Strong magnetic field symbol

How to read it

In Great Britain the sign is a warning triangle, Strong magnetic field: a black horseshoe magnet on a yellow triangle with black edging. The Health and Safety Executive’s guide to the Control of Electromagnetic Fields at Work Regulations 2016 gives one hazard of a large static magnetic field as ferromagnetic objects being pulled towards the magnet and hitting anyone in the way. It also names interference with pacemakers among the indirect effects of electromagnetic fields. Its examples of strong static fields include the main magnet of an MRI scanner, nuclear magnetic resonance spectrometers, electromagnetic lifting cranes and industrial electrolysis, such as aluminium extraction.

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At MRI scanners

The Medicines and Healthcare products Regulatory Agency’s Safety Guidelines for Magnetic Resonance Imaging Equipment in Clinical Use (fifth edition, 2026) say clearly visible warnings are displayed at each entrance to the MR controlled access area and the MR environment, and point to a free set of MR safety notices from the Institute of Physics and Engineering in Medicine (IPEM). The guidelines explain the projectile effect: the field pulls ferromagnetic objects so that they become projectiles, and the pull often increases sharply close to the opening of the scanner, the bore.

Look-alike: the MR Conditional label

Items taken near an MRI scanner are marked under ASTM F2503, which the MHRA guidelines name as the standard to use. Frank Shellock, writing in Diagnostic Imaging, describes the three marks. MR Conditional is the letters MR in black inside a yellow triangle with a black border, the one easily taken for this sign. MR Safe is MR in green in a white square with a green border, or in white on a green square, and MR Unsafe is MR in black on white inside a red circle with a diagonal red band. The triangle on a device marks the item, not the place: ASTM defines MR Conditional as an item shown to pose no known hazards in a specified MR environment with specified conditions of use.

Rules on signs in the EU

The EU’s directive on electromagnetic fields at work, 2013/35/EU, says workplaces where workers are likely to be exposed to fields above its action levels are marked with signs under the safety signs directive, 92/58/EEC, unless access is already suitably restricted for other reasons and workers are informed of the risks.