Traces of radioactive fallout from nuclear tests in the 1950s and 1960s can still be found in American honey, new research reveals.
The radioactive isotope identified, cesium-137, falls below levels considered to be harmful – but the amounts measured nonetheless emphasize the lingering persistence of environmental contaminants in the nuclear age, even a half-century after international bomb tests ended.
"There was a period in which we tested hundreds of nuclear weapons in the atmosphere," lead researcher Jim Kaste, an environmental geochemist at William & Mary university in Williamsburg, Virginia, explained last year in comments about the research.
"What that did was put a blanket of these isotopes into the environment during a very narrow time window."
One of those isotopes was cesium-137, a byproduct of nuclear fission involving the reaction of uranium and plutonium, which can often be found in trace amounts in food sources due to such nuclear contamination of the environment.
Some of these traces are much fainter than others, Kaste found out – but only by chance, as it happened, after assigning his students a Spring Break assignment in 2017.
To demonstrate to his class how radioactive contaminants from mid-20th century nuclear testing still persisted in the environment today, Kaste asked his students to bring back locally sourced foods from wherever they spent the holidays.
As expected, various samples of fruits, nuts, and other foods revealed very faint traces of cesium-137 when measured with a gamma detector, but even Kaste wasn't prepared for what happened when he ran the same test with a jar of honey from a North Carolina farmer's market.
"I measured it again because I thought something happened to the container or my detector was bonkers," Kaste says.
"I reproduced the measurement. And it was, again, 100 times hotter than any of these other foods."
To find out why honey registered such high levels of cesium-137, Kaste and his team (including one of his students, Paul Volante) began testing samples of locally made raw, pure, and unfiltered honey from markets and beekeepers located across the eastern US.
Of the 122 honey samples tested, 68 showed detectable traces of the radioactive isotope – a legacy of atmospheric nuclear tests conducted by the US, the USSR, and other nations during the Cold War era.
The majority of detonations occurred above the Marshall Islands in the Pacific Ocean and Novaya Zemlya, an Arctic archipelago in northern Russia, with other tests being conducted in New Mexico and Nevada.
According to the researchers, the cumulative effect of over 500 of these test detonations released more ionizing radiation to the atmosphere than any other event in human history – not that all the blasts were equal in scope.
"We know that the cesium-137 production from the Pacific and Russian sites was more than 400 times the production of the New Mexico and Nevada explosions," Kaste says.
"A single Russian bomb, the Tsar Bomb, was more than 50 times more powerful than all the Nevada and New Mexico tests combined."
While there's no way of knowing which of these explosions produced the fallout that can still be found in American foods today, we can at least explain how the isotope could disperse so far and wide.
"Many of the air detonations were so powerful that dozens of radioactive fission products were injected into the stratosphere and distributed globally with a residence time of [approximately] one year before deposition primarily by rainfall," Kaste and fellow researchers explain in a new study.
"The presence of radioactive pollution from nuclear testing is globally ubiquitous, and detectable on every continent and even in deep ocean trenches."
Not just rainfall
While the pollution may be globally ubiquitous, honey's high levels of cesium-137 compared to other food sources show that the fallout appears to concentrate in unexpected ways – but we can now explain that mystery too.
Rainfall might be the predominant force taking cesium-137 out of the atmosphere and depositing in on Earth's surface, but the honey samples registering the highest amounts of the radioactive isotope weren't produced in regions of the US that receive the most precipitation.
Rather, the honeys with the highest levels turned out to come from places in the US where the soil has low levels of potassium, which plants absorb as a nutrient source to fuel a range of metabolic processes.
Potassium and cesium share a number of atomic similarities, and when plants in potassium-poor soil can't get ahold of sufficient levels of their preferred nutrient, they'll absorb cesium instead – even if it's of the unstable, radioactive variety.
As a result, the isotope finds its way into plant nectar, which then gets passed to bees, who in turn magnify the concentration of cesium-137 when they make honey. Which then makes its way into your home.
The phenomenon has been previously observed in the wake of events such as the Chernobyl disaster, but such is the enduring half-life of radioactive particles, it can still be observed even several decades later, and in places located thousands of kilometers away from the site of the original nuclear tests in question.
If there's a silver lining to this unsettling discovery, it's that none of the cesium-137 levels detected in honey today are considered to be harmful to humans, falling below the 50-100 becquerels per kilogram threshold of radioactivity.
However, decades ago, the same toxic fallout would have been fresher, and potentially more hazardous to human health, not to mention other organisms too.
"What we see today is a small fraction of the radiation that was present during the 1960s and 1970s," Kaste says.
"And we can't say for sure if cesium-137 has anything to do with bee colony collapse or the decline of population."
In recent years, the ongoing disappearance of bees and other insect pollinators has sparked much concern in scientific circles, and while Cold War nuclear tests aren't often considered a primary driver of the problem, we can't afford to ignore that they too could be a contributor.
"Given that pollinating insects provide vital services to the world's ecosystem and are essential in maintaining global food security, more research is needed to help us better understand how ionizing pollution threatens their health and survival," the researchers write.
The findings are reported in Nature Communications.