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This is because the further the radioactive material travels, the more dispersed (and the less harmful) it becomes.
Although measuring levels of radioactive contaminants in the oceans is challenging, measuring health effects associated with those levels is even more difficult and controversial.
Every additional source of radioactivity carries some additional health risk, but these risks vary with many factors, including the dose (how much a person is exposed to and for how long) and which isotopes you are exposed to, as well as individual sensitivities—there is a higher concern in children, for example.
Fukushima will likely have the most significant long-term health impacts on those who had the highest exposures, so those living closest to the plant or in areas with higher fallout.
Cesium-137 has a relatively long half-life (30 years), but it is also present in the ocean as a result of nuclear weapons testing in 1950s and 1960s.
Cesium-134 is much shorter-lived, which means that any detected in seawater samples must have come from Fukushima.
Another radionuclide of concern, cesium-134, has a half-life of two years, which means that it is rapidly disappearing.
Because of its short half-life, cesium-134 is the one isotope that, if we find it, could have come only from Fukushima.
For the general public, it is not direct exposure, but uptake by the food web and consumption of contaminated fish that is the main health concern from the oceans.Since mid-2011, I have worked with Japanese colleagues and scientists around the world to understand the scope and impact of events that continue to unfold today.In June 2011, I organized the first comprehensive, international expedition to study the spread of radionuclides from Fukushima into the Pacific, and I or members of my lab have participated in several other cruises and analyzed nearly one thousand samples of water, as well as dozens of samples of sediment and biota.It was also released in equal proportion with cesium-137, so when we detect cesium-134, we first adjust for its decay over time and then we can calculate how much total contamination was released.But this "fingerprint" provided by cesium-134 is rapidly fading.
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This change is measured in half-lives—the length of time it takes for the radiation to decrease by one-half.