Radiation from the 2011 Fukushima nuclear disaster in Japan has been detected at an increased number of sites off U.S. shores, including the highest level in the area detected to date, scientists announced Thursday.

While the levels are still too low to be considered a threat to human or marine life by the government’s standards, tests of hundreds of samples of Pacific Ocean water reveal that the Fukushima Daiichi plant has continued to leak radioactive isotopes more than four years after the meltdown—and must not be dismissed, according to Woods Hole Oceanographic Institution (WHOI) marine radiochemist Ken Buesseler.

“Despite the fact that the levels of contamination off our shores remain well below government-established safety limits for human health or to marine life, the changing values underscore the need to more closely monitor contamination levels across the Pacific,” Buesseler said Thursday. “[F]inding values that are still elevated off Fukushima confirms that there is continued release from the plant.”

Scientists from the WHOI and Buesseler’s citizen-science project Our Radioactive Ocean discovered trace amounts of cesium-134, the “fingerprint” of Fukushima, in 110 new Pacific samples off U.S. shores in 2015 alone.

The isotope is unique to Fukushima and has a relatively short two-year half life, which means “the only source of this cesium-134 in the Pacific today is from Fukushima,” Buesseler said.