A three-bladed trefoil, conventionally black on a yellow background, warning of ionising radiation hazards.
Also known as: Radiation trefoil, Trefoil symbol
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
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.
Three identical black wedge shapes radiating from a small central circle against a yellow background, like a three-bladed propeller.
Both belong to the ISO safety symbols system.