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)?
The source document for this Digest states:
3.5. Extremely low frequency fields (ELF fields)
3.5.1. Sources and distribution of exposure in the population
The exposure due to electric fields and magnetic flux densities in the ELF range arises from a wide variety of sources (IARC 2002). The most prominent frequencies are 50 and 60 Hz and their harmonics, often called power frequencies. For residential exposure, the major sources are household appliances, nearby power and high voltages transmission lines, and domestic installations. In some cases trains have to be considered, too. Looking at occupational exposure, installations of the electric power industry, welding, induction heaters and electrified transporting systems are important examples of ELF exposure sources. The highest electric field strengths typically occur close to high voltage transmission lines and can reach 5 kV/m and in a few cases more. The highest magnetic flux densities can be found close to induction furnaces and welding machines. Levels of a few mT are possible.
It needs to be mentioned that the maximum possible exposure next to a specific source often differs by some orders of magnitude from the average individual exposure of a person (to specify time weighted average exposure in many cases the arithmetic mean or the geometric mean or the median value are applied). To evaluate the distribution of the exposure in the population, meters are used. For assessment of compliance with exposure limits, the maximum possible exposure next to devices must be measured. An example might be a lineman: the average exposure due to magnetic flux density could be about 4 µT (IARC 2002), but the maximum exposure close to a transmission line can reach 40 µT or more. For the general population even larger variations between maximum and average exposure can be expected. Information on ELF exposure is mainly based on data from the United States and Western Europe.
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Section 3.5 Extremely low frequency fields, 3.5.1 Sources and distribution of exposure in the population, p.30
7.2 What is the level of exposure to ELF fields?
The source document for this Digest states:
Exposure of the general population
Several fixed installed sources are operated in our environment. Prominent examples are high voltage transmission lines operated between 110 and 400 kV at 50 or 60 Hz. The exposure of bypassing people can typically reach values of 2 to 5 kV/m for the electric field strength. The exposure due to magnetic flux density depends on the actual current on the line; fields up to 40 µT are possible but are usually lower. It is important to notice that such exposure levels occur only directly below the lines; exposure decreases with the square of distance to the lines. In addition, intermediate voltage transmission lines (10 kV to 30 kV) and distribution lines (400 V) have to be considered; exposure levels are in such cases much lower. Typically values of 100 to 400 V/m and 0.5 to 3 µT can be reached, the exposure is usually instantaneous. Another approach to establish power supply is the use of underground buried cables. Electric field strength exposure can be neglected in this case; the distribution of the magnetic flux density differs compared to power lines. Substations and power plants are usually not accessible to the general public. Railway power supply installations are often operated at 16 2/3 Hz. The exposure decreases linearly with the distance. The exposure levels in areas accessible for the general public are below the limits. The highest magnetic flux densities can be found close to several domestic appliances that incorporate motors, transformers, and heaters. Such exposure levels are very local and decrease rapidly with the distance, exposure is instantaneous. An example is a vacuum cleaner: at a distance of 5 cm magnetic flux densities of about 40 µT can occur, but at 1 m the exposure will be around 0.2 µT. Looking at the individual exposure of persons, a few percent of the European population are in their homes exposed above a median magnetic flux density above 0.2 µT.
Exposure of workers
In a few locations in installations of the electric power industry the exposure limits given in the directive 2004/40/EC for occupational exposure can be reached or exceeded. Safety measures for such areas have to be implemented. An example is a peak electric field strength of more than 20 kV/m that was measured in a power station. Other examples of industrial applications in the ELF range are induction and light arc ovens or welding devices. The frequencies of such applications fall both in the ELF and in the intermediate frequency range. Exposure of workers has to be controlled for such devices. Next to welding devices maximum flux densities of several hundred µT are possible, depending on the welding current and the type of application.
Medical applications
Bone growth stimulation is used as a therapeutic application in the ELF range. In this case coils are applied where the fracture is located to stimulate the healing process. Other applications include Transcranial Magnetic Stimulation, wound healing, or pain treatment. A diagnostic application is the bioimpedance measurement for cancer detection.
