Why is radiation associated with the color green?

What color comes to mind when we think of radioactive elements? Many people probably think of green. But the truth is that very few radioactive substances glow on their own: the glow (or radioluminescence) comes from the interaction of the emitted particles with the materials around them. This glow can be yellow, or blue, or yellowish... But for some reason, it is the color green that we strongly associate with radiation.

The culprit behind this prejudice is most likely the radioactive metal radium. At the beginning of the 20th century, science made great strides forward in the study of the nature of radiation and the discovery of new radioactive elements. Curiosity and the desire to innovate made people use radioactive substances, for example, in the mass production of household goods, especially since the science of the time did not yet consider radioactive radiation as something dangerous to humans.

One of the elements of greatest interest was radium. It was used to make luminous paint, where radioactive radiation from radium created radioluminescence of green color. It is so convenient to paint the necessary places with radium paint, which will glow in the dark for many years without any problems!

Such paint was used to highlight numbers and hands in wrist or room clocks, on instruments that had to be used in the dark, in vehicle cabins and even in some design solutions. When mankind finally realized the danger of radioactive radiation, everyone began to urgently get rid of things with radium paint. And the ominous green glow became forever associated with the word "radiation" in the public consciousness.

Even without the phosphor, pure radium emits enough alpha particles to excite nitrogen in the air, causing it to glow. The color isn’t green, through, but a pale blue similar to that of an electric arc.

This glow (though not the color) entered the public consciousness through this early illustration of its appearance in Marie Curie’s lab, and became confused with the green glow of radium paints.

The myth is likely kept alive by the phenomenon of Cherenkov glow, which arises when a charged particle (such as an electron or proton) from submerged sources exceeds the local speed of light through the surrounding water.

So in reality, some radionuclides do glow (notably radium and actinium), but not as brightly or in the color people think. Plutonium doesn’t, no matter what Homer Simpson thinks, unless it’s Pu-238—which has such a short half life, it heats itself red hot.

In most consumer products, though, radioluminescence has been replaced by photoluminescence, phosphors that emit light of one frequency after absorbing photons of a difference frequency. Glow-in-the-dark items that recharge to full brightness after brief exposure to sunlight or a fluorescent light only to dim again over a couple of hours are photoluminescent, and contain no radiation.

An aside on aging radium: By now, most radium paint manufactured early in the 20th century has lost most of its glow, but it’s still radioactive. The isotope of radium used has a half life of 1200 years, but the chemical phosphor that makes it glow has broken down from the constant radiation—so if you have luminescent antiques that barely glow, you might want to have them tested with a Geiger counter and take appropriate precautions. The radiation emitted is completely harmless as long as you don’t ingest or inhale the radium—in which case it becomes a serious cancer risk. So as the tell-tale glow continues to fade, how will you prevent your ancient watch dial or whatever from deteriorating and contaminating your great, great grandchildren’s home, or ending up in a landfill and in the local water supply?