During his doctoral research, Madigan was exploring another approach. He was trying to determine whether you could tell the location from which a fish had migrated by tracking the specific combination of the nonradioactive (stable) isotopes of carbon and nitrogen in their flesh.2 To put it simply, did particular regions of the ocean convey upon their marine residents a specific isotopic signature that scientists could use to determine an animal’s original location? It seemed like a reasonable idea. In fact, anthropologists have been using a similar strategy to track ancient migration patterns of humans by characterizing the isotopic composition of the teeth from their remains. For example, anthropologists are able to tell from the isotopic signature in teeth that a person whose body is unearthed in southern England actually grew up in Scandinavia, suggesting he was likely a Viking invader rather than a local Brit.
Such isotopic signature investigations are a type of tracer study. In tracer studies, materials of interest, be they living or dead, are molecularly marked in some way, as though they had a molecular bar code. Information about movement and location of the material can then be obtained by following that marking. In Madigan’s case, a unique combination of stable isotopes that characterized a specific ocean location was the marking he followed. But detecting and measuring stable isotopes is not easy. Certainly, it is much simpler to follow radioactive isotopes because they emit distinctive types of radiation that can be readily tracked, identified, and measured with radiation detectors. This is why scientists prefer to perform tracer studies with radioisotopes, rather than stable isotopes, whenever possible.
Radioisotope tracer studies can be done on either a microscopic or a grand scale. We’ve already seen a microscopic radioisotope tracer study. You’ll recall that labeling of viral DNA and protein with different radioisotopes allowed virologists Hershey and Chase to trace the migration of the DNA, rather than protein, into the cells of the virus’s host, suggesting that DNA, and not protein, was the substance of genes. Fortunately for Hershey and Chase, it was easy to tag viruses with radioisotopes in the laboratory. Unfortunately for Madigan, it is not easy to radioactively tag fish in the open ocean; hence, his nonradioisotope tracer approach was the next best alternative.
But Madigan had a thought. Suppose, somehow, a characteristic combination of radioisotopes entered the ocean at some specific location. Would that radioactivity be taken up by local sea life, and could tracing the radioactivity in such animals be used to follow their migrations to other regions of the ocean? Perhaps.
One day in August 2011, fishermen were unloading their local catch of Pacific bluefin tuna (Thunnus orientalis) on the San Diego docks, and Madigan was there. His knowledge of bluefin growth rates allowed him to identify by size alone those fish in the one- to two-year age class. This cohort of fish would have just completed their first transpacific crossing. If Madigan were correct, these would be the most recent immigrants from the bluefin spawning grounds in the waters off Japan.
ORTOLAN OF THE SEA
Pacific bluefin tuna are big animals. They can reach weights in excess of 1,000 pounds (450 kilograms) at maturity, but few ever reach full size due to overfishing. The northern Pacific population spawns off the coast of Japan and remains there until, as juveniles (one to two years old), a subgroup of the fish begin an eastward ocean migration that ends on the west coast of the United States; here they feed on schools of baitfish, particularly the California anchovy. Their transoceanic migration takes one to four months.
FIGURE 15.1. COMMERCIAL FISHERMAN WITH A BLUEFIN TUNA. These large fish travel great distances in the ocean, consuming local marine life as they go, and incorporating into their bodies whatever stable and radioactive isotopes happen to be in the local waters. When humans consume such fish, they also incorporate these isotopes into their own bodies, where they tend to congregate in specific organs. If these isotopes happen to be radioactive, consuming the fish will result in a radiation dose to those organs.
While the tuna are in American waters, local commercial fishermen catch them and, ironically, their carcasses end up back in Japan within 24 hours, this time making their transpacific trip in the cargo hold of commercial aircraft. Most of them are sold at the Tokyo fish market. This is the same market that bought the radioactive catch of the Lucky Dragon No. 5 back in 1954, and very close to the site where that radioactive catch remains buried to this day.
The Japanese have an insatiable appetite for bluefin tuna, consuming over 90% of the world’s catch. They consider bluefin the premier fish for sushi and that appetite drives worldwide demand. In 2013, a single 489-pound (222-kilogram) tuna fetched a record $1.7 million ($3,500 per pound) in the Tokyo market. With prices like that on their heads, no wonder the stocks of bluefin tuna in the Pacific Ocean are only at 5% of their historic levels.3
Madigan collected steaks from 15 different bluefin. He then contacted fellow scientist Nick Fisher, an expert on marine radioactivity at the School of Marine and Atmospheric Sciences at Stony Brook University in New York State. Would Fisher test the fish for radioactivity? Fisher told Madigan that he would, but really didn’t expect to find anything. Soon the results were in and, to Fisher’s surprise, all 15 fish were contaminated with manmade radioactivity. But not just any radioactivity. They were all contaminated with cesium-134 and cesium-137 radioactivity, a signature of recent nuclear power plant waste. The implication was clear. The tuna were contaminated with radioactivity they had likely picked up that spring in the waters off Fukushima, Japan, the site of a major nuclear reactor meltdown.
We’ll learn a lot more about the Fukushima nuclear disaster of March 11, 2011, in the next chapter, but for now, all we need to know is that this nuclear accident resulted in a lot of radioactive waste escaping into the Pacific Ocean, and some of it got into sea life. These tuna had obviously been in the vicinity of Fukushima at the time of the accident and they carried that reactor radioactivity within their bodies across the entire Pacific Ocean. “This was just nature being amazing!” Fisher gushed. “Now, potentially, we had a very useful tool for understanding these animals.”
Madigan and Fisher realized their findings were important and rushed to publish them in the prestigious Proceedings of the National Academy of Sciences.4 In their paper, they concluded, “These results reveal tools to trace migration origin … and potentially migration timing … in highly migratory species in the Pacific Ocean.” But nobody gave a damn about that. The press and the public had a more pressing question: Are the tuna safe to eat?
To answer any question about safety of radiation-contaminated food, we need to know two things: (1) the exact radioisotopes involved; and (2) the amount eaten. Knowing exactly which radioisotopes are implicated tells us whether or not they will become concentrated in specific human organs, and will inform us as to the specific types and energies of the radiation they emit. This information will allow radiation dosimetrists to calculate the dose received by different human organs from eating various amounts of the contaminated food. Knowing those organ doses, we can then determine how much is safe to eat.
We already recognize, from the radium dial painter experience, that radioisotopes can chemically mimic dietary nutrients and become concentrated specifically in the organs that use those nutrients. In the case of radium, its chemical similarity with calcium, the mineral constituent of bone, causes ingested radium to be incorporated into bone. Likewise, as we know from the Bikini Islanders’ experience with nuclear bomb fallout, strontium seeks bone because it, too, is located in column two on the periodic table of elements, along with the calcium and radium. It was strontium that posed the major threat to the Bikini Islanders who ate coconut crabs following the hydrogen bomb tests, and it was their bones that received the bulk of the dose from the ingested strontium. Health data from the radium girls was directly applicable to the Bikini Islanders because radium and strontium behave very similarly in the body.
The Fukushima radioactivity found in the San Diego fish flesh came from two isotopes
of cesium, specifically cesium-134 and cesium-137. With the exception of cesium-133, which is stable, all other isotopes of cesium are radioactive and their half-lives are relatively short in comparison to the age of planet Earth. So all the radioactive cesium produced by the supernova that created Earth is long gone. Thus, any radioactive cesium that we now find in our environment must have come from manmade sources, either nuclear power plants or nuclear weapons testing. Cesium radioactivity produced by bomb testing is ubiquitous, having spread all over the planet, and currently contributes slightly (~0.02 mSv) to the annual background radiation exposure for everyone in the world. So how was it that scientists were able to tell the cesium in the tuna specifically came from Fukushima and not from a nuclear bomb test or another nuclear power plant leak?
