The Ultimate Guide

written by Stanley Udegbunam || Dec 21, 2020

symbol of radiation



The symbol for radiation is a three-bladed symbol called the trefoil.

radioactive symbol

The trefoil is an internationally recognized symbol used to indicate the presence of a radioactive source or device, radioactive containers and areas where radioactive materials are stored.

It denotes the need for caution to avoid contamination with undue exposure to atomic radiation.

radiation sign

radiation caution


In 1946, the trefoil symbol started as a ‘doodle’ produced by a small group of people at the Berkeley Radiation Laboratory, University of California, who were keen to develop a warning symbol for ionizing radiation.

The symbol was chosen because of its simplicity and it represents the radiating activity of an atom.

The trefoil symbol then was a magenta-colored blade on a blue background.

In 1948, Bill Ray and George Warlick at the Oak Ridge National Laboratory in Tennessee conducted some field experiments with the symbol.

They cut out the magenta symbol and stapled them onto cards of different colors.

They placed these cards outside and observed from of distance which of the background color strikes the human consciousness the most.

 From their observation, the magenta on yellow best portrays the idea of danger, and this design was standardized by Oak Ridge National Laboratory (DoE, Tennessee) later in 1948.

By the late 1950’s, this radiation symbol (magenta on yellow) was implemented by the American National Standards Institute and federal regulation.

The design is still used in the U.S. today.

The International version is the black on yellow trefoil although it’s not often used in the united state compared to the magenta on yellow.

radiation trefoil

evolution of radiation symbol


The symbol is drawn with a central circle of radius R, an internal blade radius of 1.5R and an external blade radius of 5R.

The blades are separated from each other by 60°.

radioactive symbol design


On February 15, 2007, the International Atomic Energy Agency (IAEA) and the International Organization for Standardization (ISO) jointly announced the adoption of a new ionizing radiation warning symbol to supplement the traditional trefoil symbol. 

The need for this new symbol aroused from the fact that many people don’t understand the hazard message been passed by the three-cornered trefoil.

The three-cornered trefoil was said to have no intuitive meaning.

Only a few people are aware of its significance.

The new symbol is to be placed on sealed radiation sources.

It is aimed at alerting anyone, anywhere to the potential dangers of being close to a large source of ionizing radiation.

caution radiation sign

Ionizing Radiation Warning – Trefoil Supplementary Symbol, ISO Standard (21482)

The new symbol is not intended to be generally visible, but rather to appear on internal devices that house radiation sources.

It will serve as a warning not to dismantle such a device or get any closer.

This new symbol is explicit in the sense that:

  • The black trefoil with the radiating waves – indicates the presence of radiation.
  • The skull and cross bone on the red background – warns of impending danger.
  • The running person – suggests the immediate action that should be taken to avoid the labeled material or device.

The precision of the details of this new symbol provides a more intuitive depiction of the radiating hazard.

The new radiation symbol is universally understood across varying educational backgrounds and different races.

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