Scientists who are studying the impacts of a nuclear power plant accident in Japan have discovered radiation is spreading to more sites off the Pacific Coast of the United States.

But the Woods Hole Oceanographic Institute reported Thursday that even a sample with the highest-documented radiation level to date is still far below a threshold that should cause alarm.

The institute has been tracking the spread of radiation from the March 2011 accident at the Fukushima power plant, in which an earthquake set off a tsunami that struck the plant and caused three meltdowns. That event released some radioactive material into the Pacific Ocean.

Ken Bruessler, the director of the institute’s Center for Marine and Environmental Radioactivity, is part of a research team that’s monitoring the ocean for traces of that radioactive material. Over the past four years, they’ve observed small amounts of material, cesium-134, off the coast of Alaska, British Columbia, Washington, Oregon and California.

The team recently found its most contaminated sample to date — 11 becquerals per cubic meter of ocean water, according to a news release. A becqueral is a unit used to measure radioactivity, and this sample was 50 percent higher than any other that’s ever been found off the West Coast.

Even at its highest figure yet, scientists say the sample is still more than 500 times lower than government standards for safe drinking water. It’s also OK for recreational activities like swimming, and Bruessler said in the news release that it’s also below safety limits for sea life.

Contamination levels are higher near Fukushima.

While Bruessler said in the news release that contamination levels near Japan “are thousands of times lower” than they were following the 2011 accident, recent samples collected there contain 10 to 100 times more radioactive material than those collected off the Pacific Coast. He said that indicates that the plant is still releasing radioactive material, though how much remains unclear.

The scientists also note that virtually any sample of Pacific Ocean water will have some level of radioactive material, as atomic weapons testing was performed there in parts of three decades.

Scientists say they know the contamination they’re measuring is from Fukushima and not left over from atomic bomb detonations because the specific type of radioactive material is different.