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Home Publications Radiation NRPB Archive Documents of the NRPB ›  Documents of the NRPB: Volume 12, No. 1

Documents of the NRPB: Volume 12, No. 1

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Authors:

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Publication date: 2001

ISBN: 0-85951-456-0

 

Synopsis

ELF Electromagnetic Fields and the Risk of Cancer: Report of an Advisory Group on Non-Ionising Radiation

Contents

  • Advisory Group on Non-Ionising Radiation 
  • Introduction:
    • References
  • ELF electric and magnetic fields: sources and measurements:
    • Introduction
    • Exposure assessment
    • Exposure to ELF fields
    • Summary
    • References
  • Recent cellular studies relevant to carcinogenesis:
    • Introduction
    • Initiation
    • Promotion or alteration of cellular processes which could affect the mechanisms leading to cancer
    • Summary
    • Conclusions
    • References
  • Recent animal and volunteer studies relevant to carcinogenesis:
    • Introduction
    • In vivo studies of mutagenesis
    • Animal tumour studies
    • Melatonin, cancer and electromagnetic fields
    • Immune system responses relevant to cancer
    • Conclusions
    • References
  • Recent epidemiological studies on residential electric and magnetic fields and cancer:
    • Introduction
    • Studies of leukaemia and other malignancies in children
    • Other studies of children specifically limited to CNS tumours
    • Studies of cancer in adults
    • Appliance use
    • Miscellaneous studies
    • Commentary
    • Overviews
    • Case-specular method
    • Conclusions
    • References
  • Occupational exposure to time-varying ELF electric and magnetic fields and cancer:
    • Introduction
    • Studies lacking personal exposure measurements
    • Studies with quantitative estimates of personal exposures
    • Methodological issues
    • Discussion
    • Conclusions
    • References
  • Conclusions:
    • Exposure assessment
    • Cellular studies
    • Animal and volunteer studies
    • Residential exposure
    • Occupational exposure
    • General conclusion
  • Recommendations for research:
    • Experimental studies
    • Epidemiological studies
  • Appendix: Incidence of neoplasia in life-time animal studies:
    • References
  • Glossary

Advisory Group on Non-Ionising Radiation

Chairman

Sir Richard Doll, Imperial Cancer Research Fund Cancer Studies Unit, Oxford

Members

Professor C Blakemore, University of Oxford
Professor E H Grant, Microwave Consultants Limited, London
Professor D G Harnden, Wythenshawe Hospital, Manchester
Professor J M Harrington, Institute of Occupational Health, Birmingham
Professor T W Meade, St Bartholomew's and Royal London School of Medicine
Professor A J Swerdlow, Institute of Cancer Research, London

Secretariat

Dr R D Saunders, National Radiological Protection Board, Chilton

Observer

Dr H Walker, Department of Health, London

Assessors

Dr A F McKinlay, National Radiological Protection Board, Chilton
Dr C R Muirhead, National Radiological Protection Board, Chilton
Dr J W Stather, National Radiological Protection Board, Chilton

Consultant

Mr S G Allen, National Radiological Protection Board, Chilton

Please note that the paragraph numbering from the full published document has been retained in the chapters below.

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Conclusions (chapter 7)

  1. The Advisory Group provides in this report a comprehensive review of experimental and epidemiological studies relevant to an assessment of the possible risk of cancer resulting from exposures to power frequency (extremely low frequency, ELF) electromagnetic fields (EMFs) that have been published since its first report, Documents of the NRPB Vol. 3 No. 1, in 1992. It is not concerned with exposures to high frequencies nor with other potential effects of exposure to power frequencies. The possibility of an association between neurological diseases, such as Alzheimer's disease, and magnetic field exposure is being considered separately. The report summarises the extent of exposure to power frequency electromagnetic fields at home and at work and reviews recent epidemiological investigations of cancer incidence in humans. It also reviews recently published cellular, animal and human volunteer studies.

Exposure

  1. Studies reviewed in the earlier report by the Advisory Group suffered from a lack of measurement-based exposure assessments. Since then, considerable advances have been made in methods for assessing exposure, both in the case of experimental studies and in epidemiological investigations. Instrumentation allowing personal exposure to be measured has become widely available and has been used in many of the more recently published studies. This has provided a substantially improved basis for many of the epidemiological studies reviewed by the Group.

