Radiation symbol
The trefoil: the international warning symbol for ionising radiation and radioactive material.
These identify the hazard a sign warns of. For what to do in a hazard area, follow the site’s own signs, instructions and trained staff.
How to read it
In the United States the Nuclear Regulatory Commission prescribes it as a three-bladed design in magenta, purple or black on a yellow background, and radiation areas are posted with the symbol and the words “CAUTION, RADIATION AREA”. In Great Britain a warning triangle marks radioactive material.
Also searched as: radiation trefoil, radioactive symbol.
A second radiation symbol
In 2007 the International Atomic Energy Agency and the International Organization for Standardization introduced a supplementary symbol, ISO 21482: radiating waves, a skull and crossbones and a running person. The IAEA said the trefoil has no intuitive meaning and little recognition beyond those educated in its significance. The new symbol is meant for dangerous sources capable of death or serious injury, such as food irradiators, cancer teletherapy machines and industrial radiography units. It goes on the device housing the source, where it is seen only if someone takes the device apart, not on building doors, transport packages or containers.
More hazard warning signs
Sources
- Caution signs and posting requirements (10 CFR 20.1901–20.1902), 10 CFR 20.1901, U.S. Nuclear Regulatory Commission, 10 CFR 20.1901(a), 20.1902(a)
- New Symbol Launched to Warn Public About Radiation Dangers, Press release 2007/03, 15 February 2007, International Atomic Energy Agency (Internet Archive copy)
- The Health and Safety (Safety Signs and Signals) Regulations 1996, Schedule 1, SI 1996/341, legislation.gov.uk (UK Government)
Last reviewed September 25, 2026 by Rory Hansen. How we research