Radiation symbol
The trefoil: the international warning symbol for ionising radiation and radioactive material.
Easily confused with
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.
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.
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.
Read next
- Next in Hazard warning signsHigh voltage symbolA warning of electrical danger, such as high voltage.
- In NFPA 704 fire diamondSpecial hazards (white)The white section at the bottom of the NFPA 704 fire diamond. It is blank, or holds a symbol for a hazard the numbers don't show: W with a line through it for unusual reactivity with water, OX for an oxidizer, SA for a simple asphyxiant gas.
More hazard warning signs
The same shape elsewhere
Browse symbols drawn with circles.
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)
Researched, edited and approved by Rory Hansen before publishing. Last reviewed September 25, 2026. How we research
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