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Magnetic Fields

Direct effects of exposure to large static magnetic fields include vertigo, nausea, cardiac arrhythmia and impaired mental functions, while potential indirect effects include

  • Tissue damage resulting from the movement of metallic implants e.g. hip prostheses, intracranial metallic strips or accidentally embedded metal fragments.
  • Injury due to the movement of metallic objects, such as scissors, within the field.
  • Injury due to the movement and or heating of externally worn metallic objects e.g. jewellery, rings, and wrist-watches.
  • Corruption / loss of information stored on magnetic business cards, data storage devices and some wristwatches.
  • Interference with the operation of cardiac pacemakers. The level at which this occurs depends on the type of device and the method of implantation.

Experimental evidence obtained from magnetic resonance imaging systems suggests that compliance with the relevant limits described here will avoid these effects: (See References 1 and 2)

  • Short term exposure to the head and trunk should not exceed 2tesla (T).
  • Short term exposure to the limbs may be up to 5tesla.
  • Short exposure periods totalling less than 15 minutes may be used and a number of such short exposures in any day is permissible, providing reasonable intervals of about one hour occur between exposures.
  • Whole-body time-weighted exposure to magnetic flux density (averaged over 24 hours) in should not exceed 200mT.
  • For the public a time-weighted exposure limit of 40mT is considered appropriate (although if access control measures are in place it is unlikely that members of the public would be exposed).
  • In air, the effects on small metallic objects are of little consequence where the magnetic flux density is less than 3mT.
  • Pacemaker interference is unlikely to occur if wearers are not exposed to static magnetic flux densities greater than 0.5mT over the region of the trunk.

Local control measures - to be incorporated with and in addition to other essential actions which must be taken.

  • Magnetic field flux density gradient ‘maps’ should be created by taking field strength measurements and marking lines around the source (e.g. tape or draw lines on the floor).
  • Notices must be posted where appropriate to warn staff of the presence of the magnetic field and of the possibility that it may affect the functioning of cardiac pacemakers and other electronic or metal equipment.
  • The immediate vicinity of the equipment should be classified as a controlled area with the boundary set at an appropriate value.
  • Appropriate means of physically preventing access past controlled area boundaries should be used.
  • Sample handling tool should be used where possible – to increase distance from / reduce time spent adjacent to high field strengths.
  • Loose metallic objects must never be brought into the vicinity of the source beyond a 3mT boundary.
  • Jewellery (including cosmetic piercing studs), rings, watches, keys and any other metallic objects carried on the person must not be brought into the vicinity of the source beyond a 3mT boundary.

References.
1. Advice on Limiting Exposure to Electromagnetic Fields (0 - 300GHz), Documents of the NRPB. Volume 15, No 2 (2004).
2. Guidelines for limiting exposure to time-varying electric, magnetic and electromagnetic fields (up to 300GHz). Health Physics 97(3):257-259; 2009.