Elektromagnetische Felder Aktualisierung 2009

8. Static magnetic fields like those used in medical imaging

• 8.1 What are the sources of static magnetic fields?
• 8.2 What possible health effects of static magnetic fields have been studied?

8.1 What are the sources of static magnetic fields?

A magnetic field is a force field created by a magnet or as a consequence of the movement of the charges (flow of electricity). The magnitude (intensity) of a magnetic field is usually measured in Tesla (T or mT).

Static magnetic fields do not vary over time, and as such do not have a frequency (0 Hz). Examples are the fields generated by a permanent magnet or the Earth’s magnetic field.

Man-made static magnetic fields are generated wherever electricity is used in the form of direct current (DC), such as in some rail and subway systems, in industrial processes such as aluminum production, the chloralkali process, and gas welding.

The number of artificial sources of such fields is limited, but there are rapid developments of new technologies producing static fields. The number of people with implanted metallic devices such as pacemakers that can be affected by static magnetic fields is also growing.

One prominent application of strong static magnetic fields is Magnetic Resonance Imaging (MRI) that provides three-dimensional images of soft body tissue such as the brain and the spinal cord. This medical imaging technique uses very powerful permanent magnets, which can lead to high exposure levels both for patients and for operators.

Previous health assessments looked mainly at exposure to static fields alone, but many applications, particularly MRI, can lead to exposure to strong static fields in combination with radio frequency and other fields. Recent studies have thus started to look at different field combinations and their potential effects. More...

8.2 What possible health effects of static magnetic fields have been studied?

Few studies on human populations are available on the effects of static fields and the available evidence is not sufficient to draw any conclusion about potential health effects of exposure to static magnetic fields.

A large number of experimental studies on cell cultures have been carried out in an effort to detect biological effects of static magnetic fields. Experimental data have established that static magnetic fields can result in changes in the orientation of the forces applied on biological molecules and cellular components with magnetic properties – such as haemoglobin, rhodopsin (visual pigment), free radicals, and nitric oxide. Such changes can affect these biological molecules.

Human volunteer studies indicate possible instantaneous effects on neuronal functioning when moving through a static magnetic field or field gradient as used in clinical practice. These studies need confirmation.

Recent animal studies confirm earlier findings that static magnetic fields of several milliteslas (mT) can have direct effects on neurons. Studies on cell cultures also show that exposure to static magnetic fields in the millitesla range may change membrane properties. These changes may lead to changes in neuronal functioning though the effects seem to be reversible.

The studies on pain reduction in animals by exposure to static magnetic fields in the millitesla range are interesting. The question is whether rodents are an adequate model for humans in this respect, since no pain reduction in humans was observed after exposure to static magnetic fields that were 10 times stronger.

Recent animal experiments show an effect of static fields on blood flow, vessel growth, as well as on growth and development, but some results are contradictory and do not clarify the mixed results of previous studies.

Static fields seem to have an effect on the expression of specific genes in cells of humans and other mammals and these effects may depend on exposure duration and field gradients. Damage to genetic material has been reported, although it seems that these effects can be repaired and are not permanent.

Although a fair number of studies were published in 2007 and 2008, there is still a lack of adequate data for a proper risk assessment of static magnetic fields. More research is necessary, especially to clarify the many mixed and sometimes contradictory results.

Short term effects have been observed primarily on sensory functions for acute exposure. However, there is no consistent evidence for sustained adverse health effects from short term exposure up to several Teslas. More...