During the examination of the health status of workers with exposure to high electric and magnetic field strengths about 25 years ago, evidence of the possible association between occupational exposure to extremely low frequency magnetic fields and the risk of neurodegenerative diseases such as Alzheimer disease, Parkinson disease and amyotrophic lateral sclerosis was observed. Numerous epidemiological studies have been conducted in recent decades to investigate these findings. Main focus is on the occupational group of electrical occupations which comprises electric utility workers, electricians, and electrical and electronic engineers (Bowman et al, 2007). Further groups studied were welders and train conductors.
Neurodegenerative diseases are mostly slow progressive diseases of the nervous system which can be inherited or sporadic. They result in progressive degeneration of nerve cells (neurodegeneration). This causes different neurological disorders of the brain such as dementia (e.g. Alzheimer disease) or problems with movement (e.g. Parkinson disease). Alzheimer disease, amyotrophic lateral sclerosis and Parkinson disease were investigated mainly in context of exposure to extremely low frequency magnetic fields.
Dementia is a disorder of several cognitive functions leading to an impaired memory, thinking skills and other cognitive functions (e.g. language, reasoning, orientation, behavior and personality), which interfere with a person's social, occupational and daily life and activities. The most common type of dementia is Alzheimer disease which accounts for 50–70% of cases. Alzheimer disease (named after the neuropathologist Alois Alzheimer, 1864–1915) is a neurodegenerative disease which eventually leads to a shrinking (atrophy) of the brain. It is characterized by amyloid plaques and tau tangles throughout the brain. The onset of the disease is typically after the age of 60. Women are more often affected than men. In 2013, the number of people worldwide with dementia was estimated at 44 million, and likely to reach 76 million by 2030 (Alzheimer's Disease International). At present, in Germany about 1.5 million people live with dementia, two-thirds of them with Alzheimer disease. Approximate 300,000 new cases are diagnosed every year in Germany (German Alzhemer Society). The causes for the Alzheimer disease are not yet fully understood. There are known, however unchangeable risk factors such as age, inherited conditions (e.g. in the early-onset type) and genetic variations (e.g. of apolipoprotein E). Health and life-style risk factors are diabetes, high blood pressure, atherosclerosis, stroke, traumatic brain injury, smoking and alcohol consumption.
Amyotrophic lateral sclerosis (also called motor neuron disease) is a rapidly progressive degenerative disease of motoric nerve cells (motor neurons) which are responsible for controlling voluntary muscles. The degeneration and death of the motor neurons leads to muscle twitches, muscle weakness and muscle loss (atrophy). As the disease progresses the brain loses the ability to start and control voluntary movement. The disease onset is about the age of 60. Men are affected more often than women. Amyotrophic lateral sclerosis is a rare disease. Worldwide, about 2 out of 100,000 people per year will become afflicted with this disease (German Society for Muscular Disorders). In Germany there are approximately 6,000 people with amyotrophic lateral sclerosis. In about 90% of the cases, the disease occurs at random with no clear associated risk factors, the other cases are caused by inherited factors (Jellinger, 2005).
Parkinson disease (named after the physician James Parkinson, 1755–1824) is a slow progressive neurodegenerative disease of the extrapyramidal-motor system of the brain which controls involuntary movements. The loss of dopamine-producing brain cells is characteristic for this disease. The deficit of dopamine leads to a decrease of activating effect on the motoric movements. The symptoms are muscle stiffness (rigor), muscle trembling (tremor), slowness of movement (bradykinesia), and impaired balance and coordination (postural instability). It is more common in men than in women. The typical onset age of Parkinson disease is 50–60 years. Worldwide about 7 to 10 million people are afflicted with this disease (Parkinson's Disease Foundation). Approximate 250,000–400,000 people with this disease live in Germany (Parkinson aktuell). Causes for the disease are still unknown.
There are several methodological limitations of epidemiological studies on neurodegenerative diseases. First of all, these diseases are not recorded in national registries compared to cancer registries. Therefore mortality registries are used for data on cause of death and occupation, these data are less informative than patient records (WHO, 2007"). Furthermore, in some of the studies the categorization into electrical occupations is based on the job title and for this reason it is not very precise. In more recent studies, exposure assessment was improved by applying a job exposure matrix which is based on measured magnetic flux densities in the work places of different occupations (e.g. Theriault et al (1994), Floderus et al (1996) and Bowman et al (2007)).
Electric shocks have been discussed as possible confounder in studies on electric occupations (Kheifets et al, 2009). Whether the workers in electrical occupations have received electric shocks is only included in a few studies on the exposure to magnetic fields and neurodegenerative diseases. Other risk factors which have not been considered, could be the effects of chemicals in the workplace.
The current knowledge on the association between exposure to extremely low frequency magnetic fields and the risk for neurodegenerative diseases have been evaluated by several international expert groups (<WHO (2007), ICNIRP (2003) und SCENIHR (2009)). Based on epidemiological studies they came to the conclusion that there is evidence for an association between occupational exposure to power frequency magnetic fields and amyotrophic lateral sclerosis. The findings of studies on Alzheimer disease are inconsistent. There is only a weak evidence of association between extremely low frequency magnetic fields and Alzheimer disease. Based on only a small number of studies there is no evidence for an association between occupational exposure to extremely low frequency magnetic fields and Parkinson disease. Only a few studies have been conducted on the residential exposure to extremely low frequency magnetic fields generated by power lines. There is evidence for an increased risk for Alzheimer disease, however no evidence for Parkinson disease and amyotrophic lateral sclerosis (WHO).
Expert groups and authors have comprehensively summarized and evaluated the available publications on exposure to extremely low frequency magnetic fields and neurodegenerative diseases in following statements, reviews and meta-analyses:
A biological mechanism of action to explain the possible association between the exposure to extremely low frequency magnetic fields and the risk of neurodegenerative disease is currently unknown. An overview about experimental in vivo and animal studies on this topic is given in the following: