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.
The big non-news is that the radiation levels are small — hence the amazement at being able to measure them. But say “radiation” and some fraction of the population freaks right the hell out. So that’s the take-home message, even if there are a few subtle things missing in the story.
The activity (how radioactive a sample is, measured these days in Becquerels, or number of decays per second) is not the whole story, because not all radiation damages the body the same amount, and it matters greatly if the contamination accumulates in the body or not. Their example is swimming in the ocean, but I think people might also be concerned about eating fish, who would effectively be filtering out and accumulating radioactive material. Are they twice as radioactive as the water? Ten times? Is the internal dose more damaging than an external dose (that’s true of alpha and beta radiation)? Cs-137 is a beta emitter, and in humans it accumulates in the body, with a biological half-life of 70 days. So I imagine it accumulates in fish, as well. But with such low starting levels, probably not anything to worry about.
Another nit is with the picture down near the end, the “helpful figure from the Woods Hole Oceanographic Institution” is somewhat less helpful than it could be. First of all, it doesn’t have the Fukushima incident on it as a comparison. I’ve seen a few links, and their estimates vary by around an order of magnitude, but it’s between ~4 million and ~40 million curies of Iodine and Cesium into the atmosphere, and one link had an additional amount going into the water of half of the atmospheric discharge. So there’s your comparison. (A curie is 3.7 x 10^10 decays per second, as that
was Marie Curie’s favorite number is that activity of a gram of Ra-226. But it’s a huge number and not normally useful for everyday discussion — you are usually talking about picocuries or nanocuries, or something like that.)
Another nit is that comparing activities between different isotopes is a tad dicey, for a couple of reasons. The activity is a rate, not the total amount of potential dose. 1 curie of a contaminant that has a half-life of a day is very different than a curie of something else that has a half-life of a million years — in 10 days, the one-day half-life isotope is down to 0.1% of the activity (a millicurie), while the other is still basically the same. When you look at the Uranium and Potassium numbers on that infographic, keep in mind that K-40 has a half-life of 1.3 Billion years, and Uranium-238 is 4.5 Billion years. Their respective activities aren’t going to perceptibly change in the next 30 years, while any contaminant Cs-131 will drop in half. Further, Uranium is an alpha decayer, which is the most damaging internal source (though if it’s external is pretty harmless — an alpha won’t penetrate your clothes or through the dead layer of skin on your body). Uranium also decays through a chain of radioactive daughters, which have their own decays to contribute. So the activity doesn’t tell the whole story — the reason the ocean’s Uranium and Potassium aren’t a big issue is that the ocean is huge. Fukushima’s release was concentrated, but owing to time and dilution, it’s not a problem for those of us in the US.