Sand blown north into Europe from the Sahara last month contained radioactive pollution dating from nuclear tests in the 1960s, scientists said.
A French group, the Association for the Control of Radioactivity in the West, said French atomic bomb tests were responsible for the radioactivity.
On February 6, sand and fine particles from the Sahara covered a large area of France and the sky turned orange, as did snow on the ground.
By the evening, most surfaces were covered in a fine layer of particles. Acro tested samples of orange dust at Jura, near the Swiss border in eastern central France.
“It is an artificial radio-element which is therefore not naturally present in the sand and which is a product resulting from the nuclear fission brought into play during a nuclear explosion,” Acro said.
"This radioactive contamination, which comes from far away, 60 years after the nuclear explosions, reminds us of the perennial radioactive contamination in the Sahara, for which France is responsible.”
In 1960, France carried out its first atmospheric nuclear tests in the Algerian Sahara.
Between 1945 and 1980, the US, the Soviet Union, the UK, France and China carried out 520 nuclear tests reaching stratospheric levels and dispersing large quantities of radioactive substances.
The radiation is not thought to pose a threat to people.