A new study has found that radioactive particles from nuclear tests that took place decades ago persist in the upper atmosphere.
Previously, scientists believed that nuclear debris found high above the Earth would now be negligible.
However, this research shows that plutonium and caesium isotopes are still present at surprisingly high concentrations, the BBC reported.
Lead author Dr Jose Corcho Alvarado, from the Institute of Radiation Physics at Lausanne University Hospital in Switzerland, said that 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.
However, he said that the levels were not high enough to pose a risk to human health.
At the height of the Cold War, when the nuclear arms race was in full swing, weapons were being developed and tested around the world.
But more than 50 years on, their radioactive legacy remains.
While nuclear explosions initially throw material up into the air, scientists had thought that the radioactive particles would remain for a limited time.
In the troposphere (the lower layer of the atmosphere that sits directly above the Earth), the isotopes are removed fairly quickly, as they are "washed out" by attaching to rain or snow or are drawn down by gravity.
However, in the stratosphere (the layer that sits from about 10-50km above the Earth), the Swiss team believes that some particles become trapped.
Dr Jose Corcho Alvarado said that the concentrations they measured were in the order of about 1,000 to 1,500 levels higher in the stratosphere than in the troposphere.
While the tests were carried out over Switzerland, the team said they expected similar levels would be found at the same latitude elsewhere around the world.
The study is published in the journal Nature Communications.