By far the cheapest way to transport electricity is by means of overhead cables, supported by large transmission towers or pylons. The towers themselves do not give off electromagnetic fields. It is the electric current flowing through cables that are strung between them which create magnetic and electric fields. The electric field is proportional to the line voltage, while the magnetic field depends on the load current. Typically, high voltage transmission lines carry high current and therefore give off both high electric and high magnetic fields.
In the UK, the largest powerlines (400 kV and 275 kV) are owned and maintained by National Grid. Although some 132 kV lines are owned by National Grid, most of these (and all lower voltage lines - 33kV, 11kV, 415 volts and 230 volts) are owned by the power companies. These may be British or international companies. These lines are often 'unbalanced', that is, the current on one side of the line cables is very different to that on the other. This leads to much higher electric and magnetic fields than if both sides carried equal currents.
As stated above, there are two types of field produced by the powerlines. The electric field depends on the voltage and is always present when the powerline is switched on. In contrast, the magnetic field is caused by the electric current flowing in the line when people use electrical power and can therefore vary considerably. Electric fields are stopped by most building materials, but magnetic fields penetrate most materials as if it wasn't there. The main factor that reduces magnetic fields is distance from the source.
The electric fields for underground power cables will be zero as they are screened by earth, concrete, sand etc. Usually underground cables are buried rather shallowly. As a result, the magnetic fields at the surface of the ground are very high near to the buried cable, higher than from overhead cables because they are closer to you. They fall off more rapidly than the fields from overhead wires, because the cables are closer together and cancel out each other's effects more quickly.
Hand-held, easy-to-use meters that will accurately measure both electric and magnetic fields such as the EMFields-PRO or ELF-GS3030B are available to hire or buy in the UK from EMFields.
Powerlines, extremely low frequency magnetic fields and health effects
Current research has mainly concerned magnetic fields, not only from high voltage powerlines from the electricity supply in general. The evidence points towards an association between exposure to magnetic fields and childhood leukaemia (references below), adult leukaemia [Tynes 2003, O'Carroll 2008], neurodegenerative diseases (such as amyotrophic lateral sclerosis) [Feychting 2003, Hakansson 2003, Ahlbom 2001], miscarriage [Lee 2002, Li 2002, Cao 2006], and clinical depression. A 2008 meta-analysis from Spain found a strongly statistically significant doubling in risk for Alzheimer disease (CI 1.51-2.80) [Garcia 2008].
Whilst the precise mechanisms are unknown, there are now a few theories proposing mechanisms for how the magnetic fields may cause adverse health effects [Henshaw 2002, Henshaw & Reiter 2005, Binhi 2008, Blank 2008].
High voltage powerlines also emit corona ions and some theories suggest that these may be responsible for increased risk of illnesses at distances from powerlines beyond the range of the electric and magnetic fields [Fews 1999a, Fews 1999b, Henshaw 2002, Henshaw 2008].
Leukaemia and cancer
Researchers Anders Ahlbom and Sanders Greenland separately published large meta-analyses in 2000, that both found a significant doubling in leukaemia rates associated with exposure to ELF magnetic fields of over 0.4 µT and 0.3 µT respectively [Ahlbom 2000, Greenland 2000].
In 2002, the California Department of Health produced a report in 2002 from their California EMF program, set up to review the health effects from electric and magnetic fields from powerlines, wiring, and appliances, concluding that EMFs were responsible for an increase in childhood leukaemia andadult leukaemia.
This was followed by further work in 2001 where Ahlbom et al conducted a review into EMFs and Health, and found that there was a doubling in childhood leukaemia for magnetic fields of over 0.4 µT, although he summarised that "This is difficult to interpret in the absence of a known mechanism or reproducible experimental support" [Ahlbom Dec 2001]. In 2007, the UK Health Protection Agency produced a paper showing that 43% of homes with magnetic fields of over 0.4 µT are associated with overground or underground circuits of 132 kV and above [Maslanyj 2007].
Ahlbom's findings were echoed by Draper et al in 2005 when a 70% increase was found in childhood leukaemia for those living within 200 metres (656 ft) of an overhead transmission line, and a 23% increase for those living between 200 metres (656 ft) and 600 m (1,969 ft). Both of these results were statistically significant [Draper 2005]. The authors considered it unlikely that the increase between 200 metres (656 ft) and 600 m (1,969 ft) is related to magnetic fields as they are well below 0.4 µT at this distance. Bristol University (UK) has published work on a theory that could account for this increase, and would also provide a potential mechanism, being that the electric fields around power lines attract aerosol pollutants [Fews 1999a, Fews 1999b, Henshaw 2002, Henshaw 2008].
