAuA expert assessment as rather improbable. Furthermore, when exposure to magnetic fields was assessed by the Finnish job‐exposure matrix FINJEM24
(a method less prone to misclassification by next of kin interviews), again, we did not find any significant relationship between exposure to magnetic fields and the diagnosis of dementia (results not shown; supplementary tables are available from the authors).
- In a case–control study, low‐frequency magnetic fields assessed by expert rating were not associated with the diagnosis of dementia.
- The increased risk for dementia of subjects in blue‐collar occupations potentially exposed to magnetic fields might at least partly reflect the increased risk for dementia in less challenging jobs.
- Further studies should consider the increased risk for dementia in blue‐collar occupations.
Previous studies on the relationship between electromagnetic fields and dementia lead to ambiguous results. This might depend on differences in the exposure assessment. Although, for example, three case–control studies7,8,9
assigned dressmakers, seamstresses and tailors to the “medium to high” exposure to magnetic field category, two other studies4,11
applying a Swedish job‐exposure matrix25
did not regard these occupations as exposed at all. Interestingly, in the first study conducted by Sobel et al
23 of 36 patients with Alzheimer's disease in the “medium to high” exposure category had worked as dressmakers, seamstresses or tailors as their main occupation. When dressmakers and tailors are regarded as non‐exposed in the case–control studies conducted by Sobel et al
the relationship between exposure to magnetic fields and Alzheimer's disease loses its statistical significance (own calculation based on Sobel et al7,8
). As a potential alternative explanation, the authors take into consideration that some seamstresses, dressmakers and tailors might have worked in dry‐cleaning establishments. This might be also the case in our study, in which dyers were explicitly included in the occupational group “spinners, weavers, knitters, dyers, tailors or dressmakers”. Although in our study the occupational group “spinners, weavers, knitters, dyers, tailors or dressmakers” is non‐significantly associated with dementia (OR for having worked 10 years in this occupations 1.4; 95% CI 0.9 to 2.1), the heterogeneity of this group might mask increased risks among specific occupations (eg, dyers). However, the numbers in our study are too small to further examine the risks among specific occupations. Altogether, there is a need for further epidemiological research considering the possibly increased risk for dementia of dyers, tailors and dressmakers.
In a nested case–control study using US mortality data, Savitz et al2
found a slightly increased risk for Alzheimer's disease among electricians and among power‐plant operators, but not among others in occupations exposed to magnetic fields (eg, electrical engineers). Analysing virtually the same dataset, but using “all mentions” on the death certificate rather than underlying causes alone, Schulte et al3
found increased proportionate mortality ratios for Alzheimer's disease among occupations that could have been exposed to electromagnetic fields. However, no specific assessment was conducted relating to exposure to magnetic fields. Estimating cumulative exposure to magnetic fields in a death certificate‐based cohort study of electric utility workers, Savitz et al6
did not find a significant association between exposure to magnetic fields and mortality from Alzheimer's disease. By contrast, two recently published Swedish death certificate‐based studies1,4
showed significantly increased risks for Alzheimer's disease in subjects exposed to magnetic fields. These two studies show considerable overlap in the observed study populations; moreover, they use the same job‐exposure matrix25
to assess exposure and the same method to identify occupations. In the Swedish study mentioned, 1
which included relatively young subjects (very few subjects were >76 years at the end of follow‐up), welders comprised 70% of the highest exposure group. As welders are exposed to metal fumes containing established neurotoxicants such as lead, aluminium and manganese, a possible role of other risk factors cannot be excluded in the mentioned study.26
In the second Swedish study,4
significantly increased risks for Alzheimer's disease could be found among subjects who died before 75 years, but not among subjects who died after 75 years. As our study predominantly includes patients with dementia >75 years at the time of diagnosis, our study allows no conclusions with regard to early‐onset dementia. In general, mortality studies are not able to take disease onset or age at diagnosis into account. Therefore, the results based on mortality data might be biased by a long latency and survival if the diagnosis or a long premorbid phase has an effect on the exposure.
As preclinical onset of dementia might precede its clinical manifestation by many years or even decades27,28,29
; in an additional lag‐time analysis, we restricted our analysis to exposure to magnetic fields which had occurred
30 years before diagnosis (the results of this sub‐analysis are available from the authors). However, we could not find any significant positive relationships.
