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Power lines


Image of a high voltage pylon.

The main sources of power frequency electromagnetic fields are related to the transmission, distribution and use of electricity. Transmission power lines in the UK operate principally at 400 kilovolts (kV) and 275 kV, and distribution lines operate at 132 kV, 66 kV, 33 kV, 11 kV and 400 V. Underground cables and substations can also be sources of exposure. Away from power lines, power frequency EMF in homes arise from currents and voltages associated with distribution circuits and household electrical wiring, and the use of appliances.

The strength of EMF tends to fall rapidly with distance. The relative contribution of these sources to residential exposure in the UK is variable and depends on individual home circumstances. Power frequency EMF are also produced from the use of electricity in the workplace and from electrified transport systems.

Why Is There Concern about a Possible Risk of Cancer from Exposure to ELF Electric and Magnetic Fields?
The concern about the possible role of electromagnetic fields in cancer has its origin in an epidemiological study carried out in 1979 in the USA by Wertheimer and Leeper. In this study a statistical correlation between the incidence of childhood leukaemia and proximity to electricity supply wiring was demonstrated. The study prompted many other epidemiological studies to examine cancer incidence and exposure to electromagnetic fields. Since then, extensive programmes of experimental and epidemiological research have been undertaken to search for scientific evidence of possible health effects. Some of these studies were examined in earlier reports by AGNIR and more recent studies are reviewed in the present report. Most studies have been concerned with exposure to magnetic fields.  

Does Residential Exposure to Power Frequency Electromagnetic Fields Cause Cancer?
The residential epidemiology has suggested that there may be a small risk related to leukaemia in children and young persons and in particular to those exposed at levels of average domestic exposure to magnetic fields at or above 0.4 T (400 nT). However the evidence is inconclusive. The epidemiological association may be due to chance, confounding factors or some unrecognised artefact related to the way the data have been collected. The review of experimental studies gives no clear support for a causal relationship between exposure to ELF EMF and cancer. AGNIR also concluded that there is no reason to believe that residential exposure to EMF is involved in the development of cancer in adults, and in particular of leukaemia or brain cancer.

What Is the Main Conclusion of the Recent Report from AGNIR?
AGNIR concluded: Laboratory experiments have provided no good evidence that ELF 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 the 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. (AGNIR Report, page 164, paragraph 15.)
Imgae of the sun setting behind a pylon. How Do the Conclusions of the Present Report Differ from Those Expressed Previously by AGNIR?
In general there have been considerable advances in methods for assessing exposure, both in the case of experimental studies and in the epidemiological studies. The most recent review concludes that the epidemiological studies, in particular the pooled analysis of om et al., have provided a firmer base for the possibility of a risk of leukaemia in children, but at higher average exposures (0.4 T and above) than originally thought (0.2 T and above). The evidence, however, is not conclusive (page 163, paragraph 13) and chance, bias and confounding may explain some of the more recent results. The conclusions about the experimental studies are similar to those expressed previously by AGNIR they do not establish any biologically plausible mechanism whereby carcinogenic processes can be influenced by exposure to low levels of electromagnetic fields.

Power Frequency Electric Fields and the Risk of Childhood Cancer The results of the UK Childhood Cancer Study [UKCCS] that examined the relationship between childhood cancer and exposure to residential electric fields were published recently in the British Journal of Cancer1. This pilot study provides no support for the hypothesis that residential exposure to extremely low frequency electric fields is associated with childhood cancer, either by disease category or in total. The UKCCS began, in the early 1990s, as a population based case-control study covering the whole of Great Britain. It was designed to investigate putative causes of childhood malignancy including exposure to extremely low frequency [ELF] electric and magnetic fields [EMF]2. The results of the magnetic fields study have been published3. The pilot study of electric fields was conducted as part of the main magnetic field study but restricted to the second phase for which eligibility criteria identified case-control pairs where either member had a potential for high magnetic field exposure. After a given starting date in each UKCCS region, all phase 2 assessments included measurements of ELF electric field strength. This gave a pilot study population of 473 children who were diagnosed with a malignant neoplasm between 1992 and 1996 and who were aged 0-14 at diagnosis, together with 453 controls matched on age, sex and geographical location. Spot measurements of electric field strength were made in 3 orthogonal axes. Measurement locations were those likely to be associated with major determinants of a child's average exposure level, namely on the child's bed, in the child's bedroom and in the family room. The study demonstrated good temporal stability in residential ELF electric field levels. Moderately weak correlation of bed and family room measures indicated variability in electrical field strengths around the home. Examination of potential selection biases found little difference between cases and controls in the distribution of a deprivation index, though the most deprived category was under-represented in both groups. Unconditional logistic regression analysis adjusted for deprivation index as well as matching variables. The study used the mean of pillow and bed centre spot measurements as the principal exposure metric. Part way through the study, the introduction of instrument function tests before and after assessments enabled the assigning of validity status to assessments. For the 273 cases and 276 controls with fully validated measures, and comparing those with a measured electric field exposure > 20 V m-1 to those in a reference category of exposure 10 V m-1, odds ratios of 1.31 (95% confidence interval 0.68 2.54) for acute lymphoblastic leukaemia, 1.32 (95% CI 0.73 2.39) for total leukaemia, 2.12 (95% CI 0.78 5.78) for central nervous system cancers and 1.26 (95% CI 0.77 2.07) for all malignancies were obtained. There was no significant association between any of the diseases and exposure to electric fields at 20 Vm-1 and above. Similar results were obtained when considering the larger population of 426 cases and 419 controls where measures without validity status were not excluded. The corresponding odds ratios were 0.86 (95% CI 0.49 1.51) for acute lymphoblastic leukaemia, 0.93 (95% CI 0.56 1.54) for total leukaemia, 1.43 (95% CI 0.68 3.02) for central nervous system cancers and 0.90 (95% CI 0.59 1.35) for all malignancies.

The study also investigated calculated electric fields from local overhead power lines (66 kV 400 kV) and found neither an increase in risk nor a trend in dose response. In addition the use of electric blankets (n=44; 22 cases, 22 controls) and night storage heaters (n=8; 5 cases, 3 controls) was examined and was not associated with significant increase in risk. The negative findings of the pilot study are consistent with other epidemiological studies of childhood cancer and exposure to ELF electric fields. 1. UK Childhood Cancer Study Investigators (2002). Exposure to power frequency electric fields and the risk of childhood cancer in the UK. Brit. J. Cancer 87(11): 1257-66 2. UK Childhood Cancer Study Investigators (2000). The UK Childhood Cancer Study: objectives, materials and methods. Brit. J. Cancer 82(5): 1073-1102 3. UK Childhood Cancer Study Investigators (1999). Exposure to power frequency magnetic fields and the risk of childhood cancer. Lancet 354(9194): 1925-1931.
 
Link to an external website. The Health Protection Agency


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