Cellular studies

  1. At the cellular level, there is no clear evidence that exposure to power frequency electromagnetic fields at levels that are likely to be encountered can affect biological processes. Studies are often contradictory and there is a lack of confirmation of positive results from different laboratories using the same experimental conditions. There is no convincing evidence that exposure to such fields is directly genotoxic nor that it can bring about the transformation of cells in culture and it is therefore unlikely to initiate carcinogenesis.
  2. The most suggestive evidence of an effect of exposure to power frequency magnetic fields on biological systems comes from three different areas:
    1. possible enhancement of genetic change caused by known genotoxic agents.
    2. effects on intracellular signalling, especially calcium flux.
    3. effects on specific gene expression.
  3. Those results that are claimed to demonstrate a positive effect of exposure to power frequency magnetic fields tend to show only small changes, the biological consequences of which are not clear.
  4. Many of the positive effects reported involve exposure to time-averaged fields greater than 100 µT which are unlikely to be encountered in a domestic situation where typical exposures generally fall in the range between 10 and 200 nT. It is usual to test carcinogens at levels well above those normally encountered in order to demonstrate their potential to have an effect, on the assumption of a linear dose-response relationship without threshold. However, such an assumption may not be justified with non-genotoxic agents and risk assessment is most usefully focused on realistic exposure levels. Furthermore, the induced current density may be radically different in vivo as compared with that for cells in culture.

Animal volunteer studies

  1. Overall, no convincing evidence was seen from a review of a large number of animal studies to support the hypothesis that exposure to power frequency electro-magnetic fields increases the risk of cancer.
  2. Rodents, particularly mice, have been used extensively in studies of adult leukaemogenesis; there is, however, currently no natural animal model of the most common form of childhood leukaemia, acute lymphoblastic leukaemia. Most studies report a lack of effect of power frequency magnetic fields on leukaemia or lymphoma in rodents, mostly mice. These include several recent large-scale studies of spontaneous tumour incidence in normal and transgenic mice, and of radiation-induced lymphoma and leukaemia in mice. The transgenic mice used in two of the studies mentioned above develop a disease with some similarities to childhood acute lymphoblastic leukaemia. Further studies found no effect on the progression of transplanted leukaemia cells in mice or rats.
  3. Rat mammary carcinomas represent a standard laboratory animal model in the study of human breast cancer. Three recent large-scale studies of rats found that lifetime magnetic field exposure had no effect on the incidence of spontaneous mammary tumours. The evidence concerning electromagnetic field effects on chemically induced mammary tumours is more equivocal. Two early studies suggested that exposure to power frequency magnetic fields increased the incidence or growth of chemically induced mammary tumours in female rats but two more recent studies have not corroborated these findings.
  4. Whilst there is no natural animal model of spontaneous brain tumour, a recent large-scale study reported a lack of effect of exposure to power frequency magnetic fields on chemically induced nervous system tumours in female rats. In addition, the low incidence of brain cancers in three recent large-scale rat studies was not elevated by magnetic field exposure. With regard to studies of other tumours, particularly chemically induced skin tumours, the evidence is almost uniformly negative.
  5. The possibility that the hormone melatonin acts as a natural tumour suppressor is controversial. Nevertheless, a number of studies have investigated the ability of power frequency electromagnetic fields to alter endogenous circadian melatonin rhythms. Most evidence from human volunteer studies suggests that melatonin rhythms are not delayed or suppressed by exposure to power frequency magnetic fields, although one recent study provided preliminary data indicating that exposure prior to the night-time rise in serum melatonin may have had this effect in a sensitive subgroup of the study population. In addition, the evidence for an effect of exposure to power frequency magnetic fields on melatonin levels and on melatonin-dependent reproductive status in seasonally breeding animals is largely negative. The evidence concerning power frequency electromagnetic field induced suppression of rat pineal and/or serum melatonin levels is equivocal and the physiological relevance of any effect (if any is produced) remains unclear.
  6. There is no consistent evidence of any inhibitory effect of power frequency magnetic field exposure on those aspects of immune system function relevant to tumour suppression that have been examined. In addition, two studies were unable to correlate possible electromagnetic field induced changes in tumour incidence with significant changes in immune function.

