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
Most electricity carried in power lines, wiring and household appliances is alternating current (AC). Alternating current (AC) moves back and forth in cycles of either 50 or 60 times per second that is at a frequency of 50Hz and 60Hz (the latter predominantly in US). Such electromagnetic fields are classified as Extremely Low Frequency (ELF) fields, as their frequency is below 300 Hz.
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 powerline, 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 the set limits.
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 1m 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 (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?
7.3.1 In 2002, the International Agency for Research on Cancer (IARC) classified ELF magnetic fields as “possibly carcinogenic to humans” (Group 2B). ((Toolbox will explain IARC categories)) 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 few studies conducted since then have shed little light on the subject. Laboratory studies do not support a link between childhood leukaemia and extremely low frequency magnetic fields, and how these fields might cause childhood leukaemia remains unknown. This highlights a need for more research to reconcile the results.
In 2004 possible hypothesis explaining the childhood leukaemia findings have been put forward. One recent study has observed a decreased survival in children with leukaemia being exposed to average ELF magnetic fields above 0.3 µT, but confirmation studies have to be awaited before drawing conclusions.
Most new epidemiological studies have looked into breast cancer or brain tumour risk. Breast cancer caught particular interest because of experimental results suggesting that melatonin synthesis was related to ELF field exposure and because melatonin might play a role in the development of breast cancer. The hypothesis of a link between ELF field exposure and breast cancer risk has essentially been written off following big and well-controlled studies. While some new data on brain tumours have appeared, firm conclusions can still not be drawn. More...
7.3.2 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. More...
7.3.3 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. Because evidence from theoretical and experimental studies suggest that ELF fields are unlikely to damage the genetic material directly, most studies have examined possible effects on the cell membrane, gene expression, and on the transmission of signals by the cell. In addition, a large number of studies have been performed to investigate possible effects on processes such as cell proliferation, cell cycle regulation, cell differentiation, metabolism, and various physiological characteristics of cells. There is a need for independent replication of certain studies suggesting genotoxic effects and for studies with improved design. A better understanding is also needed on possible combined effects, on ELF effects on cell regulation, as well as on the inhibition of a breast cancer treatment. More...
7.4 Can exposure to ELF cause headaches or other health effects?
A variety of symptoms have been suggested to be caused by ELF field exposure: redness, tingling and burning sensations of the skin, 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 and it seems clear that ELF field exposure is neither a necessary nor a sufficient factor to trigger health complaints in individuals reporting symptoms. Whether ELF fields may be a contributing factor under some conditions remains to be determined.
Following the initial epidemiological study on childhood cancer, a great number of other diseases have also been studied in relation to ELF fields, but no strong support for a link between extremely low frequency fields and those diseases was found. Nevertheless, several diseases that affect cells of the brain and spinal cord are still considered worthy of study in this respect, and this refers particularly to ALS (amyotrophic lateral sclerosis) and Alzheimer’s disease.
Although some experimental studies on laboratory animals have described ELF magnetic field effects on the nervous system, animal development and melatonin production, the evidence for such effects is weak and ambiguous. No conclusions concerning possible human health risks can be drawn from these 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. Basic studies were conducted to understand the various interaction mechanisms, but at this stage the data is insufficient to allow an extrapolation to specific symptoms or conditions. More...
7.5 What can be concluded about ELF fields?
ELF magnetic fields have been classified as possibly carcinogenic by the International Agency for Research on Cancer (IARC). This conclusion is mainly based on epidemiological studies indicating that exposure to relatively strong ELF magnetic fields might be a cause of childhood leukaemia. Now these findings must be reconciled with experimental studies, which so far have provided little support. The big question remains: How exactly could the fields cause leukaemia?
For some other diseases, notably breast cancer and cardiovascular diseases, recent research indicates that a link with extremely low frequency fields is unlikely. For yet some other diseases, such as those affecting the brain and spinal cord, the issue of a link to ELF fields remains open and more research is called for.
No relationship between extremely low frequency fields and self-reported symptoms such as fatigue, headache, and concentration difficulties has been demonstrated so far.