Documents of the NRPB: Volume 3, No. 1
Recently reported epidemiological studies and laboratory research have raised questions about the adequacy of the existing guidelines for limiting exposure to electromagnetic fields. These have to take into account the balance of benefits and risks. The benefits associated with the sources of the fields are, however, clear, while the magnitude of the risks associated with low level exposures, if indeed there are any such risks, is certainly not. If, therefore, it is only sensible at the moment to retain the present guidelines, which are based on known and quantifiable effects, it is essential that the results of the recent studies on low level effects are kept under constant review, to ensure that the guidelines are based on the best available information. Many of these studies were reviewed in a draft document issued by the American Environmental Protection Agency (EPA) on an 'Evaluation of the Potential Carcinogenicity of Electromagnetic Fields'. This report has not yet been formally published but has received considerable attention. Following the issue of the EPA draft report, the Director of the Board set up an Advisory Group on Non-Ionising Radiation which has as its terms of reference a remit
to review work on the biological effects of non-ionising radiation relevant to human health and to advise on research priorities.
The first objective of the Advisory Group has been to review epidemiological and laboratory studies relevant to the possible carcinogenic effect of electromagnetic fields and to determine the extent to which the weight of evidence suggests they should be treated as a potential carcinogen. The emphasis of the present work has been on exposure to time varying fields. The Advisory Group has also made recommendations about further studies that need to be undertaken in the field of epidemiology and some preliminary recommendations about further experimental studies. The following text has been taken from the Conclusions of the report. In summary, the epidemiological findings that have been reviewed provide no firm evidence of the existence of a carcinogenic hazard from exposure of paternal gonads, the fetus, children, or adults to the extremely low frequency electromagnetic fields that might be associated with residence near major sources of electricity supply, the use of electrical appliances, or work in the electrical, electronic, and telecommunications industries. Much of the evidence that has been cited is inconsistent, or derives from studies that have been inadequately controlled, and some is likely to have been distorted by bias against the reporting or publishing of negative results. The only finding that is at all notable is the consistency with which the least weak evidence relates to a small risk of brain tumours. This consistency is, however, less impressive that might appear as brain tumours in childhood and adult life are different in origin, arising from different types of cell. In the absence of any unambiguous experimental evidence to suggest that exposure to these electromagnetic fields is likely to be carcinogenic, in the broadest sense of the term, the findings to date can be regarded only as sufficient to justify formulating a hypothesis for testing by further investigation.
Recommendations for research
- The Advisory Group recognises that the current paucity of fundamental knowledge on the biological effects of low level electromagnetic fields and the multitude of experimental variables involved make it difficult to propose a specific programme of research aimed resolving possible carcinogenic effects. In general, however, it was thought that further directed work on fundamental biophysical interactions and on possible promotional/co-carcinogenic effects of electromagnetic fields should be emphasised.
- A feature of the current experimental database is the difficulty experienced in reproducing some apparent biological effects and a lack of coherence between different investigations of similar biological endpoints. The Advisory Group suggests that more emphasis is needed on the consolidation of 'positive' findings and the formulation of the testable hypotheses necessary for the whole field to progress beyond the largely phenomenological position it currently occupies.
- The following set of research areas, whilst not comprehensive, serves to highlight further studies that should be considered.
- While some biophysical models of possible cellular and tissue interactions have been formulated there is need for further theoretical consideration of how electromagnetic fields might operate at the bimolecular level. Here, the possibilities of long-range co-operative interactions and ionic resonance effects on the cell membrane leading to physiological changes within and between cells have been suggested. It was also considered important to explore further the possibility that specific frequency and amplitude windows exist for the induction of biological effects. If these could be unambiguously shown to exist then it might be necessary to revise the conventional view that a simple relationship should exist between the level of electromagnetic field exposure and potential biological effect. In addition, since electromagnetic fields are known to affect chemical radical-mediate reactions in simple organic solutions 1, 2, it would be valuable to know whether such effects modulate cellular biochemical reactions involving radical intermediates, particularly those thought to play a role in tumour promotion 3. In general, however, current theoretical models are not sufficiently developed or lack convincing experimental support. A major potential benefit of experimentally supported theoretical models is that they may provide guidance on the appropriate exposure conditions under which further experimental and epidemiological evidence of electromagnetic field effects might be sought.