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Section 3.5 Extremely low frequency fields, 3.5.1 Sources and distribution of exposure in the population, p.30-31
7.3 Can ELF fields increase the risk of childhood leukaemia and other cancers?
7.3.0 Epidemiology
The source document for this Digest states:
3.5.2.Cancer
3.5.2.1. Epidemiology
What was already known on this subject?
In 2002, the International Agency on Research on Cancer (IARC) published a monograph on the evaluation of carcinogenic risks of static and extremely low-frequency (ELF) electric and magnetic fields to humans (IARC, 2002). ELF magnetic fields were classified into group “2B” (“possibly carcinogenic to humans”). While the outcome of this evaluation was already known at the time of the last opinion, the IARC reasons for this decision were not yet published. The justification states limited evidence in humans based on consistent results from sound epidemiological studies showing an association with an increased leukaemia risk in children at average field strengths above 0.3/0.4 µT
(Ahlbom et al. 2000, Greenland et al. 2000), but bias could explain some of the raised risk. Thefindings from observational studies arenot supported by studies in experimental animals, which provide inadequate evidence of carcinogenicity.
Furthermore, the IARC monograph concluded, there was no evidence for an association of ELF magnetic fields with any other type of cancer. ELF electric fields were grouped into
“3” (“is not classifiable as to its carcinogenicity to humans”).
What has been achieved since then?
Only a few studies on childhood leukaemia have been conducted since the adoption of the previous opinion, and they did not add anything substantial to the previous studies. At a workshop of WHO in 2004, possible explanations for the childhood leukaemia finding have been put forward (summarized in Kheifets et al. (2005)). None of them reaches a level beyond hypothesis. One recent study has observed a decreased survival in children with leukaemia being exposed to average ELF magnetic fields above 0.3 µT (Foliart et al. 2006). This finding, however, is based on small numbers and no mechanism has been proposed, so confirmation studies have to be awaited before conclusions should be drawn. Most new ELF studies have been looking 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. Several studies also reported an increased breast cancer risk among subjects with elevated ELF exposure. However, later big and well controlled studies have been entirely negative and the hypothesis of a link between ELF field exposure and breast cancer risk is essentially written off (Forssen et al. 2005). While some new data on brain tumours have appeared since the previous opinion, firm conclusions can still not be drawn.
Discussion
Little data that have an impact on the evaluation have appeared since the previous opinion. Therefore, the previous assessments stay the same. The fact that the epidemiologic results for childhood leukaemia have little support from known mechanisms or experimental studies is intriguing and it is of high priority to reconcile these data.
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Section 3.5 Extremely low frequency fields, 3.5.2 Cancer, p.31-32
7.3.1 In vivo
The source document for this Digest states:
3.5.2.2. In vivo
What was already known on this subject?
The previous opinion did not evaluate evidence of carcinogenicity from animal studies. However, such data were included in the monograph by IARC that classified ELF magnetic fields into group 2B, “possibly carcinogenic to humans”, based on epidemiological studies showing an association between residential ELF magnetic fields and childhood leukaemia (IARC 2002). The long-term animal carcinogenicity studies reviewed by IARC provided very little evidence that exposure to ELF magnetic fields alone could induce any type of cancer, including hemopoietic, mammary, brain and skin tumours. Negative results were also obtained from studies that evaluated the effects of ELF magnetic fields on growth of transplanted tumour cells. Animal studies that combined magnetic fields with known carcinogenic agents produced more equivocal results, although also these co-carcinogenicity studies were mostly negative. Among the few positive findings are enhanced development of UV-induced mouse skin tumours in one study (Kumlin et al. 1998) and accelerated development of rat mammary tumours induced by 7,12-dimethylbenz(a)anthracene (DMBA) in several experiments by a German research group (Löscher et al. 1993, Baum et al. 1995, Mevissen et al. 1996, Mevissen et al. 1998, Thun-Battersby et al. 1999). The latter findings were not substantiated in independent replication studies (Anderson et al. 1999, Boorman et al. 1999), but there are differences in experimental details that could potentially explain the differences in results (Anderson et al. 2000, Löscher 2001). Based on the available experimental studies, IARC concluded that there is inadequate evidence for carcinogenicity of ELF magnetic fields in experimental animals.
