a) Physical Hazards
Danger is frequently encountered where loose magnetic or magnetisable objects are in the vicinity of strong magnetic fields or where magnetic field gradients are high. The field may be strong enough to attract such objects and to cause them to fly along the field lines towards the magnet – the ‘missile effect’ or simply a crushing effect. Therefore metallic objects such as rings, glasses, watches, coins, steel toe caps and in particular those with sharp edges, keys, scissors, tools, gas cylinders, trolleys, vacuum cleaners etc. may become dangerous projectiles and their use should be controlled in any areas where the magnetic field exceeds 3 mT (30 Gauss) in the stray field from a strong >100 mT (>1000 Gauss) magnet. Consideration should be given to establishing systematic search protocols before electrically generated magnetic fields are started up to ensure that relevant areas are free from loose magnetic objects.
Permanent magnets, particularly rare earth magnets, can pose extra hazards since, by their nature, they are always generating a strong static magnetic field and gradient. Extra precautions need to be taken when handling them as the risk of pinching skin and crushing fingers is high. The magnets should only be handled one at a time, unless special fixtures are being used to restrain them, and non-magnetic tools should always be used in the vicinity of permanent magnet blocks or magnet assemblies that are powered by permanent magnets.
Movement of conducting materials in static magnetic fields
The movement of electrically conducting materials in strong static magnetic fields can result in the generation of eddy currents in the conductor which should be considered if assessing hazards.
Effect on medical implants
See reference 4.2.
Passive medical implants that contain ferromagnetic materials may be subject to forces and torques in the presence of strong static magnetic fields, which can result in movement of the implant that could result in injury to the wearer. The following types of implant may be susceptible, although it should be noted that this list is not necessarily exhaustive and not all implants of a given type will be manufactured from the same materials:
- artificial joints
- aneurysm clips
- metal surgical clips
- heart valve prostheses and annuloplasty rings
- contraceptive implants
- cases of active implants
- dental implants
Active medical devices may be subject to electromagnetic interference from strong external static magnetic fields. This is a particular issue where the device contains a magnetic switch, which is often included in the design to permit the device to be switched from outside the body. The following non-exhaustive list gives examples of devices that may be susceptible:
- cardiac pacemakers
- implanted cardiac defibrillators
- neuromuscular stimulation devices
- cochlea implants
- electronically operated prosthetic devices
- hormone infusion pumps
In general, medical implants and body-worn devices are not normally affected by fields less than 0.5 mT (5 Gauss).
b) Biological Hazards
See reference 4.2.
Movement through a strong magnetic field generates electric fields in the body that may affect excitable tissues. The organs of balance are particularly sensitive, leading to feelings of dizziness (vertigo), when walking through, or quickly moving the head in the field. The tongue may also be affected with a metallic taste in the mouth often reported. Other symptoms can include nausea. These are all considered to be sensory effects and normally only occur at fields in excess of 2 T. Where it is necessary for exposures to exceed 2 T, effects can be limited by avoiding rapid movements, although there is limited recent evidence to suggest that these effects may occur in the absence of movement.
As noted above, when a conductor moves in a static magnetic field, an electric field will be induced in the conductor. As blood is conductive, its flow in a strong field will result in the induction of electric fields even when the person is not moving. The magnitude of the induced flow potential depends on the strength of the magnetic field, the velocity of blood flow and the diameter of the blood vessel; the strongest electric fields will be induced by flow through the aorta and have the potential to disrupt the sinoatrial node. These electrodynamic effects should not occur to any significant extent for fields less than 8 T.
When a static magnetic field is applied perpendicular to the direction of blood flow, magnetohydrodynamic forces will act to reduce the flow rate. The magnitude of this effect is approximately proportional to the square of the magnetic field strength; aortic flow rates would be reduced by around 5% at 10 T. Similar effects are possible in other major blood vessels.
Collectively electrodynamic and magnetohydrodynamic effects are considered to be adverse health effects, but should not be significant provided exposures do not exceed 8 T.