Studies on cell membrane function and gene expression
- Theoretical and some experimental studies point towards the cell membrane as a possible target for electromagnetic fields to affect. Mammalian cells interact with their environment through specific membrane protein receptors. Such receptors allow the transduction of extracellular signals from growth factors and hormones into the cytoplasm where they interact with biochemical pathways known to be involved in both tumour promotion and the activation of tumour-associated genes. In addition to this, some tumour promoters are now believed to interfere with cell-to-cell communications through specialised membrane structures thus terminating the signals that may normally suppress pre-neoplastic cell proliferation 4. Studies on electromagnetic field effects in these crucial areas of cell physiology and growth response may yield valuable information on whether these fields have biophysical properties consistent with a tumour promotion mechanism.
Cell transformation studies
- Many of the currently used cell transformation systems are based on grossly abnormal mammalian cell lines and may not provide a reliable guide to in vivo carcinogenic potential. It is therefore recommended that studies on possible electromagnetic field effects be extended to include newly developing transformation systems based on more normal cultured mammalian cells and grafted tissue explants 5 where more directly relevant data may be expected.
Whole animal experiments
- It was felt that large-scale animal studies would be necessary to explore further possible promotional/co-carcinogenic effects and while there is a paucity of quantitatively reliable animal models of tumour promotion it is suggested that a limited number of such studies should be considered. A murine model of skin carcinogenesis 6 might offer the best immediate opportunities but animal models of leukaemogenesis and brain tumorigenesis might also be sought. In addition, the experimental studies revealing a possible association between electromagnetic field exposure, melatonin synthesis and breast cancer were considered to be potentially important; further research in this area is also recommended.
- In conclusion, the Advisory Group believes that, while existing data are inconclusive, further experimental studies can play a significant role in resolving current uncertainties on the possible association between electromagnetic fields and carcinogenesis and, in this respect, provide some guidance for the design and interpretation of more definitive epidemiological studies.
- Evidence strong enough to justify the conclusion that exposure to greater than normal electromagnetic fields can cause cancer in humans or to allow the hypothesis to be rejected is unlikely to be obtained without much high quality epidemiological research that includes appropriate exposure measurements. Two approaches are most likely to be fruitful: study of cohorts of workers occupationally exposed to different fields and case-control study of the history of children who develop the disease in comparison with that of unaffected controls.
- Interest in potential occupational hazards focuses primarily on the risk of developing brain cancer and leukaemia. Both are rare diseases, large numbers of exposed workers will need to be studied, and detailed proposals for further research cannot be made until more information is obtained about the numbers of workers in different industries who are likely to have been materially exposed in the past or are likely to be so exposed in the future. The immediate need is, therefore, for surveys to be carried out in the industries that are suspected of causing exposure substantially above normal to determine the characteristics of personal exposures and the number of workers likely to be exposed to different levels and to different extents.
- Children's cancers have the advantage from the point of view of epidemiological investigation that past exposures of the children relevant to the causation of the disease can have been only brief and, in most cases, it is possible to identify and make measurements in all the locations in which the children have lived.
- Informative studies will need to provide objective observations about a group of control children that constitutes a truly representative sample of the population from which the affected children are drawn. At the same time, information will need to be obtained about the extent to which the children have been exposed to other suspected aetiological agents. A study of this sort is complex and expensive, but it could be carried out in a reasonable time more easily in the United Kingdom than in most other countries - for not only is the population of children sufficiently large to provide enough cases for the detection of quite a small hazard within a few years, but the structure of the national records also provides a suitable sampling base from which a satisfactory control group can be readily obtained.
- It is recommended that research along both these lines should be undertaken as soon as practicable.
Last reviewed: 6 August 2013