"The portion of the total radiation dose (in air) contributed by each isotope versus time after the Chernobyl disaster depicting caesium-137 becoming the largest source of radiation about 200 days after the accident." Image credit:Wikipedia
I am oversimplifying the science in this post to get to a risk comparison: climate forcing CO2 emissions vs widespread radiation hazards from nuclear plant failure.
There is much free-floating anxiety outside of Japan as a result of the ongoing Japanese nuclear incident. US citizens are ordering potassium iodide tablets, for example, which will do nothing to mitigate what is likely the greatest long-term human exposure risk, indirect contact with Cesium-137. Key point: outside and inside Japan, widespread exposure to radionuclides from the Japanese incident may be mostly indirect, and would potentially be highest this coming fall - assuming that lessons learned from Chernobyl are applicable. See the above chart for Chernobyl comparison. Read on for details.
First, closely read this paragraph from Wikipedia and I'll explain a bit.
The [cesium] isotopes 134 and 137 (present in the biosphere in small amounts from radiation leaks) represent a radioactivity burden which varies depending on location. Radiocaesium does not accumulate in the body as effectively as many other fission products (such as radioiodine and radiostrontium). As with other alkali metals, radiocaesium washes out of the body relatively quickly in sweat and urine. However, radiocaesium follows potassium and tends to accumulate in plant tissues, including fruits and vegetables. Accumulation of caesium-137 in lakes has been a high concern after the Chernobyl disaster. Experiments with dogs showed that a single dose of 3800 μCi (4.1 μg of caesium-137) per kilogram is lethal within three weeks; smaller amounts may cause infertility and cancer. The International Atomic Energy Agency and other sources have warned that radioactive materials, such as caesium-137, could be used in radiological dispersion devices, or "dirty bombs".When Radiocesium falls onto the land as particulate matter or in precipitation, snow melt and spring or early summer runoff will eventually carry the isotopes and deposit them in ponds, wetlands, and lakes, where they may be incorporated into biomass or bound to suspended sediment, ending up either in the food chain or in bottom sediments.
Something analogous has happened before.Because Cs-137, a dominant radiocesium, has a half-life of 30+ years, sediment cores taken from lake bottoms anywhere in the world can sometimes be 'dated' by the presence of a distinct Cs-137 layer laid down by late 1960's nuclear bomb testing; and, overall sedimentation rates can be estimated with this information.
Local and regional risk potential.
Before the Cesium isotopes erode into sedimentation basins, though, what can happen? Apparently, Cs-137 can be taken up by vegetables and fodder crops for days-to-years after it has fallen out of the sky: depending on erosion or 'wash-off' rate.
I have not looked at the literature to see if Cs-137 falling on the ocean may be taken up by algae or seaweed. Seems reasonable to suppose so. Seaweed, Japanese cooking. You see the point, I presume. This needs corroboration, mind you. It is merely an abstract possibility until proven out as a risk.
Groundwater recharge: again I have not looked into it.
Global comparative risk: excessive greenhouse gases vs all potential nuclear plant-emitted radionuclides.
Summarizing the above: radionuclide particles originating from failed nuclear power reactors do not disperse and come into contact with people only by breathing or from touching contaminated clothing - which are the only vectors that mass media seem now to focus on.
Cesium isotopes, once released, can disperse also indirectly, by surface erosion and wash-off, and pose risks for many days, or years, afterward.
Let's compare the cumulative indirect exposure risk of high Cs-137 emissions with the risk of excessive CO2 emissions. As a friend pointed out to me:
"CO2 is very like radiation in that it has a half-life of [indirect] toxicity in the air of about 200 years and in the oceans the half-life of [direct] toxicity is more like 2000 years."