How long does Cesium 137 from Fukushima plant stay in human body?

The maximum amount of Cesium allowed in food produced in Japan  has recently been lowered from 500 Bq/kg to 100 Bq/kg. This very conservative level is causing several problems to vegetable and meat producers, that have to throw away larger quantities of food. 

Even though the value of bananas is 125 Bq/kg of potassium 40 and up to 260 Bq/kg of Radium in brazilian nuts is higher than the current limit, it is sometimes argued that the values are unrelated. 

A conservative (and rough) estimate would give a factor 4 to "convert" from Cesium to Potassium (1Bq of) Cs = 4 Bq od K40. 

In other words  eating  100 g of Shitake with 50 Bq of Cesium   is equivalent to eating a couple (200g) of bananas. This takes into account decay modes, physiology and time to expel it from the body.  Calculations are below. Some points worth remembering are: 

Decay scheme of Cs137:
either one electron (beta-)
or one electron and one gamma.

But radioactivity from bananas is "natural" and good! There is no "natural" (good) and "artificial" (bad) radioactivity. All radioactive elements decay in the same ways and are potentially dangerous. Ceisum and Potassium decay in the same way (electron or gamma) and there is no difference among the two. 

Becquerels and Sieverts are two different things, to be used in different contexts. Becquerels represent the amount of radiation in an object (food, ground, trees...). Sieverts represent the equivalent dose to which the human body is exposed. Think of Becquerels as the luminosity of an UV light bulb of a tanning machine and Sieverts as the amount of tanning we might get from it.  It is not straightforward to pass from Becquerels to Sieverts, since it depends on the exposure time, if the food is ingested etc...

Decay scheme of K40,
either one gamma or one electron

Does Cesium accumulate in human body? No, it is expelled in one to four months, compared to one month of Potassium:

                               Cesium 137 and 134. The biological half-life of Cesium is between one and four months. ICRP standard  give 110 days for man and 65 days for women.  Some quotations:  (53+-12 days in children). 112 days in adults. Some differences between population may arise: biological half time is somewhat shorter among Scandinavian males (84 days), not in females (64 days).

                                Potassium 40. Our body contains about 3700 Bq of potassium 40, corresponding to about 0.32 mSv in an year. The biological half-life of potassium, that is the time it takes to expel half of it from our body is 30 days. (data from here).

So, how do I compare radiation from potassium and from Cesium? 

On average, and very roughly, 100 Bq from Cesium results in 4 times more Sieverts than 100 Bq from Potassium. This takes into account the factor 2 of the decay (Cesium decays in 82% of the time in an electron and then one gamma) and another factor 2 for the biological half-life (two months for Cesium vs one month of Potassium). In the most conservative case  you might assume a factor 8 (2 for the decay and 4 months of maximum half-life).

Nuclear Plants, Radiation and Food in Japan - updates

Currently in English, Italian translation will follow. 

Power Production

No electricity is generated from nuclear plants in Japan. All but one reactors are down, officially for maintenance. Whether they will be reactivated is still debated. Oki reactors are now less likely to be put back on line, pending the complaints by local government and some still unclear safety issues.

As of now, both Kanto and Kansai (that are on different grids) are consuming about 80% of the full power production. If this figures are correct, there is the clear possibility to have scheduled black-outs this summer, especially if it will be warmer than last year. The situation might be more critical for Kansai  which had a larger nuclear-produced energy fraction and that last summer had still the nuclear power plants online. 

Environmental radiation in case of no decontamination
in 10 and 20 years from now

Environmental radiation

Outside the exclusion zone, Japan has  an average  lower environmental radiation than Italy.  

Recent simulations show that - in absence of any decontamination - the Fukushima expelled Cesium will leave the exclusion zone unsafe for more than 10 years. This is hardly surprising given the 30 years half-life of Cesium 137. Decontamination strategies are under study, either by removing the ground, by letting radiation penetrate deepen in the soil in case of hot spots, or by physically separating the radioactive material from the rest. A truck from Toshiba has been set up to demonstrate the principle. The first method seems more expensive but is much more reasonable,  provided that all the material is stored around the site of the now destroyed power plant. The truck option would be the cleanest one, but cost of the procedure and the possibility of large scale production are not clear. 

Is there radiation in food in Japan?

A vegetable wholesaler has been found relabeling the origin of Fukushima cucumbers in other prefectures. He was just scolded, no fine or charges. The cucumbers were not dangerous in themselves, since it still is below the new limit of 100Bq/kg for Cesium, but the lack of legislation is worrying. This new lower limit (previously was 500Bq/kg) is causing a lot of trouble to fruit and vegetable companies, since the value is extremely low too meet. .  All food (and our body) contains radioactive material: as a reference it is worth remembering that bananas  have 125 Bq/kg of  Potassium 40 (even though it is expelled faster from the huma. body).

Left and right: radiation data of Coop food products

All are below the new limit of 100Bq/Kg (mentioned in the top page). The food with highest radiation content are Shiitake mushrooms. Note that Germanium detectors have an higher resolution than those of NaI  and can distinguish between the isotopes of Cs134 and Cs137. 

The total amount of radiation in Ge-measured products is the sum of the two isotopes. Cs134 has a shorter half-life, two years, compared to the 30years of Cesium 137, so it is expected that Cs134 content will be negligible by next year.

Rate of disappearance of Cesium from human body
from here