Radiation Protection Service

Static magnetic fields

Introduction

This section is concerned with the 'static' magnetic fields produced by large permanent magnets. For information and guidance on time-varying magnetic fields see the EMF/RF/Microwaves section.

Small permanent magnets have a wide variety of uses in modern equipment, e.g. speakers, microphones, computer hard disks, etc, etc, and there are no significant health consequences from their use.

However large magnets producing high magnetic fields are used in the university for applications such as Nuclear Magnetic Resonance (NMR) spectroscopy and in other specialised equipment and these have some health and safety implications.

Hazards / risks

Transient sensory effects, e.g. disorientation, dizziness, vision distortion, etc. can be experienced when a person is exposed to high magnetic fields, although there has been no evidence of any long-term health implications from such exposures.

High magnetic fields can however cause the following risks:

• electromagnetic interference can affect the operation of implanted medical devices such as pacemakers, cardiac defibrillators, hormone infusion pumps, neuromuscular stimulation devices, neurostimulators, electronically stimulated prosthetics, etc.
• mechanical forces can be induced on ferromagnetic implants such as hip prostheses, aneurysm clips, stents, surgical clips, etc. or on any other ferromagnetic foreign objects present in the body.

• High magnetic flux densities (>3mT) can create mechanical forces on metallic objects (e.g. tools) and depending on their size and content of ferromagnetic material can cause them to rapidly accelerate and cause a projectile or trapping hazard.

Exposure limits

The International Commission on Non-ionizing Radiation Protection (ICNIRP) has published the following guidelines for limits of exposure to static magnetic fields in Tesla (T):

• Occupational exposure of head and trunk = 2T
• Occupational exposure of limbs = 8T
• General public exposure of any part of the body = 400mT.

In addition to the above levels ICNIRP recommend a limit of 0.5mT (5 Gauss) to protect individuals who have implanted ferromagnetic or electronic medical devices, and this level will also protect against the induced movement of metal objects.

Therefore, in practice, there needs to be an exclusion zone for employees with medical devices or members of the public where the magnetic flux density is greater than 0.5mT.