Radiation trefoil
Also called radioactive trefoil, ☢
Doodled in 1946 at Berkeley; magenta on yellow settled in 1948
Modern symbolsRadiation trefoil meaning
The radiation trefoil was drawn in 1946 at the University of California Radiation Laboratory in Berkeley. Magenta on yellow became the standard pair in 1948. The mark means ionizing radiation. It is not a logo for the atomic age as a mood. ISO 361 specifies it. A harsher supplementary sign, ISO 21482, came in 2007.
A mark for a source that can injure you without looking like fire.
The name. Trefoil means three leaves. The designers were not naming a plant. Nels Garden, who oversaw the work, said the motif that interested the group was activity radiating from an atom. The English name on a warning sign is radioactive. The picture came first.
Where it comes from
In 1946 a small group at the Radiation Laboratory in Berkeley needed a sign for radioactive material that would not be confused with existing plant codes. Nels Garden, head of the Health Chemistry group, described the choice in a 1952 letter: several motifs were tried, and the one that held interest was meant to show activity radiating from an atom. The first signs printed there were magenta, a Roman violet, on a blue ground. Garden liked the magenta because it was distinctive and expensive enough that other people would not splash it around. Blue failed at a distance and outdoors. At Oak Ridge in 1948 Bill Ray and George Warlick, working under K. Z. Morgan, cut the magenta blades off the Berkeley signs and tried them on other grounds. A committee, looking from about twenty feet, chose magenta on yellow. That pair is the one the world learned. The International Organization for Standardization wrote it into ISO 361. In 2007 ISO 21482 added a different supplementary sign for very dangerous sources: a red triangle with a skull, a running figure, and waves. It does not replace the trefoil. It is for a source so large that the ordinary trefoil was not stopping scrap-metal theft and accidental exposure. Unicode encodes the trefoil as RADIOACTIVE SIGN, U+2622, ☢. The lexicon draws the three blades and the center. It does not copy a particular agency’s file.
Modern
A warning, not a genre
The trefoil means a source of ionizing radiation is here, or that an object is contaminated, or that a room is controlled. It does not mean science, progress, or a music genre, though all three have borrowed it. The borrow is easy because the mark is memorable and, at the start, meant nothing. That emptiness was the brief. Once it meant something, the emptiness was over. Using it as a fashion mark does not restore the emptiness. It ignores a warning that was paid for in exposure incidents.
A hard history
The sign exists because the material can kill, burn, and sicken at a distance and after a delay. Magenta on yellow is not a vintage palette. It is the pair a committee could still see from twenty feet. Treating the trefoil as nostalgia is a way of not reading it.
In popular culture
game · 1997–
Fallout series
Vault-Tec, Nuka-Cola and the Children of Atom build a retro-atomic world where hazard signs are set dressing.
The sign as nostalgia for a war that already happened.
Sourcefilm · 2018
Red Sea Action (红海行动)
A yellowcake package bears a red trefoil in the Chinese action film.
A yellowcake package bears a red trefoil in the Chinese action film, which is a later reading of the mark.
SourceWorth knowing
- The trefoil was drawn in 1946 at Berkeley’s Radiation Laboratory and first printed magenta on blue. Source
- It warns of ionising radiation, X-ray machines included, and the character is U+2622. Source
- In 2007 the IAEA and ISO added a supplementary sign with a skull and a running figure because the trefoil alone was not understood as 'danger'. Source
Questions
What does the Radiation trefoil mean?
The radiation trefoil was drawn in 1946 at the University of California Radiation Laboratory in Berkeley. Magenta on yellow became the standard pair in 1948. The mark means ionizing radiation. It is not a logo for the atomic age as a mood. ISO 361 specifies it. A harsher supplementary sign, ISO 21482, came in 2007.
How do I type the Radiation trefoil?
The character is ☢ (U+2622).
Why does Fallout series use the radiation trefoil?
Vault-Tec, Nuka-Cola and the Children of Atom build a retro-atomic world where hazard signs are set dressing. The sign as nostalgia for a war that already happened.
Why does Red Sea Action use the radiation trefoil?
A yellowcake package bears a red trefoil in the Chinese action film. A yellowcake package bears a red trefoil in the Chinese action film, which is a later reading of the mark.
Near this
- BiohazardA mark chosen because people would remember it and because it did not already mean anything.
- Doomsday ClockA clock face used as a judgment. Midnight is the catastrophe. The hands are a decision.
- Mushroom cloudThe shape of a nuclear explosion after the camera made it a symbol.
- AtomA nucleus and three orbits. A picture of a teaching model, not of an atom.
Sources
- ORAU, origin of the radiation warning symbol
- Radiation warning symbol
- ISO 21482 supplementary ionizing radiation warning
Named when the entry depends on a text, a date, or a standard. Catalog numbers are not invented.
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symbol.wiki, edited as Sigilla. Last reviewed 20 October 2026. A reading of public tradition, not a ruling.