Scientists have discovered that radioactive particles from nuclear tests decades ago at sites such as Mururoa in the Pacific remain in the upper atmosphere.
Researchers in Switzerland say plutonium and caesium are present in much higher concentrations than previously thought.
At the height of the Cold War, nuclear weapons were being developed and tested around the world. But more than 50 years on, their legacy remains, the BBC reports.
Samples taken from the upper atmosphere show that radioactive particles from these tests are still there.
Previously, scientists had thought that natural atmospheric processes would have removed most of the caesium and plutonium isotopes.
While scientists say the long-term effects are not clear, the amounts of radioactive material they are detecting are not high enough to pose a risk to human health.