What has been achieved since then?
Motivated by the epidemiological findings of increased leukaemia risk in children, Sommer and Lerchl (2004a) investigated the influence of 50 Hz (1 or 100 µT) magnetic fields in the AKR/J mouse strain genetically predisposed to thymic lymphoblastic lymphoma. There was no effect of magnetic field exposure on survival, and the time to lymphoma development did not differ between exposed and sham-exposed animals. The results do not support the hypothesis that chronic exposure to 50 Hz magnetic fields increases the risk of hemopoietic malignancy in this experimental model. However, the relevance of the model to human childhood leukaemia is limited.
New results have been published by German researchers who have reported accelerated development of DMBA-induced rat mammary tumours. In their most recent study
(Fedrowitz et al. 2004) they tested the hypothesis that use of different sub strains of SD rats explains the difference between their previous results and those of the replication studies. The results were consistent with the hypothesis: exposure to a 100 µT, 50 Hz magnetic field enhanced mammary tumour development in one sub strain of SD rats, but not in another sub strain obtained from the same breeder. The tumour data were supported by the finding that exposure to MF increased cell proliferation in the mammary gland of the MF-sensitive strain, but no such effect was seen in the insensitive sub strain. The finding is potentially important for explaining the inconsistent results, if the sub strain-specific effect of MF exposure is confirmed in further independent experiments.
Although short-term animal studies are considered less relevant for cancer risk assessment than long-term carcinogenicity and co-carcinogenicity studies, they can provide important contributions to understanding the mechanisms of carcinogenic effects. Genotoxicity of ELF magnetic fields was studied by Lai and Singh (2004b), who reported significantly increased DNA damage after exposure to a 60 Hz, 10 µT magnetic field for24 or 48 hours. Although the effect was relatively small, it was seen in several independent experiments. The effects were blocked by treatment with a radical scavenger, a nitric oxide synthase inhibitor and an iron chelator, suggesting involvement of free radicals and iron in the effects of magnetic fields. The same authors have previously reported similar effects after short (2 hour) exposure to higher magnetic flux densities of 0.1-0.25 mT. Environmental agents can promote the development of cancer also through non-genotoxic mechanisms such as stimulation of cell proliferation and inhibition of apoptosis. In support of their previous results suggesting co-carcinogenic effects of ELF magnetic fields (described above), two research groups have reported increase in cell proliferation markers in rat mammary gland (Fedrowitz et al. 2002) and inhibition of UV radiation-induced apoptosis in mouse skin (Kumlin et al. 2002) after short-term exposure to magnetic fields at 100 µT. The results of the short-term animal studies are interesting and, if confirmed in further independent experiments, potentially important for understanding possible cancer-related effects of magnetic fields.
Discussion
Overall there is no evidence from animal studies that ELF magnetic field exposure alone causes tumours or that it enhances the growth of implanted tumours. There is some inconsistent evidence that ELF magnetic fields of about 100 µT may enhance the development of tumours induced by known carcinogens, but the majority of studies evaluating such co-carcinogenic effects have been negative. 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.
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Section 3.5 Extremely low frequency fields, 3.5.2 Cancer, p.32-33
7.3.2 In vitro
The source document for this Digest states:
3.5.2.3. In vitro
What was already known on this subject?
There are many observations of cellular responses induced by ELF magnetic fields in vitro. A large number of cellular components, cellular processes, and cellular systems can conceivably be affected by EMF exposure. However, because evidence from theoretical and experimental studies suggest that ELF fields are unlikely to induce DNA damage directly, most studies have been conducted to examine effects on the cell membrane, general and specific gene expression, and signal transduction pathways. In addition, a large number of studies have been performed to investigate effects on processes such as cell proliferation, cell cycle regulation, cell differentiation, metabolism, and various physiological characteristics of cells.