It turns out that cesium-134 and cesium-137 are produced in roughly equal amounts by the fission of uranium-235 (i.e., nuclear power plant fuel). But the 134 radioisotope has a half-life of just 2 years, which is very short in contrast to the 137’s half-life of 30 years. So the 134 disappears from the environment relatively quickly while the 137 persists. That’s why it’s the 137 and not the 134 radioisotope that currently contributes to our background dose from atmospheric nuclear bomb testing years ago.5 The cesium-134 decayed away long ago, so there are no currently detectible amounts of cesium-134 in our environment. Consequently, when you do find cesium-134 somewhere in the environment, you know that it was produced by a relatively recent fission reaction. But how recent?
As already mentioned, cesium-134 and 137 are produced by fission reactions in relatively equal amounts. In fact, the cesium released into the Pacific Ocean at Fukushima had almost exactly a one-to-one ratio (134 to 137, respectively). Since 134 has a 2-year half-life and 137 has a 30-year half-life, it can be readily seen that two years following a release to the environment only half the 134 would have remained, but the 137 would have hardly decayed away at all. Thus, at two years, the ratio of 134 to 137 should be about 0.5 to 1.
You may recall that the ratio of carbon-14 to carbon-12 in biological material can be used to accurately date archeological finds (radiocarbon dating; see chapter 3). The same approach can be used here to date the cesium found in the fish. So that’s what Madigan and Fisher did. They determined the ratio of 134 to 137 in the fish to find out how long ago the cesium had been produced. They discovered it had been produced five months before the fish were caught. Since the fish were caught in August 2011, they must have picked up their cesium contamination around March 2011, the month of the Fukushima accident. Given that time frame, combined with the fact that the fish are known to migrate into Californian waters from Japan, the findings are pretty strong evidence that the cesium contamination in the fish came specifically from Fukushima.
Now, having identified cesium-134 and 137 as the Fukushima radioisotopes being ingested when eating bluefin tuna, how big a risk does eating the fish pose to sushi eaters?
Cesium is not in column two of the periodic table (i.e., the column with calcium, radium, and strontium). Rather, it resides in column one, which is inhabited by the biological nutrients sodium (element Na) and potassium (element K).6 As mentioned earlier when we discussed nuclear fallout, sodium and potassium are electrolytes critical to cellular function and muscle activity (including cardiac muscle). As might therefore be expected, the body’s sodium and potassium supplies tend to be somewhat enriched in muscle. And, yes, as you may have guessed, cesium is enriched in muscle as well. So cesium consumed as fish flesh (muscle) ends up in human muscle because it is mistaken for sodium or potassium by the body’s physiological processes. Thus, cesium is more likely to be found in muscle, rather than bone. But there are other important differences between cesium and radium.
The turnover of the minerals in bone is extremely slow. So radium incorporated into bone stays there a very long time. Sometimes scientists express the residence time of an element in the body in a manner similar to how they express the decay of radioactivity. They say that the element has a certain biological half-life, and the interpretation of this value is the same as for a radiological half-life. For example, a biological half-life of one year for element X in a specific organ would mean that after one year the amount of X in the organ would have decreased to half of the original amount through biological turnover processes and body excretion. For radium, its radiological half-life is 1,600 years, and its biological half-life in the human body is about 28 years.7
So for radium, the bones get a double whammy. The radium decays away slowly, plus it stays in the bone for a long time. In other words, the radium is going to give a whopping dose to the bones because the exposure time is so very long. In contrast, for radioisotopes with either a short radiological half-life or a short biological half-life, the exposure time is going to be too short to deliver a significant radiation dose (all else being equal). Fortunately, cesium falls in this latter category of radioisotopes. Its radiological half-lives, 2 years and 30 years (cesium-134 and 137, respectively) are much shorter than radium’s (1,600 years). More importantly, cesium’s biological half-life is just 70 days (0.20 years),8 as opposed to radium’s 28 years. Of these two determinants, its very short biological half-life, rather than its moderately long radiological half-lives, is the primary determinant of cesium’s low radiation dose. In effect, most of the cesium radioisotopes are excreted from the body before they get the chance to decay.
