Directorate-General for Health and Food Safety
8. Static magnetic fields like those from battery devices and high voltage overhead DC power lines
- 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?
MRI scanners use static magnetic fields
Credit: Kasuga Huang
A magnetic field is created as a consequence of the movement of electric charges (electric current) and characterised by the force acting upon resting or moving electric charges. The strength of a magnetic field is usually measured in ampere per meter (A/m), alternatively, by accounting for the magnetic properties of material, as magnetic induction, in tesla (T).
Static magnetic fields do not oscillate, and therefore have a zero frequency (0 Hz). Examples of fields of natural origin are the magnetostatic fields generated by permanent magnets 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 aluminium production, the chloralkali process, and gas welding or where permanent magnets are sometimes technically used such as for clasps and closures in necklaces, underwear or handbags.
The variety 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 strong static magnetic fields is also growing.
One prominent application of strong static magnetic fields is Magnetic Resonance Imaging (MRI) that high-resolution cross-sectional images of the body including the head without shadowing by bony structures. This medical imaging technique uses very high static magnetic fields of several Tesla, 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.
8.2 What possible health effects of static magnetic fields have been studied?
In daily life, geomagnetic fields are too weak to generate relevant effects. The strong magnetic fields in and around Magnetic Resonance (MRI) scanners at present operating with 0.3 – 9.4 T are strong enough to produce relevant effects, which may necessitate taking protective measures such as using detector gates for control access to avoid having ferromagnetic devices brought in the vicinity of MRI scanners, to prevent from adverse interference with implanted cardiac pacemakers and – at magnetic fields at or above 4T - to prevent from dangerous stimulation of nerves and muscles.
In most of the available in vitro studies, SMF above 30 μT induced neuronal effects, although in some cases the effects were temporary and reversible. Gene expression was affected in all studies with predominantly several genes involved in cell growth and division.
A number of studies are reporting that effects of SMF exposures occur in animals, at levels ranging from mT to T. However, since many of the findings are limited to single studies, they do not provide any firm foundation for risk assessment. Some animal studies show an effect of static MF on blood flow, vessel growth and on growth and development, but some results are contradictory and do not clarify the mixed results of previous studies.
Since the previous SCENIHR Opinion (2009) a meta-analysis of studies, which have assessed the health effects of static magnetic fields, identified four studies which reported effects including dizziness, nausea, and vertigo. The exposure was not found to have a significant effect on cognitive function at any field strength. The frequency of occurrence of these symptoms seemed mainly to be associated with the strength of the MRI systems, the time spent in their vicinity, and the speed with which workers move through these fields. These effects can be explained by established interaction mechanisms and are more likely to occur in fields above 2 T. The relevance of these effects for the health of personnel remains unclear but, according to some studies, these dose-dependent effects could theoretically lead to an increased risk of accidents and errors by workers that are harmful for themselves or for patients under their care.
While these new studies confirm the conclusion that these effects can be repaired and are not permanent, there is also some evidence of genotoxic effects in patients undergoing MRI examination but it seems unlikely that the static field alone could cause such effects. Further studies on DNA integrity and MRI exposure are thus needed. Magnetic fields in and around MRI scanners are thus strong enough to enable relevant side-effects such as accelerating ferromagnetic objects or magnetic interference with electronic devices. These justify protective measures such as access control by detector gates or restricting access to pacemaker patients with not MRI-compliant implants to prevent adverse interference with implanted cardiac pacemakers.
Globally, there is no consistent evidence for sustained adverse health effects from short-term exposure up to several Teslas.