A worker walks past first storage tanks of radioactive contaminated water at tsunami-crippled Tokyo Electric Power Co.'s Fukushima Daiichi Nuclear Power Plant. (EPA/Kimimasa Mayama)

The 2011 Fukushima Daiichi crisis was the worst nuclear disaster in decades, and people in Japan are still living with its consequences. One team of scientists estimated in 2012 that the radiation released from Fukushima's four reactors may ultimately claim 130 lives and cause 180 additional cases of cancer (in addition to the exposures suffered by workers on site). Radiation releases into the ocean near the Fukushima plant also led to fisheries closures and bans,  and a tightening of acceptable limits for radiation in Japanese seafood.

Naturally, in light of all this, many Americans have been concerned -- sometimes overly so -- that radiation from Fukushima, traveling through the vast Pacific ocean, would eventually make its way to the waters off the West Coast of the United States and Canada. And according to a new scientific paper just out in the Proceedings of the National Academy of Sciences, that has indeed happened.

The paper, by John N. Smith of Fisheries and Oceans Canada (a government agency) and several colleagues, is the "first systematic study...of the transport of the Fukushima marine radioactivity signal to the eastern North Pacific," and concludes that radiation reached the continental shelf of Canada by June of last year, and has increased somewhat since.

But-- and here's the good news -- the levels of radiation are very low, well below levels that public health authorities cite as grounds for concern. The radiation "does not represent a threat to human health or the environment," reports the paper.

The new study is not the first to reach that conclusion. "We came up with something like 500 to 1000 times less of a dose, the hazard of the radiation of swimming in the Pacific, as a dental X Ray," says senior scientist Ken Buesseler of the Woods Hole Oceanographic Institution, who was not involved in the current study. Buesseler heads a crowd-funded, citizen research project, Our Radioactive Oceans, which first reported the presence of small quantities of Fukushima radiation in a sample taken in August 2014 100 miles off the coast of Eureka, Calif. The radiation was at low levels, similar to that reported in the current research.

In fact, what's truly amazing about the work is that scientists are able to actually measure these very low levels of radiation at all -- as well as to chemically fingerprint them and thereby prove that certain radioisotopes of the chemical element Cesium, which arise as a by-product of nuclear fission, actually arrived off of North American waters after traveling all the way from Fukushima.

In the current paper, the researchers accomplished this  by taking a series of measurements, from 2011-2014, on ocean vessels which traveled 1,500 kilometers out from Canadian coastal waters into the Pacific. Water samples were taken to look for two radioisotopes, Cesium-134 and Cesium-137, both of which were released as part of the radioactivity from Fukushima.

Cesium-134 has a two-year half-life (meaning half of it will have decayed within 2 years), whereas Cesium-137 has a 30 year half life. What that means is that the presence of Cesium-134 allows scientists to conclusively separate out Fukushima-generated radiation from the other major human source of radiation in the Pacific -- nuclear weapons testing, which happened decades ago (there would be no more Cesium-134 detectable from this source). This simple fact allows for "unequivocal" detection of radiation originating from Fukushima, even thousands of miles away, noted the new study.

In light of this, the paper found that radiation from Fukushima was definitely detectable in waters of the Canadian continental shelf by June 2013, and had apparently increased somewhat by February of 2014. However, the levels of radiation are quite low. For nuclear power nerds, the Fukushima radiation levels were under 1 Becquerel per cubic meter of ocean water, where a Becquerel refers to one nuclear decay per second and a cubic meter of ocean is 1000 liters (or 260 gallons).

"A Becquerel per cubic meter is not a lot of radioactivity," says Stony Brook University's Nicholas Fisher, another researcher who has published on Fukushima radioactivity's contamination of the oceans, but was not involved in the current study. For reference, the background levels of Cesium-137 in many parts of the world ocean is actually higher than that:

Indeed, the number underscores just how much the Fukushima radiation has been diluted and dispersed in its three-year journey across the Pacific. In the waters right off the Fukushima plant, just after the accident, radiation levels were 50 million Becquerels per cubic meter -- extremely dangerous -- explains Woods Hole's Ken Buesseler. A few months later, levels were in the low thousands, he adds -- still worrisome if you are consuming seafood.

But now that the radiation has reached the waters of the West Coast, levels of 1 or 2 Becquerels per cubic meter are pretty tiny and dilute. "It’s tens of millions of degrees on the sun, versus the temperature on the Earth," says Buesseler. "So that’s the difference in having 1 or 2 of these Becquerels, versus what was off Japan."

The radiation levels on the West Coast from Fukushima may still grow a tad higher, and actually reach the beaches. But they are not expected to approach levels that would worry public health authorities or scientists, who are constantly mindful of the fact that there is radiation all around us.

"Most of the radioactivity in the oceans is natural radioactivity, and it has nothing to do with nuclear power plants or atomic weapons or anything like that," explains Stony Brook's Nicholas Fisher,. "Something on the order of 99 percent of all the radioactivity in the oceans is natural."

Here's a helpful figure from the Woods Hole Oceanographic Institution showing as much:

Nonetheless, there has long been public concern about risks from low level radiation, which is understandable in light of the fact that you may be exposed and yet never know it. And there has often been overblown concern about radiation from Fukushima in particular, including the circulation of numerous scary (and often misleading) images purporting to show radiation flowing across the Pacific.

Scientists actually studying the matter have a very different outlook on the Fukushima radiation and its long range travels. "My take home is always, don’t trivialize it or dismiss it, but also don’t exaggerate what the effects might be," says Woods Hole's Ken Buesseler. "Some people are adamantly anti-nuclear, and that’s fine, but don’t scare people from swimming in the Pacific."