Trace amounts of radiation from the 2011 Fukushima nuclear disaster in Japan have been detected on North American shores for the first time, but researchers say the amount of radiation is not a concern.

Radioactive forms of the element cesium that could only have come from Fukushima were detected in samples collected on Feb. 19 in Ucluelet, on the west coast of Vancouver Island, with the help of the Ucluelet Aquarium, scientists at Woods Hole Oceanographic Institution reported today.

The private, independent research organization is based in Cape Cod, Mass. It has been monitoring radiation levels along North American shores over the past 15 months with help from citizen scientists who are collecting samples from 60 sites along the U.S. and Canadian west coast and Hawaii, along with a Canadian-funded organization called inFORM led by University of Victoria oceanographer Jay Cullen.

Levels not alarming, expert says

Ken Buessler, the researcher who leads the Woods Hole monitoring program, said he expects more of the monitoring sites to show detectable levels of cesium-134 in coming months. However, the amounts of cesium-134 and cesium-137 in the B.C. sample were extremely low — just 1.4 and 5.8 becquerels per cubic metre of water respectively. Canada allows up to 10,000 becquerels of cesium-137 per cubic metre in drinking water.

"Today's report is not alarming at all. It's kind of to be expected," he said.

"We knew four years later it would be reaching our shoreline, and we had seen it offshore, and these numbers are quite small.

"As an example, even if they were twice as high and I was to swim there every day for an entire year, the dose I would be exposed to is a thousand times less than a single dental X-ray. So the risk is never zero, but when I think of health risk, I always think of the Japanese side of the Pacific instead of ours."

​Cesium-134 and cesium-137 are both produced exclusively from human activities. Cesium-137 decays slowly, so most detectable amounts are from nuclear tests decades ago. However, cesium-134 decays quickly — after two years, only half of it is left, and after four years, just a quarter remains. That means all the cesium-134 in the ocean today has to be from Fukushima, the only source recent enough to produce detectable amounts of cesium-134.

Previously detected farther from shore

Radioactive cesium had previously been detected off the west coast of North America, but farther from shore.

In June 2013, scientists from the Department of Fisheries and Oceans detected cesium-134 and cesium-137 as close to shore as the Canadian continental shelf. The shelf stretches 20 to 80 kilometres from the western shore of Vancouver Island.

The Department of Fisheries and Oceans says normal levels of cesium-137 in the Pacific Ocean are about one becquerel per cubic metre. Fukushima is expected to boost levels off the North American coast to a peak of three to five becquerels per cubic metre in 2015 to 2016, before declining to normal levels by 2021.

The Fukushima Daiichi nuclear plant in Japan suffered multiple meltdowns following a massive earthquake and tsunami in March 2011. Radiation began seeping from the plant when the magnitude 9 earthquake and a tsunami on March 11 knocked out its cooling systems.

It was a Level 7 "major accident" and the worst since the nuclear disaster in Chornobyl, Ukraine, in 1986.