The Trefoil (Tri-Foil)

This short essay is by Paul Frame, Ph.D., CHP

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Oak Ridge Institute for Science and Education

Trefoil or Radiation Warning Symbol

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First, the radiation warning symbol should not be confused with the civil defense symbol designed to identify fallout shelters.

Figure 1. Civil defense symbol

The latter [Fig. 1] consists of a circle divided into six equal sections, three of these sections are black and three yellow. There is no central circle. The civil defense people originally wanted to use the radiation warning symbol with the circle in the center and the three radiating magenta propeller blades but this was objected to because a fallout shelter represents safety whereas the rad warning symbol represents a hazard.

Figure 2. Tre-foil

The rad warning symbol [Fig 2] as we currently know it (except for the colors used) was doodled out at the University of California radiation lab in Berkeley sometime in 1946 by a small group of people. The key guy responsible was Nels Garden. They had these printed up, magenta on blue, and use of the design spread around the country. Blue as a background was a poor choice, since blue was not to be used on warning signs and besides, it faded, especially outdoors. The use of yellow as a background color was probably standardized by Oak Ridge National Lab in early 1948.That is informed speculation. Modifications to the Berkeley design were suggested and implemented locally into the early 50s e.g. the addition of straight or wavy arrows between or inside the propeller blades but an ANSI standard and federal regs finalized the current version by the mid 50s. By the way, at present, the color black is an acceptable substitute for magenta.

Why did the Berkeley people chose this symbol? There is no answer, only speculation. One idea has it is that this symbol was used at a naval base dry dock near Berkeley to warn of spinning propellers. Others imagine that the central circle is a radiation source and that the three blades represent radiation, perhaps one blade each for alpha, beta and gamma. To me, it has a striking similarity to a commercially available radiation warning sign used before 1947 at some labs that consisted of a small red do with four or five red lightening bolts radiating outwards. The latter design was very similar to that on electrical hazard warning signs. Another thought I had was that this design was created one year after WWII and the symbol has some resemblance to the Japanese battle flag which would have been familiar to folk on the west coast.

Whatever the reason, it was a good choice because it is simple, readily identifiable, i.e. not similar to other warning symbols, and discernible at a large distance.

The key reference is "A Brief History of a 20th Century Danger Sign" by Stephens and Barrett that appeared in Health Physics Vol. 36 (May) pp. 565-571. This paper with some additional material appears in the wonderful book "HealthPhysics: A Backward Glance" by Kathren and Ziemer published by Pergamon Press.