7. Extremely low frequency fields like those from power lines and household appliances
- 7.1 What are the sources of extremely low frequency fields (ELF fields)?
- 7.2 What is the level of exposure to ELF fields?
- 7.3 Can ELF fields increase the risk of childhood leukaemia and other cancers?
- 7.4 Can exposure to ELF cause headaches or other health effects?
- 7.5 What can be concluded about ELF fields?
7.1 What are the sources of extremely low frequency fields (ELF fields)?
Power lines generate ELF fields Credit: Miguel Saavedra
In this assessment, extremely low frequency (ELF) fields designate electromagnetic fields with frequencies below 300 Hz, the frequencies that are lower than intermediate frequencies. The main source of extremely low frequencies is alternating current carried in power lines, wiring and household appliances. The electromagnetic field generated has the same frequency as the current that causes it, i.e.50Hz or 60Hz (the latter predominantly in US).
Besides power lines and household appliances, important sources of extremely low frequency fields include power plants and substations, welding machines, induction heaters, and railway, tramway and subway systems.
Extremely low frequency fields have an electric and magnetic component:
- An electric field is the force created by the attraction and repulsion of electric charges (the cause of electric flow), and is measured in volts per meter (V/m).
- A magnetic field is a force created as a consequence of the movement of the charges (flow of electricity). The magnitude (intensity) of a magnetic field is usually measured in tesla (T).
The intensity of both electric and magnetic fields decreases with distance from the field source.
ELF electric fields tend to be strongest close to high voltage power lines (up to 5 kV/m and in a few cases more), and ELF magnetic fields are particularly strong near induction furnaces and welding machines (up to a few mT).
To determine compliance with exposure limits, the maximum possible exposure next to the source must be measured. Maximum exposure is often much higher than average exposure. This is true not just for those who live and work far from the source. Even a lineman who installs or repairs power lines could be expected to have an average exposure on the order of 10 times lower than the maximum. For the general population, the average exposure could be expected to be hundreds or thousands of times lower.
For assessment of compliance with exposure limits, the maximum possible exposure next to devices must be measured. However, the maximum possible exposure next to a specific source is often tens, hundreds or thousands of times higher than the average individual exposure of a person.
For example, for a lineman who installs or repairs electrical lines, the average exposure due to magnetic fields could be more than ten times lower than the maximum exposure close to a transmission line. For the general population which lives and works further away from the source, the difference between maximum and average exposure can be expected to be even greater. More...
7.2 What is the level of exposure to ELF fields?
The general public can be exposed to extremely low frequency (ELF) fields from various fixed sources that are operated in our environment, such as power lines.
When people are passing directly below a high voltage power line, they can be exposed to an electric field between 2 to 5 kV/m and to magnetic fields of less than 40 µT. The strength of the electric and magnetic field diminishes rapidly with distance to the line.
Low voltage power lines cause much lower exposure (100-400 V/m and 0.5-3 µT), and buried cables virtually none. Power plants and distribution stations are off limits to most people and so are not considered a source of exposure for the general public. The same goes for railway power supply installations. The exposure levels in the areas that are accessible to the public are below safety limits set by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) to provide protection against known adverse health effects. At home the magnetic fields tend to be strongest close to certain domestic appliances that contain motors, transformers, and heaters, and fields quickly decrease with distance. For instance, the magnetic field close to a vacuum cleaner is 200 times weaker at 1 m distance than at 5 cm distance (up to 40 µT).
Workers in the electric power industry can be exposed to high levels of electromagnetic fields on the job. Extremely low frequency fields reach or exceed the recommended limits for workers (directive 2004/40/EC). In some areas within power plants and distribution stations, appropriate safety measures are needed. Extremely low frequency (as well as intermediate frequency) fields are also generated by induction and light arc ovens and welding devices, and exposure of workers has to be controlled for such devices. For certain welding devices, magnetic field strengths of up to several hundred µT are possible.
Some medical applications that use electromagnetic fields in the extremely low frequency range include: bone growth stimulation to promote the healing of fractures, Transcranial Magnetic Stimulation to trigger brain activity or treat certain health conditions, wound healing, and pain treatment. ELF can also be used for cancer detection through bioimpedance measurements, a non-intrusive diagnostic method. More...
7.3 Can ELF fields increase the risk of childhood leukaemia and other cancers?
In 2002, the International Agency for Research on Cancer (IARC) classified ELF magnetic fields as “possibly carcinogenic to humans” (Group 2B). This was based on statistical studies indicating children are more likely to develop leukaemia if their exposure to extremely low frequency magnetic fields exceeds 0.3-0.4 µT, which would be relatively strong. Experimental studies on animals did not support these findings.
