Radioactive debris caused by nuclear bomb tests during the Cold War still exist in the Earth's atmosphere, a new study has warned.

Scientists previously believed that nuclear debris high above the Earth would now be negligible.

The research, however, found that plutonium and caesium isotopes are still present at surprisingly high levels, 'BBC News' reported.

"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," lead author Dr Jose Corcho Alvarado, from the Institute of Radiation Physics at Lausanne University Hospital in Switzerland, said.

However, according to Alvarado, the levels were not high enough to pose a risk to human health.

Researchers said that 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.

However, more than five decades after, their radioactive legacy remains.

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 - some particles become trapped, researchers said.

Researchers said they expected similar levels would be found at the same latitude elsewhere around the world, the report said.

Scientists also found that this material can be moved around in the atmosphere by natural events such as volcanic eruptions.

The study was published in the journal Nature Communications.