The international radiation symbol

The international radiation symbol (also known as trefoil) first appeared in 1946, at the University of California, Berkeley Radiation Laboratory. At the time, it was rendered as magenta, and was set on a blue background. The modern version used in the US is magenta 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°. The trefoil is black in the international version, which is also acceptable in the US
The sign is commonly referred to as a radioactivity warning sign, but it is actually a warning sign of ionizing radiation. Ionizing radiation is a much broader category than radioactive radiation alone, as many non-radioactive sources also emits potentially dangerous levels of ionizing radiation. This includes x-ray apparatus, medical beam cannons and particle accelerators. Non-ionizing radiation can also reach potentially dangerous levels, but this warning sign is different from the trefoil ionizing radiation warning symbol
On February 15, 2007, two bodies—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 new symbol, to be used on sealed radiation sources, is aimed at alerting anyone, anywhere to the danger of being close to a strong source of ionizing radiation. It depicts, on a red background, a black trefoil with waves of radiation streaming from it, along with a black skull and crossbones, and a running figure with an arrow pointing away from the scene. The radiating trefoil suggests the presence of radiation, while the red background and the skull and crossbones warn of the danger. The figure running away from the scene is meant to suggest taking action to avoid the labeled material. The new symbol is not intended to be generally visible, but rather to appear on internal components of devices that house radiation sources so that if anybody attempts to disassemble such devices they will see an explicit warning not to proceed any further