GUIDELINES ON LIMITS OF EXPOSURE TO STATIC MAGNETIC FIELDS


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INTRODUCTIONThe rapid development of technologies in industry and medicine using static magnetic fields has resulted in an increase in human exposure to these fields and has led to a number of scientific studies of their possible health effects. The World Health Organization (WHO) recently developed a health criteria document on static electric and magnetic fields within the Environmental Health Criteria Program (WHO 2006). The document contains a review of biological effects reported from exposure to static fields and, together with other recent publications [mainly International Commission on Non-Ionizing Radiation Protection (ICNIRP) 2003, McKinlay et al. 2004, and Noble et al. 2005], serves as the scientific database for the development of the rationale for the guidelines described in the current document, which supersede those published by ICNIRP in 1994 (ICNIRP 1994).SCOPE AND PURPOSEThese guidelines apply to occupational and general public exposure to static magnetic fields. The guidelines do not apply to the exposure of patients undergoing medical diagnosis or treatment. Detailed consideration of protection of patients is given in an ICNIRP statement on protection of patients undergoing a magnetic resonance imaging (MRI) examination (ICNIRP 2004; ICNIRP in preparation).QUANTITIES AND UNITSWhereas electric fields are associated with the presence of electric charge, magnetic fields result from the physical movement of electric charge (electric current). Similarly, magnetic fields can exert physical forces on electric charges, but only when such charges are in motion. A magnetic field can be represented as a vector and may be specified in one of two ways: as magnetic flux density B or as magnetic field strength H. B and H are expressed in teslas (T) and amperes per meter (A m−1), respectively. In a vacuum and with good approximation in air, B and H are related by the expressionThe constant of proportionality μo in eqn (1) is termed the permeability of free space and has the numerical value 4π × 10−7 expressed in henrys per meter (H m−1). Thus, to describe a magnetic field in air or nonmagnetic materials (including biological materials) with an adequate approximation, only one of the B or H quantities needs to be specified.The magnitude of the force F acting on an electric charge q moving with a velocity v in a direction perpendicular to a magnetic flux density B is given by the expressionThe direction of the force (the Lorentz force) is determined from the vector product of the velocity of the charge and the magnetic flux density and is therefore always perpendicular to the direction of the flow of electric charge. As a result, the interaction of a magnetic field with electric charge will result in a change of direction of the flow of the charge, but never a change in velocity. Static magnetic fields do not deposit energy into tissues.The magnetic flux density, measured in teslas, is accepted as the most relevant quantity for relating to magnetic field effects. The magnetic flux within a given area of surface is the product of the area and the component of the magnetic flux density normal to its surface.A summary of magnetic field quantities and units is provided in Table 1.Standard international (SI) units are the internationally accepted units for expressing quantities in the scientific literature.

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