Radiation from Japan’s 2011 Fukushima nuclear disaster has apparently traveled across the Pacific.

Researchers reported that radioactive matter — in the form of an isotope known as cesium-134 — was collected in seawater samples from Tillamook Bay and Gold Beach in Oregon. The levels were extremely low, however, and don’t pose a threat to humans or the environment.

In 2011, a 9.0-magnitude earthquake triggered a wave of tsunamis that caused colossal damage to Japan’s Fukushima Daiichi nuclear power plant. The disaster released several radioactive isotopes — including the dangerous fission products of cesium-137 and iodine-131 — that contaminated the air and water. The ocean was later contaminated by the radiation.

But cesium-134 is the fingerprint of Fukushima due to its short half-life of two years, meaning the level is cut in half every two years.

“It [cesium-134] disappears much more quickly,” said Dr. Kathryn A. Higley, the head of the School of Nuclear Science and Engineering at Oregon State University.

Cesium-137 has a 30-year half-life. Particles from Chernobyl, nuclear weapons tests, and discharge from other nuclear power plants are still detectable — in small, harmless amounts.

While this is the first time cesium-134 has been detected on US shores, Higley said “really tiny quantities” have previously been found in albacore tuna.

The Oregon samples were collected by the Woods Hole Oceanographic Institution in January and February. Each sample measured 0.3 becquerels, a unit of radioactivity, per cubic meter of cesium-134 — significantly lower than the 50 million becquerels per cubic meter measured in Japan after the disaster.

But those levels have already dropped enough that Japanese fisheries have started to discuss reopening, said Higley.

Cesium-134 was first detected in North America off the shores off Canada in September 2015. One sockeye salmon from a lake in British Columbia tested positive in November, according to the Fukushima InFORM project. The level was “more than 1,000 times lower the action level set by Health Canada,” according to the Statesman Journal.

InFORM — a coalition of scientific and nonprofit organizations, including Woods Hole — has been tracking the course of the radiation plume across the Pacific. They suspect a rise in levels over the coming year, but Ken Buesseler, a chemical oceanographer at Woods Hole, told the Statesman Journal that not even the peak would be a health concern.

Ultimately, the samples helped scientists determine that any more radiation from Fukushima would take between four and five years to reach the West Coast.