Controlled Magnets

Risks and Hazards

Controlled magnet systems present a significant risk and are therefore subject to a set of safety procedures. On this page we will look at the risks and hazards presented by these systems and general safety advice.

Magnet classification scheme and hazards

Controlled magnet systems are those which create controlled areas, i.e. conditions where a magnetic field of 0.5mT (0.5 millitesla, or 5 gauss) and above exists in the workplace. Magnet systems such as NMRs can create controlled areas that present hazards if certain items are brought into them. These items can be placed in two categories: things outside the body and things inside the body.

Things outside the body: within the controlled area, ferromagnetic objects will become attracted to the magnetic field and could become projectiles - they could strike a part of the body and trap it against the magnet or impact and puncture the magnet itself.

It is also possible that, should an object strike an NMR's magnet, the impact will cause a fracture and cryogenic fluid will leak from it. The rapid release of cryogens into a warm atmosphere can be explosive, or the expanding nitrogen cloud it creates could exclude oxygen from the atmosphere, asphyxiating those in the room.

There is also the risk of items containing magnetic data - such as credit cards and memory sticks - becoming corrupted from magnetic interference.

Things inside the body: high magnetic fields can interfere with the function of pacemakers. Whilst this won't prevent them from working, their rhythms can be affected. Ferromagnetic implants such as stents and prosthetic devices could be caused to move, which could cause serious injury.

The Radiation Protection Service has created a list of typical objects, inside and outside the body, which can be affected by controlled magnet systems.

Occupational exposure limits

Generally speaking, magnetic radiation does not pose a risk in terms of occupational exposure, and risks only arise due to the presence of certain materials in a strong magnetic field. However, in cases of extreme magnetic radiation, workers can be at risk.

The International Commission on Non-ionizing Radiation Protection (ICNIRP) has established limits of exposure for certain body parts and the body in general. The head and torso should not be exposed to more than 2T, limbs should not be exposed to more than 8T, and, in general, the body should not be exposed to more than 400mT on a day-to-day basis. Consequently, there are no risks either inside or outside of the 0.5mT line as these field levels are not approached in any areas of the university.

General safety

Areas in which the magnetic field is at a high enough level to produce significant risk are designated 'exclusion zones' and will be identified by signs, on-floor marking and chain barriers. Ferromagnetic objects should be kept out of this area, as should people with pacemakers or other medical implants.

In the event that an object strikes the magnet, notify the facility staff. If the object is attached to the magnet do not try to remove it yourself.

Safety instructions for the operation of NMRs

Most of the magnetic field produced by an NMR magnet system is reduced by the shielding that surrounds it. This means that only strongly-magnetic items will be affected by the magnet. However, this shielding doesn't completely neutralise the magnetic field and the shielding doesn't always cover the top and the bottom of the magnet.

In addition to the hazards created by objects striking magnets, NMRs present the risk of a cryogen leak if impacted and fractured. If you notice a sudden release of gas from the magnet (indicating a cryogen leak), evacuate the room and immediately notify facility staff.

Additional safety instructions for contractors

One of the greatest risks for workers is the use of iron-based tools near magnets. To avoid this risk, only bring non-magnetic equipment (e.g. brass or aluminium tools) into the exclusion zone.

Building work should be well-planned in advance with care to minimise disruption to the operation of NMR machines. You should refer to the protocols and the hierarchy of controls in the Standard Operating Procedure when planning and undertaking contracting.