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 3mT (30 Gauss). Consideration should be given to establishing systematic search protocols before such 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.
The following signs should be employed to warn those using magnetisable materials of the hazard of their use in strong static magnetic fields.
Effect on medical implants
See reference 4.2.
Persons wearing artificial metallic implants may feel painful sensations. Wearers of heart pacemakers, for instance, should be aware of the possibility of interference from magnetic fields.
"ICNIRP recognises that practical policies need to be implemented to prevent inadvertent harmful exposure of people with implanted electronic medical devices and implants containing ferromagnetic materials, and injuries due to flying ferromagnetic objects, and these considerations can lead to much lower restriction levels, such as 0.5mT (IEC, 2002)." - ICNIRP Fact Sheet, 2009 (PDF - link opens in a new window).
The following signs should be employed to warn those with medical implants of the hazards posed by strong static magnetic fields.
b) Biological Hazards
See reference 4.2.
Although present knowledge of the possible biological effects of strong static magnetic fields is still somewhat uncertain, some evidence has been obtained which indicates that simple biological systems may be affected by exposure to strong static magnetic fields. There is no specific information regarding possible long term health effects from exposure and none have been observed so far.
Investigations have been carried out to ascertain the effect of magnetic fields on humans, very few have shown any effect; in one or two cases associated with high magnetic fields it has been claimed that slight headaches, disorientation and slight feelings of nausea have ensued especially when there is movement in a static magnetic field. The only effect on which there appears to be some agreement is that of a taste sensation, possibly due to loose metal fillings in teeth, when exposed to high magnetic fields.
"For static magnetic fields in excess of 2-4 T, physical movement in static field gradient will induce sensations of vertigo and nausea that, although transient, may adversely affect people. Together with possible effects on eye-hand co-ordination, the optimal performance of workers executing delicate procedures could be reduced, with a concomitant impact on safety. Other acute effects are less clearly established; cardiovascular responses, such as changes in blood pressure and heart rate, have occasionally been observed in volunteer and animal studies, but lie within the normal range below 8 T." - Health Protection Agency (HPA) website (link opens in a new window) 2010
It is therefore recommended, wherever possible, that reasonable and simple measures to limit such exposure should be adopted, particularly exposure of the head or whole body. Where exposure to high magnetic fields in working situations cannot be avoided then the exposure limit values detailed in reference 4.2 should be considered.