The concentration of radioactive material in some waters of the Pacific Ocean has increased one-thousand times since the nuclear disaster at the Fukushima Daiichi nuclear power plant last March.

The U.S.’s Woods Hole Oceanographic Institution said the level of radioactive cesium was ten to one-thousand times higher than usual in samples collected in the Pacific three months after the Fukushima nuclear disaster.

Cesium was found in some samples taken in waters 600 kilometers away from the disaster site.

But the institute added that leaked radioactive substances have been diluted by the sea water and don’t pose a serious threat to the human body.