Summaries of in vitro studies are found in Portier and Wolfe (1998) and IARC (2002). In particular, studies focusing on cell cycle kinetics, proliferation, differentiation, gene expression, DNA damage, signal transduction pathways, apoptosis and membrane characteristics have received attention and are useful in carcinogen evaluation.
What has been achieved since then?
It is generally accepted that ELF fields do not transfer energy to cells in sufficient amounts to cause direct DNA damage and subsequent genotoxic effects. However, it is possible that certain cellular processes, such as DNA repair, are altered by exposure to EMF, which could indirectly affect the structure of DNA causing strand breaks and other chromosomal aberrations, including sister chromatid exchange, or micronucleus formation.
A recent review of genotoxic effects after ELF field exposure (Vijayalaxmi and Obe 2005) analysed studies published 1990-2003 and found a very mixed picture. Overall, studies with positive or negative, or inconclusive, findings were more or less equal in frequency.
By analyzing studies using combinations of ELF and other factors (chemical as well as physical) with known carcinogenic or mutagenic effects, a recent review suggests that effects of these co-exposures are far more frequently appearing in the literature than effects of pure ELF exposure (Juutilainen et al. 2006). This finding suggests a possible interaction of ELF magnetic fields with other agents. Furthermore, this review suggests that since effects frequently appear from 0.10 mT and higher, the radical pair mechanism (Brocklehurst and McLaughlan 1996) could explain the presence of positive findings at such flux densities.
Regarding more recent experimental findings, studies on genotoxic effects performed as part of the REFLEX project have received considerable attention. Different types of human and other mammalian cells (including human fibroblasts and lymphocytes) were exposed to a range of frequencies, flux densities and exposure regimes (Ivancsits et al.
2003a, Ivancsits et al. 2003b, Ivancsits et al. 2005, Winker et al. 2005). Chromosomal damage (DNA strand breaks, micronucleus formation) due to exposure was found in some, but not all cell types (e.g. lymphocytes not affected), after intermittent but not after continuous exposure. Flux density, frequency, and exposure time were important for observed effects, as well as age of cell donors. Similar studies have been performed to ascertain the replicability of the results. The outcome of these studies are at present not completely available and do thus not allow for final interpretation of the data, although at least one study could not confirm the initial findings (Scarfi et al. 2005). Other recent studies using human cells have also shown inconsistent results regarding DNA damage after ELF exposure (alone or in combination with chemical or other physical agents). These studies vary considerable both in exposure conditions and in techniques employed to test for clastogenic effects, making it difficult to draw firm conclusions at present. However, Mairs et al. (2007) recently showed that by using the very sensitive microsatellite sequence analysis, 50 Hz EMF at 1 mT could alone increase mutation rate in human glioma cells, as well as increase the mutagenic capacity of ionizing radiation. Also a study by Wahab et al. (2006) has recently indicated genotoxic actions of exposure to 50 Hz EMF. In this study it was seen that frequencies of sister chromatide exchanges were elevated in EMF exposed human lymphocytes. Any mechanism responsible for these possible genotoxic effects is not shown.
During the last years, there has been increased attention towards effects by ELF fields on free radical homeostasis as an indirect mechanism for several biological responses (Simkó and Mattsson 2004). Experiments with several cellular systems have shown that exposure leads to increased radical levels (e.g. Simkó et al. 2001, Rollwitz et al. 2004, Lupke et al. 2004). Interestingly, DNA damage in human cells (Wolf et al. 2005) exposed to ELF magnetic fields was counteracted by addition of antioxidants, suggesting that ELF magnetic fields can indirectly, possibly via changes in radical homeostasis, affect integrity of DNA.
