Hazard symbols or warning symbols are recognizable symbols designed to warn about hazardous materials, locations, or objects, including electric currents, poisons, and radioactivity. The use of hazard symbols is often regulated by law and directed by standards organizations. Hazard symbols may appear with different colors, backgrounds, borders and supplemental information in order to specify the type of hazard. Warning symbols are used in many places in lieu of or addition to written warnings as they are quickly recognized (faster than reading a written warning) and more universally understood (the same symbol can be recognized as having the same meaning to speakers of different languages).

The international radiation symbol (also known as the 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 emit 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.

Source: Hazard symbol from Wikipedia

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