Magnets generating intense magnetic fields are becoming the standard in research. They now push back the limits of material science, refine the resolution of MRIs and increase considerably the frequency range of NMR spectrometers and other devices.
If some of the older magnets remains traditional (copper wiring and water cooling) with moderate fields ranging from 0.1 T to 2 T (Tesla, 1 T = 10k Gauss), the new techniques (superconductors) allow to reach static magnetic fields that now exceed 20 T. At the same time there are also large differences in magnetic shielding of these devices; residual fields outside the magnet can pose a risk to users and nearby equipment.
Work with magnetic fields
The occupational exposure limit values for static magnetic fields in Switzerland are given in SUVA’s directives ‘Valeurs limites d’exposition aux postes de travail’, 1903.f. These limit values are 0.2 T for continuous exposure (8 hours working day), 2 T for short-term whole-body and 5 T for exposure to arms & legs. These field levels are high and indicate the insufficiency of evidence for biological effects (especially accumulated) from static fields.
The limit value for exposure (42 hours per week, 8 hours a day, for long periods) is 40 mT for pregnant women.
A warning sign should be posted at the entrance of laboratories or spaces where magnetic fields exceed one of the limits listed above.
All public spaces are limited to less than 0.5 mT (5 G) for static fields.
Ground marking is also needed to define the areas of magnetic field limits for the protection of people and equipment.
Three zones are defined according to a map of magnetic field. They are represented by marking tapes of different colors on the ground, which represent the limits of access for the public and for the professionally exposed workers.
|Magnetic flux density||Limit / restriction description||Entrance forbidden to|
|0.5 mT (5 G)||Maximum value allowed for the public, prosthesis / pacemaker wearers.||– Public
– Pacemaker wearers
– Non-authorized people
|3 mT (30 G)||Value starting from which the field can drag ferromagnetic objects.||Any ferromagnetic object|
|0.2 T (2 kG)||8 hours per day/5 days a week occupational exposure limit.
Value starting from which the access is unauthorized without medical recommendation.
|Any, excepted with medical recommendation|
- The tools must be suitable for working in magnetic fields because the forces are strong enough to cause their attraction, acceleration and therefore dangerous injuries. Asthenic steel (non-magnetic), aluminum etc. are recommended.
At 3 mT (30 G), ferromagnetic objects can be driven by the magnetic field.
- Keep magnetic field equipment such as magnetic cards, mechanical watches and field-sensitive equipment in a safe place out of the field.
Magnetic cards are damaged at around 100 mT, it is recommended not to expose them to more than 10 mT.
- People with passive (prosthetic) and active (cardiac, insulin pump, etc.) implants can’t be exposed to fields exceeding 0.5 mT (5 G).
Purchase of material generating a magnetic field
In order to limit the danger area, the DSPS-SCC performs a mapping and marking of the material generating magnetic field exceeding 0.5 mT (5 G). These areas are marked by bands of different colors that represent the access limits for public and professionally exposed workers. Please contact the DSPS-SCC before purchasing any material generating such fields.
Static field hazards
The most important dangers to the human body are:
- Sensory effects: From 2T, effects such as nausea, vertigo and metallic taste in the mouth may appear.
- Electrocardiogram: Above 8T, a reversible modification of the electrocardiogram is possible.
- Implants: Passive implants (screws, pins, etc.) and ferromagnetic foreign bodies (shrapnel, etc.) can move in the body. Active implants such as pacemakers, defibrillators and other neurological stimulators may experience electrical and / or electronic dysfunctions. The same problem may also arise for Insulin pumps.
The physical dangers are typically:
- Kinetics: From 3 mT, the attraction and projection of moving metal objects can be dangerous if not deadly.
Variable magnetic field hazards
The effects of variable fields over time are similar to those of static fields with some major differences:
- Visual disturbances: Above 2-3 mT and 20 Hz, some people may experience visual disturbances, characterized by the perception of luminous spots called magnetophosphene.
- Hyperthermia: When exposed to high-frequency fields (greater than 100 kHz), the energy absorbed by biological tissues can lead to hyperthermia characterized by an increase in body temperature. High powers can cause superficial or deep burns. In addition, some metallic implants could be heated by induction which would result in heating of the tissues in contact.
- Hypersensitivity: Some hypersensitive people may have physical or muscular asthenia or even muscle aches, tiredness, memory loss or abnormal irritability, sleep disorders, headaches.
- Electronic failure: Electronic components (chip, hard disk, magnetic cards, etc.) can be damaged. Some sensitive instruments may be magnetized and lose their basic characteristics.
- Cryogenic: The auto perturbation generated by a ferromagnetic object can generate a “quench” of superconducting magnets which would result in the rapid evaporation of the cryogenic cooling liquid. If the system is poorly designed, an extremely cold gas / liquid can reach users;
- Explosion: In potentially explosive areas the heating of components or sparks may present a risk of ignition.