Finally, based on data obtained with modern high-throughput screening methods and real-time PCR, Lupke et al. (2006) have suggested a comprehensive pathway by which ELF fields could influence cells of the immune system. The suggested pathway includes that membrane-associated events are affected by the fields, causing changes in radical homeostasis, and leading to down-stream events that include changesin gene expression, which could be of importance for regulation of proliferation.
Other biological endpoints relevant for carcinogenesis (e.g. cell cycle regulation, proliferation, apoptosis, gene expression) have been investigated in a number of studies. There are mixtures of positive and negative findings, not allowing for a general conclusion to be made regarding the overall potency for ELF fields to participate in the carcinogenic process. However, an interesting exception is three replication studies of an older study showing that low intensity 60 Hz MF can inhibit the antiproliferative effect of tamoxifen on a specific subclone of human MCF-7 breast cancer cells (Blackman et al. 2001, Ishido et al. 2001, Girgert et al. 2005). These are among the few EMF studies that have yielded reproducible results in several independent laboratories.
Discussion
The value of in vitro studies is in providing information on mechanisms of damage to cells and tissues. Published in vitro studies cannot explain epidemiological findings, but do not contradict them either. There is a need for independent replication of certain studies suggesting genotoxic effects and for better understanding of combined effects of ELF magnetic fields with other agents, their effects on free radical homeoastasis, as well as of the possible implications of ELF field inhibition of tamoxifen effects. Studies with improved design are also needed.
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Section 3.5 Extremely low frequency fields, 3.5.2 Cancer, p.33-35
7.4 Can exposure to ELF cause headaches or other health effects?
The source document for this Digest states:
3.5.3. Symptoms
What was already known on this subject?
A variety of symptoms (dermatological symptoms such as redness, tingling and burning sensations as well as for example fatigue, headache, concentration difficulties, nausea, heart palpitation) have been suggested to be caused by ELF field exposure. The term “electromagnetic hypersensitivity” (EHS) has come into common usage based on the reported experience by the afflicted individuals that electric and/or magnetic fields, or vicinity to activated electrical equipment trigger the symptoms.
In the CSTEE opinion of 2001, the possibility of hypersensitivity in some individuals was said to require confirmation and the reports of such health problems did not provide a basis for changes in exposure limits.
What has been achieved since then?
Since the CSTEE opinion of 2001 only few new provocation studies have been published on symptoms and ELF fields (for EHS and RF fields see Chapter 3.3.3). As stated in the WHO Fact sheet on electromagnetic hypersensitivity, well controlled and conducted double-blind studies have not shown any correlation between symptoms and EMF (WHO 2005). Rubin et al. (2005) reviewed 31 provocation studies (using different frequencies and EMF sources) testing more than 700 individuals reporting EHS (Rubin et al. 2005). The results in 24 of these studies did not support a relationship between the health problems and EMF. In seven of the other studies some supporting evidence was found, but in two cases the same research group failed to replicate their own findings. For another three studies Rubin and co-authors suspected that the results were statistical artefacts and in the final two studies the results were mutually incompatible.
Discussion
A relationship between ELF field exposure and symptoms has not been shown in scientific studies. From these results 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.
3.5.4 Other Health Effects
3.5.4.1. Epidemiology
Following the initial epidemiological study on childhood cancer a great number of other diseases have also been studied in relation to ELF fields. These diseases include cardiovascular disease, neurodegenerative disease and psychiatric disorders. An effect of heart rate variability seen in laboratory studies was the basis for a hypothesis that ELF exposure might affect the risk of cardiovascular disease and some initial epidemiologic results supported this. However, later well controlled studies have dismissed this hypothesis. For several of the other outcomes the support was never strong. Nevertheless, several neurodegenerative diseases are still considered worthy of study in this respect, and this refers particularly to ALS (amyotrophic lateral sclerosis) and Alzheimer disease (Ahlbom et al. 2001).
3.5.4.2. In vivo
What was already known on this subject?
The previous opinion did not evaluate evidence of health effects from animal studies. However, such data have been reviewed by IARC (2002) and ICNIRP (Bernhardt et al. 2003).