To get to our goal of determining dose, we now need to employ some logic extrapolated from what we’ve already learned. For any ingested radioisotope, knowing its rate of uptake into the body, its target organs, its radiological half-life, its biological half-life, and the energies of its decay emissions (i.e., the energies of its alpha particles, beta particles, and gamma rays, etc.), dosimetrists are able to calculate the radiation dose rate to any particular target organ. These calculations involve calculus, but we need not get into any of that math here. Nevertheless, the strategy is simply to calculate the average length of time that one atom of the radioisotope spends in the organ, using its radiological and biological half-lives, and then determine the average radiation dose that a single atom would deliver during its residence time in the organ. Once that is known, you can simply multiply the dose delivered by a single radioisotope atom by the total number of radioisotope atoms ingested, and you have the total dose.
What the dosimetrist has actually calculated is an organ dose, which is not unlike a partial-body dose from a diagnostic x-ray, as we’ve already discussed for broken arm x-rays and mammography. So we now ask the dosimetrist to convert the organ dose (partial-body dose) from the radioisotope, into an effective dose (a whole-body dose equivalent), just like we previously did for partial-body external x-rays. She does the calculation, gives us the number, and we send her on her way, because effective dose is all we need to make our risk estimate. How is that? Because, as you remember from our earlier discussions of diagnostic x-rays, effective dose allows us to calculate our personal risk using the atomic bomb survivor findings.
Even if the details seem complicated, the take-home message is simple and short: With a little knowledge of the metabolic physiology of the elements and the emission characteristics of their radioisotopes, the effective dose for any ingested radioisotope can be directly calculated, and that’s very important because effective dose allows us to predict cancer risk.
Before we leave all this internal dosimetry stuff, we should acknowledge one more thing. In this chapter, we’re focusing on ingested radioisotopes. But the same approach works for inhaled radioisotopes (e.g., radon), and for radioisotopes absorbed through the skin. The approach works exactly the same, and it works well. How well we’ll get to shortly. But now, let’s get back to the tuna.
The scientists found that the San Diego tuna contained 4 and 6 becquerel per kilogram (Bq per kg) of Fukushima radioactivity, from cesium-134 and cesium-137, respectively.9 To keep this simple, let’s just combine these numbers and say that there were 10 Bq per kg of Fukushima cesium
in the tuna. Is that a lot of radioactivity?
Everything is relative, so let’s take that value of 10 Bq per kg and make some comparisons. In terms of health regulations, it is not high. The US Food and Drug Administration (FDA) allows up to 1,200 Bq per kg of cesium in fish, so the bluefin are at less than 1% of the regulatory limit. How do the folks at the FDA determine the regulatory limit? They simply perform the effective dose calculations we described above and then they make a worst-case assumption about a typical American’s fish consumption. That is, they assume that people are eating a lot of the contaminated fish. In this case, they assume that the typical American eats one pound (0.45 kg) of fish per week and all of that fish is cesium-contaminated bluefin tuna. Then they determine how high the cesium radioactivity concentration would need to be in the fish before an individual’s effective dose would exceed the US Nuclear Regulatory Commission (NRC) radiation-dose safety limits for the general public, and presto, they have a regulatory limit for consumption of cesium in fish. Basically, this is where the 1,200 Bq per kg regulatory limit comes from.10
What does all this mean in terms of public health safety? It means that it is virtually impossible for Americans to eat enough bluefin to exceed federal regulatory limits for effective dose, as long as the cesium radioactivity concentrations of the fish are <1,200 Bq per kg, even if they are eating extremely large amounts. However, the reality is that no one in the United States is eating that much bluefin since it is not readily available on the American market. Thus, the FDA’s assumptions about Americans’ bluefin consumption, like the EPA’s assumptions about the time Americans spend in radon-contaminated homes, don’t pass the smell test. Even for the sushi-eating Japanese, it would be extremely hard to eat enough cesium-contaminated bluefin sushi to get themselves into trouble, even if they could afford to do so.11
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