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Much remains undiscovered about the causes of Alzheimer's disease (AD). Several genetic mutations appear to be associated with early-onset as well as late-onset disease [1] and environmental factors are implicated, particularly for the development of late-onset AD [2]. The best established risk factors identified so far are age, familial aggregation, isoform 4 of apolipoprotein E gene (APOE*4), autosomal-dominant mutations in hereditary AD and Down's syndrome. Occupational agents studied include aluminium, solvents, pesticides, lead and electromagnetic fields (EMF) but the evidence is inconsistent. Other genetic and environmental factors remain controversial but exposure to extremely low-frequency electric and magnetic fields (ELF-EMF) has been postulated to increase the risk for AD.
The February edition of the International Journal of Epidemiology contains a meta-analysis on the association of occupational ELF-EMF exposure and the risk for AD. García et al. [3] looked at 12 papers with results from 14 studies published between 1995 and 2004. They calculated a pooled odds ratio of 2.03 [95% confidence interval (CI) 1.38–3.00] from nine case–control studies and a pooled relative risk of 1.62 (95% CI 1.16–2.27) from five cohort studies. However, they found indication of publication bias (small studies effect) and a large statistical heterogeneity between study results, suggesting this resulted from methodological differences between the reviewed studies. However, they found little evidence that heterogeneity was explained by study type, gender or differences in ascertained exposure levels.
Most of the epidemiological research on occupational exposure to non-ionizing EMF and long-term effects on health has focused on workers exposed to ELF-EMF, with frequencies ranging between 3 and 3000 Hz, and primarily on workers with occupational exposure to power-frequency fields (50–60 Hz). Magnetic field exposure levels for occupations are usually measured in units of Gauss (mG; 1 Gauss = 1000 mG) or Tesla (µT; 0.1 µT = 1 mG). Even though sources of ELF-EMF produce both electric and magnetic fields, research has mostly focused on potential health effects of magnetic field exposure. This is because initial epidemiological studies reported increased cancer risk associated with estimates of magnetic field exposure and because many of the studies examining biological effects of electric fields were equivocal.
All epidemiological studies included in the meta-analysis used retrospective exposure data. Collecting accurate retrospective exposure data based on recollection from AD patients is inherently problematic if investigators are unable to retrieve the information from more objective sources, such as census data or occupation records. In 7 of 14 reviewed epidemiological studies, exposure information had been obtained by proxy interviews (e.g. from relatives). Proxies may not be well informed about the exposure-relevant circumstances or may overestimate the ELF-EMF exposure of cases if they are aware of the study hypothesis.
A further challenge of AD research is the case ascertainment. Differentiation between AD and other forms of dementia needs considerable diagnostic effort and is often inaccurate. Five studies relied on diagnosis data from death certificates. It is well recognized that AD diagnosis is substantially underreported on death certificates [4–6].
Present recommended safety limits for ELF-EMF exposure remain over observed effect levels in this meta-analysis, ranging between 2.75 µT for general public (6.15 µT for exposed workers) at the Canadian Safety Code 6 of 1999, up to 1600 µT at 50 Hz at the UK National Radiological Protection Board [7].
Further research is recommended on relevant duration and latency of exposure, on potential interactions between EMF exposure and other risk factors for AD and on the biological mechanisms potentially underlying ELF-EMF exposure to neurodegenerative changes in the development of AD.
In an accompanying editorial, Röösli [8] cites three additional studies that have reported an association between occupational ELF-EMF exposure and AD, since completion of García's review, whereas only one small case–control study did not.
The potential association between ELF-EMF and AD merits further exploration.
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- Lambert JC, Amouyel P. Genetic heterogeneity of Alzheimer's disease: complexity and advances. Psychoneuroendocrinology (2007) 32(Suppl 1):S62–S70.[CrossRef][Web of Science][Medline]
- Serretti A, Olgiati P, De Ronchi D. Genetics of Alzheimer's disease. A rapidly evolving field. J Alzheimer's Dis (2007) 12:73–92.[Web of Science][Medline]
- García AM, Sisternas A, Perez Hoyos S. Occupational exposure to extremely low frequency electric and magnetic fields and Alzheimer disease: a meta-analysis. Int J Epidemiol (2008) 37:329–340.
[Abstract/Free Full Text] - Ganguli M, Rodriguez EG. Reporting of dementia on death certificates: a community study. J Am Geriatr Soc (1999) 47:842–849.[Web of Science][Medline]
- Kay DW, Forster DP, Newens AJ. Long-term survival, place of death, and death certification in clinically diagnosed pre-senile dementia in northern England. Follow-up after 8–12 years. Br J Psychiatry (2000) 177:156–162.
[Abstract/Free Full Text] - Hug K, Röösli M, Rapp R. Magnetic field exposure and neurodegenerative diseases—recent epidemiological studies. Soz Praventiv Med (2006) 51:210–220.[CrossRef]
- Habash RWY, Brodsky LM, Leiss W, Krewski D, Repacholi M. Health risks of electromagnetic fields. Part I: evaluation and assessment of electric and magnetic fields. Crit Rev Biomed Engineer (2003) 31:141–195.[CrossRef]
- Röösli M. Commentary: epidemiological research on extremely low frequency magnetic fields and Alzheimer's disease—biased or informative? Int J Epidemiol (2008) 37:341–343.
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