Residential exposure

  1. Recent large and well-conducted studies have provided better evidence than was available in the past on the relationship between power frequency magnetic field exposure and the risk of cancer. Taken in conjunction they suggest that relatively heavy average exposures of 0.4 µT or more are associated with a doubling of the risk of leukaemia in children under 15 years of age. The evidence is, however, not conclusive. In those studies in which measurements were made, the extent to which the more heavily exposed children were representative is in doubt, while in those in Nordic countries in which representativeness is assured, the fields were estimated and the results based on such small numbers that the findings could have been due to chance. In the UK, very few children (perhaps 4 in 1000) are exposed to 0.4 µT or more and a study in the UK, with much the largest number of direct measurements of exposure, found no evidence of risk at lower levels. Nevertheless, the possibility remains that high and prolonged time-weighted average exposure to power frequency magnetic fields can increase the risk of leukaemia in children. Data on brain tumours come from some of the studies also investigating leukaemia and from others concerned exclusively with these tumours. They provide no comparable evidence of an association. There have been many fewer studies in adults. There is no reason to believe that residential exposure to electromagnetic fields is involved in the development of leukaemia or brain tumours in adults.

Occupational exposure

  1. Study of populations exposed occupationally to electromagnetic fields can include groups exposed generally at much higher levels than members of the public. They may therefore have a greater potential to detect any adverse health effects. Although recently published studies of occupational exposure to electromagnetic fields and the risk of cancer are, in the main, methodologically sound, and some of them have considerable statistical power, causal relationships between such exposure and an increase in tumour incidence at any site are not established. The excesses, where they exist, are generally modest and are largely restricted to the two cancers that were noted in the 1992 report of the Advisory Group - that is, leukaemia and cancer of the brain. Conflicting evidence exists for the particular cell types of leukaemia associated with the greatest risk but acute myeloid leukaemia is the most cited. The evidence of any risk for brain cancer is conflicting, even that from the most powerful of the studies.

General conclusion

  1. Laboratory experiments have provided no good evidence that extremely low frequency electromagnetic fields are capable of producing cancer, nor do human epidemiological studies suggest that they cause cancer in general. There is, however, some epidemiological evidence that prolonged exposure to higher levels of power frequency magnetic fields is associated with a small risk of leukaemia in children. In practice, such levels of exposure are seldom encountered by the general public in the UK. In the absence of clear evidence of a carcinogenic effect in adults, or of a plausible explanation from experiments on animals or isolated cells, the epidemiological evidence is currently not strong enough to justify a firm conclusion that such fields cause leukaemia in children. Unless, however, further research indicates that the finding is due to chance or some currently unrecognised artefact, the possibility remains that intense and prolonged exposures to magnetic fields can increase the risk of leukaemia in children.

 

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Recommendations for research (chapter 8)

  1. The Advisory Group recognises that the scientific evidence suggesting that exposure to power frequency electromagnetic fields poses an increased risk of cancer is very weak. Virtually all of the cellular, animal and human laboratory evidence provides no support for an increased risk of cancer incidence following such exposure to power frequencies, although sporadic positive findings have been reported. In addition, the epidemiological evidence is, at best, weak. Nevertheless, considering the ubiquitous nature of power frequency electromagnetic field exposure and the concern about possible adverse health effects, the Advisory Group considers that the following areas of research merit further investigation.

