There has been an increase in radioactivity measured in the air across parts of Central and Western Europe over the past week, Germany's Office for Radiation Protection said.

Since September 29, a slight increase in the isotope Ruthenium-106 has been measured in the air in Germany, Italy, Austria, Switzerland and France.

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The low levels do not pose a threat to human health, the Office for Radiation Protection said. It also stressed that the source could not be an accident at a nuclear power plant.

Officials do not know the cause of the elevated radiation levels, but they believe it may have originated from Eastern Europe.

Among other things, Ruthenium-106 is used for chemotherapy to treat eye tumors. It is also occasionally used in so-called radioisotope thermoelectric generators (RTGs), which provide power to satellites. It's one of the more stable, least dangerous, ruthenium isotopes.