The potential link between extremely low frequency fields and childhood leukaemia has been addressed by a number of epidemiological studies, which have not found any conclusive evidence, and further studies are needed. No new influential study has appeared over the last few years concerning any other type of cancer.
A recent study on human populations suggests a link between defects in DNA-repair systems and childhood leukaemia caused by exposure to extremely low frequency fields at home. There are however too many weaknesses in this study to allow any conclusions to be drawn.
Studies on laboratory animals have shown little evidence that exposure to ELF magnetic fields alone could induce any type of cancer or would affect existing tumours. There is some inconsistent evidence that ELF magnetic fields of about 100 µT may enhance the development of tumours induced by other known carcinogens, but the majority of studies evaluating such combined effects did not find such a link. Results from recent studies are potentially helpful for explaining mechanisms and inconsistencies of previous findings, but they lack confirmation in independent experiments, and are not sufficient to challenge IARC’s evaluation that the experimental evidence for carcinogenicity of ELF magnetic fields is inadequate. This means that the experimental studies cannot be interpreted as showing either the presence or absence of a carcinogenic effect because of major qualitative or quantitative limitations.
Laboratory studies on isolated cells and tissues (in-vitro studies) can provide information on mechanisms of damage to cells. At this stage, published in-vitro studies cannot explain epidemiological findings, but do not contradict them either. They have shown many effects of ELF fields, and a large number of cellular components, cellular processes, and cellular systems can conceivably be affected by EMF exposure. The fact that the epidemiological findings of childhood leukaemia are neither supported by experimental studies nor explained by known mechanisms is intriguing and it is of high priority to overcome this contradiction. More...
7.4 Can exposure to ELF cause headaches or other health effects?
A variety of symptoms, often self-reported, have been suggested to be caused by ELF field exposure: skin redness, tingling and burning sensations, as well as fatigue, headache, concentration difficulties, nausea, and heart palpitation. The term “electromagnetic hypersensitivity” (EHS) has come into common usage based on the reported experience by the afflicted individuals that electric and/or magnetic ELF fields, or vicinity to activated electrical equipment trigger the symptoms. A relationship between ELF field exposure and those symptoms has not been shown in scientific studies.
Over the last few years, studies on electromagnetic hypersensitivity have come to focus on identifying various possible factors influencing the well-being of the group reporting symptoms. It appears that people who reported electromagnetic hypersensitivity, among other things, tended to have specific personality traits, be more anxious and more susceptible to stress, and report more other health complaints when compared to reference groups.
It remains unclear if there is a link between extremely-low frequency field exposure and some neurodegenerative diseases such as Alzheimer’s, but some recent data suggests there might be such a link.
Laboratory studies on animals have looked at possible effects of ELF magnetic fields on various parts of the body. Although some studies have observed effects on the nervous system, animal development, and melatonin production, the evidence for such effects was found to be weak and ambiguous, and inadequate for drawing conclusions concerning possible human health risks. Recent studies have suggested a link between magnetic fields and brain activity, but no conclusion can be drawn from this data.
Studies on isolated cells and tissues (in-vitro studies) are rather scarce when it comes to ELF fields and their possible role in diseases other than cancer. The data available suggests that exposure to EMF activates the expression of certain proteins, but the biological significance of these findings is still unclear. There is a need for hypothesis-based in vitro studies to examine specific diseases. More...
7.5 What can be concluded about ELF fields?
The main conclusions remain unchanged:
ELF magnetic fields have been classified as “possibly carcinogenic” by the International Agency for Research on Cancer (IARC). This conclusion is mainly based on studies on human populations indicating that exposure to relatively strong ELF magnetic fields might be a cause of childhood leukaemia. Laboratory studies on cell tissues have not yet provided an explanation of how exactly these fields might cause leukaemia.
No consistent relationship between extremely low frequency fields and self-reported symptoms such as fatigue, headache, and concentration difficulties has been demonstrated.
For some other diseases, such as cardiovascular diseases, recent research indicates that a link with extremely low frequency fields is unlikely. For yet other diseases, such as those affecting the brain and the spinal cord, the issue of a link to ELF fields remains open and more research is called for.
There is a need for hypothesis-based studies on cell tissues (in vitro studies) to examine specific diseases. It is notable that animal and in vitro studies show effects at exposure to ELF fields at levels (from 0.10 mT and above) that are considerably higher than the levels encountered in the epidemiological studies (µT-levels) which showed an association between exposure and diseases such as childhood leukaemia and Alzheimer's disease. This warrants further investigation. More...