Experimental studies

  1. Further biophysical studies might suggest conditions of exposure more liable to affect carcinogenic processes. Particular attention should be given to weak magnetic field effects on biochemical processes involving radical pair intermediates. Consideration should also be given to the possibility that exposure parameters such as the higher frequencies associated with switching transients might be more biologically relevant than experimental data based only on the time-weighted average exposure. Additional dosimetric studies are required using improved tissue conductivity data in order to quantify more accurately the magnitude and distribution of induced current in the body. Consideration needs also to be given to the possible effects that might result from the dispersal of corona ions and the way any such effect might be assessed.
  2. At the cellular level, further studies should be carried out of possible enhancement of genetic change caused by known genotoxic agents, effects on intracellular signalling and effects on specific gene expression. These studies should focus, where possible or appropriate, on the replication of studies that have previously suggested positive results.
  3. For animal carcinogenesis studies, future work should be based on carefully designed, hypothesis-driven investigations. Such hypotheses may be derived from consideration of mechanistic investigations at the cellular level and epidemiological investigations. With regard to the epidemiological observations concerning possible increased risks of childhood acute lymphoblastic leukaemia, the absence of a natural animal model has imposed significant restrictions on experimentation. However, there are various transgenic mouse models of leukaemia which develop a disease having some similarities to childhood acute lymphoblastic leukaemia which may prove useful in future studies. It would in addition be valuable to study possible power frequency effects on the cellular structure and development of the prenatal and neonatal haemopoietic system and any implications for cellular differentiation and clonal growth. There is no strong epidemiological or experimental evidence concerning increased risks of brain or mammary tumours and therefore there is less imperative for further study. However, a recently developed model of spontaneous medulloblastoma in Ptch-knockout mice and, more particularly, a mouse model of astrocytomas, a leading cause of brain cancer in humans, may prove useful in the investigation of electro-magnetic field effects on spontaneous brain tumour incidence. In addition, further investigation should resolve present uncertainties concerning possible electromagnetic field effects on chemically induced mammary tumours.
  4. With regard to possible effects on circulating melatonin levels, there is further scope for longer term volunteer studies in the laboratory and volunteer or observational studies in the workplace. However, careful consideration must be given to individual variability in melatonin fluctuation in addition to differences in lifestyle, night-time light exposure and other possible confounding factors.
  5. Whilst the evidence concerning possible electromagnetic field effects on the immune system is mostly negative, the effects on tumour rejection per se have not been investigated and further study should be carried out using classical tumour rejection models.

Epidemiological studies

Residential studies

  1. Residential studies published to date have mostly been difficult to interpret because of the potential for the control data to be biased. Further work is required to investigate the extent to which the methods of control selection that have been used could have affected the frequency with which relatively high exposures were recorded.
  2. Nothing would seem to be gained by further study of more cases of childhood leukaemia in relation to exposure to extremely low frequency electromagnetic fields in the UK, as the number likely to have been exposed to fields of the strength that may cause a material increase in risk (namely fields of 0.4 µT or more) is too small to provide any useful information. There are, however, parts of the European Union, notably Denmark and Sweden, where such exposures are more common and, moreover, where unbiased evidence can be obtained through the use of national registers. It is, therefore, to be hoped that the European Union will fund an extension of the studies that have been reported from the Nordic countries, which alone might provide clear evidence of the existence of a risk (if one does in fact exist). If parts of the world can be identified where yet greater exposures to children occur frequently, and where good quality epidemiological studies are practical, then study of leukaemia risk in relation to electromagnetic field exposures in those places would be valuable.
  3. If relatively high residential magnetic fields do not produce a risk directly, it is possible that they might do so in association with some specific (or near specific) alteration in the cell's DNA. It might therefore be helpful to compare the characteristics of the DNA in cases of acute lymphoblastic leukaemia that occurred after exposure to such fields with the DNA in the general run of the disease. Because there would be so few relevant cases in the UK, the research would be worthwhile only with international collaboration.

Occupational studies

  1. Although occupational studies based on job title suggest a consistent link to excess risks of leukaemia and possible brain tumours, occupational cohort studies have not confirmed this association and are at best equivocal. The more recent cohort studies using better exposure characterisation for magnetic and electric fields either have not shown an association with leukaemia or brain cancer, or the association has been weak. Better quality exposure assessment is needed, preferably with detailed personal records of exposure in large well-characterised cohorts. In addition to cumulative exposure assessments, consideration should be given to the use of metrics such as rate of change of exposure, exposure peaks, duration of exposure above predefined exposure levels and rapid changes in exposure (transients). The paucity of good quality exposure data hampers progress and research within industry is required to correct this deficiency, define the most heavily exposed groups and quantify their exposure. In future cohort studies of exposed workers, note should be taken of the individual's residential history and, when residence had been near a high power transmission line, measurements of exposure at home should also be included.

 

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Last reviewed: 5 August 2013