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A three-bladed trefoil, conventionally black on a yellow background, warning of ionising radiation hazards.
Also known as: Radiation trefoil, Trefoil symbol, Ionising radiation symbol, ISO 7010 W003
Devised in 1946 at the University of California Radiation Laboratory in Berkeley, the radiation trefoil was later standardised internationally and remains the primary warning symbol for ionising radiation.
Warns of the presence of ionising radiation or radioactive material.
Originally reported to have been sketched in magenta on a blue background at its 1946 creation, later standardised in its now-familiar black-on-yellow form.
Origin account is widely repeated across secondary sources but traced here to a single mirror source; treated as well-documented rather than primary-verified.
1946 onward
Is registered in ISO 7010 as W003, warning: radioactive material or ionizing radiation, and is drawn as three blades at a hundred and twenty degrees around a central disc, black on yellow in the general convention and magenta on yellow in much American practice.
Contexts: Laboratory, Public information
Is seen on sealed-source housings, radiotherapy and radiography rooms, isotope stores, transport packages and the doors of controlled areas. It marks the presence of a source or the boundary of an area, not a level of danger: the same trefoil appears on a smoke-detector component and on a hospital treatment head.
Contexts: Medicine, Laboratory, Public information
Was joined in 2007 by a supplementary symbol — radiating waves, a skull and a running figure, inside a red triangle — issued by the IAEA and the ISO after testing showed the trefoil meant nothing to people who had not been taught it. The new mark is deliberately not a replacement: it goes inside the device, on the source housing, where it is seen only by somebody who has already opened something they should not have.
2007 onward · Contexts: Laboratory
Related forms that are not treated as separate canonical symbols — see our variant rules.
IAEA/ISO supplementary radiation warning symbol (2007)
A separate high-visibility symbol (radiating wave, skull-and-crossbones and running figure) added in 2007 for dangerous radioactive sources, intended to be placed on the source device itself rather than as a general area sign.
ISO 21482 supplementary ionizing-radiation symbol
A red triangle containing radiating waves, a skull and crossbones and a figure running from them, adopted in 2007 for large sources in categories 1 to 3. Placed on the source housing rather than on the door, so that it warns whoever has got past every other control.
Magenta on yellow
The trefoil is drawn magenta or purple-red on yellow in much United States practice and black on yellow elsewhere. Both are the same registered sign; neither colour carries a different meaning.
Three identical black wedge shapes radiating from a small central circle against a yellow background, like a three-bladed propeller.
Three blades around a disc is radiation; three interlocking broken rings is biological hazard.
Both belong to the ISO safety symbols system.