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.