Most of the plutonium and cesium isotopes from those blasts have since been rinsed out of the lower levels of the atmosphere by falling in rain or snow, or by being brought down by gravity.
The stratosphere was also thought to be relatively fallout-free before the Swiss team found its contamination to be about 1,000 to 1,500 levels higher than in the troposphere, the layer just above the surface.
Jose Corcho of the Swiss Federal Office for Civil Protection told AFP that the contamination probably poses no danger to humans, and is "comparable to the levels measured at ground level air at the end of the sixties and in the seventies."
“Most of the radioactive particles are removed in the first few years after the explosion, but a fraction remains in the stratosphere for a few decades or even hundreds or thousands of years,” said Corcho.
The study, published in the journal Nature Communications, used aerosol samples taken by Swiss military aircraft over the past several decades.
A previous study, conducted by the U.S. Centers for Disease Control and Prevention and the National Cancer Institute, concluded that nuclear fallout from atmospheric testing might have led to approximately 11,000 deaths, mainly from thyroid cancer.
Photo: Archive - U.S. Government