The international radiation symbol (also known as trefoil)first appeared in 1946, at the University of California, Radiation Laboratory. At that time, it was rendered as magenta, and was set on a blue background. Before, they use the blue color to signify the presence of radiation.

Later, they changed it to reddish-purple against a yellow background, and it is drawn with a central circle of radius R, an internal radius of 1.5R and an external radius of 5R for the blades, which are separated from each other by 60°.

On February 15, 2007 the IAEA and the ISO announced this new ionizing radiation symbol to supplement the traditional trefoil symbol. The new symbol is aimed at alerting anyone, anywhere to the potential dangers of being close to a large source of ionizing radiation.

Experts have felt that the trefoil symbol had little intuitive value and was less likely to be recognized by those not educated in its significance. According to the IAEA, in a survey conducted at an international school, many children mistook the trefoil for a non-threatening propeller. Hence, the Agency, along with the International Organization for Standardization has devised this symbol for sealed radiation sources.

It depicts, on a red background, a black colored trefoil radiating waves, a skull and crossbones and a person running away from the scene. The radiating trefoil suggests the presence of radiation and the red background, skull and crossbones warn of the danger. More importantly, the person running away from the scene suggests the action of avoiding the labeled material. The symbol had been tested in 11 countries with different population groups, age and educational background to ensure that it clearly conveys the message “Danger- Stay away”.

The new symbol is to be displayed prominently on the device that actually houses the radiation sources, so that, even by mistake, if some one attempts to disassemble the device it provides an explicit warning not to proceed any further.