In their methodologically striking long‐term cohort study, Qiu et al13
based their exposure assessment on the above‐mentioned Swedish job‐exposure matrix,25
completed with direct measurements for occupations that were not listed in the matrix. The authors did not find an association between exposure to magnetic fields in the last job and the diagnosis of dementia, whereas exposure to magnetic fields in the primary occupation is related to late‐onset dementia in men, but not in women. As a possible explanation for the sex difference, the authors discuss a generally lower exposure to magnetic field level combined with a greater misclassification of the exposure among women. When our case–control study was analysed separately for both sexes, we did not find any significant increase in risk either among men or women exposed to magnetic fields. However, numbers are low particularly in the analysis of men alone.
The strengths of our study include the calculation of cumulative exposure to magnetic fields during the entire worktime and adjustment for multiple potential confounders. However, limitations of this analysis should be considered when interpreting the results. In our study, the moderate agreement between the job‐exposure assessment and the expert rating of exposure to magnetic fields suggests a considerable misclassification of exposure. Furthermore, we were not able to control for potential confounding by residential exposure to magnetic fields. However, the community degree of urbanisation (rural, mixed or urban) did not constitute a significant predictor of the exposure to magnetic fields in a measurement‐based German study.22
Subjects who were exposed to high magnetic fields might have died before they had a chance to develop dementia. In a cohort of utility workers, Savitz et al30
found evidence of increased risk of mortality from acute myocardial infarction and arrhythmia‐related heart disease in workers exposed to magnetic fields. However, several other epidemiological studies have failed to confirm this result.31,32,33
Summarising, there is no conclusive evidence for an increased cardiac mortality of workers exposed to magnetic fields.
Recall bias might have been introduced into the study by the choice of the interviewed persons: in cases, mostly children or partners were interviewed; in controls, predominantly self‐interviews were conducted. To further elucidate this potential bias, we evaluated the agreement between the participants and next of kin on exposure to magnetic fields among controls by conducting 49 additional next of kin interviews (39 partner interviews, 7 child interviews and 3 other next of kin interviews). Agreement was relatively weak for expert‐rated cumulative exposure to magnetic fields. However, we found no evidence for a systematic overestimation or underestimation of exposure to magnetic fields on the basis of the next of kin job history. We nevertheless admit that the partner might remember the job and duration of the job better than a child or another person. In fact, with regard to expert‐based cumulative exposure, agreement tended to be higher between partner and self‐interviews than between child and self‐interview. However, when we restricted cases to patients whose partner had been interviewed (n
42; excluding, among other things, child interviews), this did not substantially alter the risk estimates for exposure to magnetic fields (results not shown). Finally, we cannot totally exclude that the presence of a patient with dementia in the family might have influenced the next of kin report of the job titles. However, we believe that “bare facts” such as job titles and industries might, to a low extent, be influenced by the knowledge about the dementia status.
The results of the occupational group analysis have to be interpreted with caution, as multiple testing might have led to false‐positive results. The occupational group analysis should therefore be regarded as hypothesis‐generating.
Owing to its relatively small sample size, the ability of our study is strongly limited to detect high‐dose magnetic field effects. However, in three recently conducted cohort studies estimating magnetic field levels, a pronounced increase in risk for dementia was found even for relatively low exposure to magnetic field levels. Hakansson et al1
found a relative risk for mortality from Alzheimer's disease of 4 (95% CI 1.4 to 11.7) for people exposed to >0.53 μT. Feychting et al4
observed an increased risk of mortality from Alzheimer's disease of 2.3 (95% CI 1.5 to 3.3) among men exposed
0.5 μT both in 1970 and 1980; the risk of mortality from Alzheimer's disease was 1.5 (95% CI 1.1 to 2.1) among men exposed to
0.3 μT. Among men whose exposure to magnetic fields in the lifetime principal job was
0.2 μT, Qui et al13
found a relative risk of 2 (95% CI 1.1 to 3.7) for dementia in total and a relative risk of 2.3 (95% CI 1.0 to 5.1) for Alzheimer's disease. Owing to small numbers, the power of our study is limited to detect slight increases in risk: if the prevalence of peak exposure to magnetic fields
0.2 μT (10.9%) among the controls was equal to the true prevalence, the power of our study would be sufficient (β
80%) to detect an OR of 2.3; the power would be 66% to detect an OR of 2. The power of our study might therefore have been insufficient to detect a slightly increased risk for dementia. Furthermore, the power of our study is limited to detect increased risks for dementia subgroups. Mainly owing to the limited power of our study, we cannot exclude an aetiological relevance of high‐dose electromagnetic fields on dementia. According to our data, we nevertheless regard a strong effect of low‐dose electromagnetic fields on the development of late‐onset dementia as rather improbable.