Do not enter
Hazard and warning symbols
Marks that tell a body to stop, leave, or count the time left.
Marks that tell a body to stop, leave, or count the time left. The radiation trefoil was doodled at the University of California Radiation Laboratory in 1946. Magenta on yellow became the standard pair. Biohazard is 1966, a Dow design for the National Cancer Institute, chosen because it was memorable and meant nothing yet. The fallout-shelter sign is 1961, three triangles in a circle, a promise of shelter rather than a warning of a source. Civil Defense is older, 1939, the letters C and D in a triangle and a circle, and the mark was retired in 2006. The Doomsday Clock is 1947, Martyl Langsdorf’s cover for the Bulletin of the Atomic Scientists. It is a clock face used as a judgment. The setting moves. On 27 January 2026 it stood at 85 seconds to midnight. The mushroom cloud is the photograph of a bomb becoming an icon, from Trinity on 16 July 1945. The power symbol is the only one in the line that is not about a war. It is an IEC convention: a 1 inside a broken 0 means standby. It sits here because people read it as a warning and because the line is about signs that tell you the state of a machine. The hard histories in this line are short on purpose. They do not console.
Start here. Radiation trefoil. The trefoil is the hazard mark that needs no caption once you have seen it. The page says who doodled it.
- 1Radiation trefoilA mark for a source that can injure you without looking like fire.
- 2BiohazardA mark chosen because people would remember it and because it did not already mean anything.
- 3Fallout shelterThree triangles in a circle, meaning a place rated to keep fallout out.
- 4Civil DefenseThe letters C and D in a triangle and a circle. A program’s face, then a relic.
- 5Doomsday ClockA clock face used as a judgment. Midnight is the catastrophe. The hands are a decision.
- 6Mushroom cloudThe shape of a nuclear explosion after the camera made it a symbol.
- 7Power symbolA one breaking a zero. It means standby, not off.
Questions
What are the main symbols in this line?
The line is the radiation trefoil, the biohazard mark, the fallout-shelter sign, Civil Defense, the Doomsday Clock, the mushroom cloud, and the power symbol. The first six are twentieth-century public signs about danger, shelter, or time. The power symbol is a standard for the state of a device. It is here because standby is a condition people misread as off.
What is the oldest symbol in this line?
The Civil Defense insignia is the oldest authored mark in the line. Charles T. Coiner designed it in 1939. The mushroom cloud, as a photograph of Trinity, is 16 July 1945. The radiation trefoil is 1946. The Doomsday Clock is 1947. Fallout shelter is 1961. Biohazard is 1966. The clock’s setting is not original to 1947. It moves, and the page dates the setting it quotes.