Cesium-137 From Fukushima Meltdown I've been reading up on the Fukushima nuclear meltdown and its effects it had on the environment. The iodine-131 initially released from the incident decayed after 8 days, but other isotopes such as cesium-137 and strontium-90 have a half-life of 28.7 and 30.2 years (about 300 years to completely decay).
Apparently, large quanties of cesium-137 were found in neighbouring countries as well as in seafood near Japan. 
Now, for something like nuclear isotopes, not having the required knowledge can lead someone to fear something that might be harmless (or confirm some doubts about the issue), hence why I am here. 
From my research on scientific websites and different governmental regulations, I found out that food and water in Japan have a legal limit as to how much cesium-137 a food item can contain. Especially since cesium-137 ressembles potassium and therefore being easily taken up by the body, this is worrying. 
To my understanding, the way a radioactive isotope creates havoc in the body of an individual is that:


*

*It ressembles a normal element we usually consume, like iodine, potassium for cesium-137 and calcium for strontium-90.

*The body digests it and it is stored in the body. 

*It emits radation for a certain period of time, anywhere from 8 days for Iodine-131 to years for cesium-137.

*The prolonged radioactivity from these isotopes causes cancerous cells to occur. 

*A tumour or other form of cancer develops. 


If this is what happens for isotopes like cesium-137, how can it be possible to eat food or drink water containing ANY cesium whatsoever? Sure, the radiation isn't so bad, but what is important to take into consideration is that this is internal and highly centralized radiation. This is a completely different thing to external radiation. 
For my questions about the topic, I would like to know three things (which could really put many doubts to rest).
QUESTIONS 
The first is, is my understanding described above correct?
The second is, how does this affect us healthwise? We are stuck with these debris of cesium-137 and strontium-90 for the next 30 or more years, but what effect do they have on us?
If someone ingests an atom (or a small amount) of cesium-137, what affect will this have on this person's body?
Finally, for someone living in North America, how much of these debris are there, either that got here from the wind, products, food, water, etc.?
EDIT: If someone knowledgeable on the subject could answer the questions directly (highlighted by means of italics), it would be very much appreciated.
 A: 
The first is, is my understanding described above correct?

Basically, yes.

The second is, how does this affect us healthwise? We are stuck with these debris of cesium-137 and strontium-90 for the next 30 or more years, but what effect do they have on us?

Stochastic health effects (induced tumors, cancers, leukemia,…)  of ionizing radiation are quantified by the quantity called committed effective dose. There are many studies regarding these effects, very good and reliable source of information is UNSCEAR, for example the 2006 Report. 
Your question "what effects do radionuclides on us" is too broad to answer. It depends on the amount, contamination pathway (ingestion/inhalation), age of person,... and milion of other parameters.

If someone ingests an atom (or a small amount) of cesium-137, what affect will this have on this person's body?

It is assumed, that any amount of ingested or inhaled radioactivity increase the risk of negative health effects. But the question whether the ingestion of small amount of Cs-137 is dangerous is not as straightforward as it seems- you need to compare it with the natural level of ionizing radiation. There is indeed no reason to worry about ingestion of a small amount of Cs-137, let’s say in order of µSv since it is comparable to the ingestion of one banana (due to high content of the K-40). 

Finally, for someone living in North America, how much of these debris are there, either that got here from the wind, products, food, water, etc.?

According the USGS report, maximum detected Cs-137 fallout from Fukushima in US was found to be 240 Bq/m2, which is a rather low value compared to the content of natural radionuclides in the soil. Regarding the Cs-137 in the food, FDA used the limit value of 1200 Bq/kg. Lower values are regarded as safe and as far as I know, there were no contaminated food products detected in USA. Regarding the water contamination, the measured values were approx. 5 Bq/m3, which is safely below any health concerns. For sake of comparison, the tap water is likely to contain much higher (few orders of magnitude) concentration of natural radionuclides, mainly Rn-222. 