Despite these findings, the World Health Organisation have published a factsheet maintaining that there is limited support for childhood leukaemia (insufficient for causality). However, it is also important that causality is not required for precautionary action, so this statement should not affect government decisions to apply more precautionary public policy where they deem appropriate.
Other health concerns
The California EMF program report also concluded that EMFs were responsible for an increase in childhood leukaemia, adult leukaemia, adult brain cancer, Lou Gehrig's disease, and miscarriage. This differs to a review by the International Agency for Research on Cancer in 2001, and the National Radiological Protection Board (now part of the UK Health Protection Agency) review in the same year. However, there have been serious criticisms of the metholodogy and criteria used in the IARC review [O'Carroll 2008].
The reasoning given by the California Department of Health panel for the difference of opinion was that "there were reasons why animal and test tube experiments might have failed to pick up a mechanism or a health problem; hence, the absence of much support from such animal and test tube studies did not reduce their confidence much or lead them to strongly distrust epidemiological evidence from statistical studies in human populations. They therefore had more faith in the quality of the epidemiological studies in human populations and hence gave more credence to them."
However, the California report concluded that they did not find there was a strong enough association between EMFs and birth defects and low birth weight, and were divided on the evidence for suicide and adult leukaemia.
UK SAGE report
The Stakeholder Advisory Group on ELF EMFs (SAGE) has been set up by the UK Department of Health to explore the implications and to make practical recommendations for a precautionary approach to power frequency electric and magnetic fields as a result of the HPA recommendations in March 2004.
The first interim assessment of this group was released in April 2007, and found that the link between proximity to power lines and childhood leukaemia was sufficient to involve a precautionary recommendation, including an option to lay new power lines underground where possible and to prevent the building of new residential buildings within 60 m (197 ft) of existing power lines.
The latter of these options was not an official recommendation to government as the cost-benefit analysis based on the increased risk for childhood leukaemia alone was considered insufficient to warrant it. The option was considered necessary for inclusion as, if found to be real, the weaker association with other health effects would make it worth implementing.
Hakansson N et al
, (July 2003) Neurodegenerative diseases in welders and other workers exposed to high levels of magnetic fields
, Epidemiology. 2003 Jul;14(4):420-6; discussion 427-8 [View Author's abstract conclusions
Li DK et al
, (January 2002) A population-based prospective cohort study of personal exposure to magnetic fields during pregnancy and the risk of miscarriage
, Epidemiology. 2002 Jan;13(1):9-20 [View Author's abstract conclusions
Cao YN et al
, (August 2006) Effects of exposure to extremely low frequency electromagnetic fields on reproduction of female mice and development of offsprings
, Zhonghua Lao Dong Wei Sheng Zhi Ye Bing Za Zhi. 2006 Aug;24(8):468-70 [View Author's abstract conclusions
Garcia AM et al
, (April 2008) Occupational exposure to extremely low frequency electric and magnetic fields and Alzheimer disease: a meta-analysis
, Int J Epidemiol. 2008 Feb 2 [Epub ahead of print] [View Author's abstract conclusions
, (July 2008) Do naturally occurring magnetic nanoparticles in the human body mediate increased risk of childhood leukaemia with EMF exposure?
, Int J Radiat Biol. 2008 Jul;84(7):569-79 [View Author's abstract conclusions
Henshaw DL et al
, (April 2008) Can disturbances in the atmospheric electric field created by powerline corona ions disrupt melatonin production in the pineal gland?
, J Pineal Res. 2008 Apr 1. [Epub ahead of print] [View Author's abstract conclusions
Greenland S et al
, (November 2000) A pooled analysis of magnetic fields, wire codes, and childhood leukemia. Childhood Leukemia-EMF Study Group
, Epidemiology. 2000 Nov;11(6):624-34 [View Author's abstract conclusions
Maslanyj MP et al
, (March 2007) Investigation of the sources of residential power frequency magnetic field exposure in the UK Childhood Cancer Study
, J Radiol Prot. 2007 Mar;27(1):41-58 [View Author's abstract conclusions