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Static Fields

3. What are the sources of static magnetic fields?

    The source document for this Digest states:

    The geomagnetic field varies over the Earth's surface between about 35 - 70 µT and is implicated in the orientation and migratory behaviour of certain animal species. Man-made static magnetic fields are generated wherever DC currents are used, such as in some transportation systems powered by electricity, industrial processes such as aluminium production and in gas welding. Magnetic flux densities of up to 2 mT have been reported inside electric trains and in developmental magnetic levitation (MagLev) systems. Workers are exposed to larger fields of up to around 60mT in the electrolytic reduction of alumina, and electric arc welding produces around 5mT at 1 cm from the welding cables.

    The advent of superconductors in the 1970s and 1980s facilitated the use of much larger magnetic fields in medical diagnosis through the development of Magnetic Resonance Imaging (MRI) and spectroscopy (MRS)1, and nuclear magnetic resonance (NMR), for research. It is estimated that some 200 million MRI scans have been performed worldwide. The static magnetic field of MRI scanners in routine clinical systems is generated by permanent magnets, superconducting magnets and combinations thereof in the range of 0.2 - 3 T. In research applications, higher magnetic fields up to 9.4 T are used for whole body patient scanning. The stray magnetic fields around the magnets for MRI studies are well defined and can be minimised in the shielded magnet versions. In terms of exposure, at the operator's console the magnetic flux density is typically about 0.5mT, but may be higher. However, occupational exposure up to and exceeding 1 T can occur during the construction and testing of these devices, and during medical procedures carried out in interventional MRI. Various physics research and high-energy technologies also employ superconductors where workers can be exposed regularly and for long periods to fields as high as 1.5 T.

    Source & ©: WHO "Environmental Health Criteria 232: Static Fields" (2006)
     Chapter 1, Section 1.1.1 Natural and Human-made sources

    For more information on   See EHC 232
    Natural magnetic fields    Chapter 3, Section 3.1.2 
    Man-made magnetic fields   Chapter 3, Section 3.2.2  

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