Nervous system and behaviour. While strong ELF fields are known to affect nerve and muscle cells and can be perceived, little evidence was found for effects on the nervous system or behaviour at environmental exposure levels. Effects of ELF magnetic fields on the EEG, cognition, behaviour and neurotransmitter levels have been described in a few studies, but there are inconsistencies in these data.
Reproduction and development. Several independent studies have suggested effects of ELF magnetic fields on the embryonic development of birds and other non-mammalian species, but the results are inconsistent. The evidence in mammalian species is restricted to minor skeletal anomalies seen in some studies with rats and mice. No consistent effects have been seen in any other reproductive or developmental endpoints in mammals. Minor skeletal variations are relatively common findings in teratological studies on rodents and often considered biologically insignificant.
Endocrine system. There is limited evidence of effects on melatonin production in experimental animals exposed to ELF magnetic fields, but such effects are not supported by other animal studies, and no statistically significant effects [5%-significance level] have been seen on human volunteers under controlled laboratory conditions.
Other effects. No consistent evidence has been found for cardiovascular or immune system effects of ELF fields.
What has been achieved since then?
Two recent animal studies have provided evidence that ELF magnetic field exposure may affect melatonin production by modifying the response of dairy cows to the length of photoperiod (Rodriguez et al. 2004) and by affecting the sensitivity of mice to circadian light variations (Kumlin et al. 2005). The results of two new studies are interesting biological observations suggesting EMF interactions with the effects of light (photoperiod) on melatonin production. These observations may help to explain the inconsistencies of earlier research on EMFs and melatonin. However, the results of both studies are only suggestive and should be confirmed in further experiments. The suggested EMF effects on melatonin are subtle and apparently observable only in specific conditions. For these reasons, these results are not helpful for human health risk assessment.
Discussion
Although some studies 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.
3.5.4.3. In vitro
What was already known onthis subject?
Very few in vitro studies have been directed at answering the question if ELF fields are involved in the onset of other diseases than cancer (Portier and Wolfe 1998). Naturally, many basic cell and molecular studies were performed, mostly to understand more about fundamental interaction mechanisms, but also to understand how certain ELF fields can be used for therapeutic purposes (bone and wound healing especially).
What has been achieved since then and discussion
Few studies are available that directly address any specific disease or group of disease. This is partly due to that few diseases are characterised in such a way that specific disease models exist on the cell level, but also due to that ELF fields have not been convincingly shown to be involved in specific non-cancerousdiseases.However, continuously there are reports showing that ELF fields during certain circumstances can give rise to cellular responses that are relevant for diseases of the nervous system, the immune system, endocrine organs, the skeleto-muscular apparatus, etc. Such studies do not at the present time allow extrapolation from the in vitro finding to any specific health state.
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Section 3.5 Extremely low frequency fields, 3.5.3 Symptoms & 3.5.4 Other health effects, p.35-37
7.5 What can be concluded about ELF fields?
The source document for this Digest states:
3.5.5. Conclusions about ELF fields
The previous opinion came to a similar conclusion regarding carcinogenicity of ELF fields as IARC’s evaluation, namely that ELF magnetic fields are possibly carcinogenic. This conclusion was mainly based on epidemiologic results indicating that exposure to ELF fields might be a cause of childhood leukaemia. This assessment is still valid. The fact that the epidemiological results for childhood leukaemia have little support from known mechanisms or experimental studies is intriguing and it is a high research priority to reconcile these data.
For some other diseases, notably breast cancer and cardiovascular diseases, later research has indicated that an association is unlikely. For yet some other diseases, such as neurodegenerative disease and brain cancer, the issue of an association to ELF fields remains open and more research is called for. A relation between ELF fields and symptoms has not been demonstrated.
Of current interest is to arrive at a better understanding of recently published genotoxicity results including those from the REFLEX study.
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Section 3.5 Extremely low frequency fields, 3.5.5. Conclusions about ELF fields, p.37
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