Static Magnetic Fields
Characteristics of the field and its use
Static magnetic fields are constant fields, which do not change in intensity or direction over time, in contrast to low and high frequency alternating fields. Hence, they have a frequency of 0 Hz. They exert an attracting force on metallic objects containing, for example, iron, nickel or cobalt, and so magnets are commonly used for this purpose. In nature, the geomagnetic field of the earth exerts a force from south to north that allows, for example, the operation of a compass. Much stronger fields are generated by some types of industrial and medical equipment, such as in Medical Resonance Imaging (MRI) devices.
The strength of a static magnetic flux density is expressed in tesla (T) or in some countries in gauss (G). The strength of the natural geomagnetic field varies from about 30 to 70 µT (1 µT is 10-6 T). Household magnets have strengths in the order of several tens of millitesla (1 mT = 10-3 T). By contrast, the fields of MRI equipment vary from between 1.5 to up to as much as 10 T.
Effects of static magnetic fields on the body and health implications
There are several known mechanisms by which magnetic static fields can influence biological systems. Magnetic fields not only exert physical forces on metallic objects but also on moving electric charges. With respect to biological functioning, exposure to static magnetic fields will affect electrically charged particles and cells in the blood when moving through the field. The magnetic force can accelerate or reduce the movement of susceptible particles. An example is a reduction in the velocity of blood cells flowing through blood vessels. A further mechanism is via complex electronic interactions that may affect the rate of specific chemical reactions.
Only when humans are exposed to strong magnetic fields such as those generated by MRI equipment, or in some specialized research facilities, will perceptible effects on the human body occur. Fields of 2-3 T or higher can evoke transient sensations such as vertigo and nausea. These result from the generation of small electrical currents in the ear's balance organ. The currents generate signals to the brain that provide different information to that obtained through vision, resulting in the sensations of vertigo and nausea. These effects are not adverse health effects in themselves, but they may be annoying and they may impair normal functioning. There is no evidence for adverse effects of exposure to fields up to 8 T, except for limited information on minor effects on hand-eye coordination and visual contrast.
The ICNIRP guidelines for exposure to static magnetic fields protect against established health effects. In situations involving exposure to very high fields, specific working procedures should be developed to minimize the impact of transient symptoms such as vertigo and nausea. In particular, when the exposure in occupational situations involves movement through a strong static magnetic fieldit is recommended in certain circumstances, that the speed of movement through the field is restricted. For patients undergoing MRI diagnostic procedures, ICNIRP has also provided specific recommendations for the safe performance of such procedures.
In addition, safety authorities need to ensure that there are provisions to protect individuals who are wearing implanted ferromagnetic or electronic medical devices sensitive to magnetic fields.