What needs to happen for one to ingest radioactive particles and how likely is this? There are many stories about radioactivity and the relative danger of it in the news lately, but very little actual information. The radioactivity levels around Fukushima Daiichi are high, but seem negligible in just somewhat removed locations.
The real danger seems to stem from ingesting radioactive particles. Just how likely is it for that to happen in any considerable distance from the reactor, say in Tokyo, and how dangerous for the human body is it really? How far can these particles travel in any dangerous concentration?
 A: I would like to add that radioactive isotopes are a constant background in all natural environments, and more so in stone  and concrete buildings. The relevant question is how much more than the natural background is the artificial radiation induced by human activities.
In a recent viewpoint in BBC news  professor Alison gives relevant numbers. For example, the human body has about 50 becquerel per kilogram in the natural state. An interesting chart that puts radiation in perspective is here, and this is a graph from the scientist who provided the numbers for the chart.
So when you see animated maps of how the radiation is spread from Japan, read the scales. You will see that the numbers are within natural variations.
The immediate danger of death come for huge doses, look at the chart. Even for people next to the reactors there has been no such exposure.  There are long term effects of ingesting or breathing in isotopes for the people in the region, which have to do with a larger cancer rate over twenty or thirty years. The rest of the world is nowhere close to such levels of exposure.
A: Ingesting alpha emitters is bad, but fortunately, we don't do that very often. Ingesting beta-emitters at LOW levels is something we have evolved to cope with, since even as necessary an element as potassium has naturally occurring beta-emitter isotopes.
But both beta and alpha emitters are more harmful to us once ingested or inhaled. Fortunately, in every day life, we get exposed to very little of that. Even after Fukushima and Chernobyl, most of us are more exposed to such emitters left over from atmospheric testing than from these two accidents. But even that source is swamped by naturally occurring isotopes (again, at low levels) in our food.
Now true, Cs-137 is being found in fish from the Tohoku area, but still at low levels, too low to keep me from enjoying sushi! Besides: the biological half-life of these elements (I and Cs) in the body is low enough (1-4 months) to keep it from accumulating in humans at a dangerous rate.
As for the reports of plutonium appearing after Fukushima, that was another initial, unreliable report: later reports showed that trace amount was from earlier fallout, most likely from that dark period of atmospheric nuclear bomb tests.
A: How far they travel depends mostly on weather. How much you ingest also depends on chemistry - you are more likely to ingest something that is water soluble then a heavy metal atom.
Then how much damage it does depends on biochemistry, a heavy metal atom that doesn't have any biological role may just go straight through you while something like iodine or strontium that are used in the body will be absorbed and may stay in you long enough to do some damage.
Remember a few particles don't do much harm, we can detect radioactivity at amazingly low concentrations, down to single atoms, so simply detecting radioactivity on the other side of the pacific doesn't mean much from a health perspective.
The most dangerous radioisotope for health is Radon (in terms of killing most people/year). It's a gas that comes out of the ground and gets trapped in houses, and because it's a gas you breathe it in so it's easily ingested. 
A: The big danger is less from eating radioactive substances than from breathing them.  The gut is a continuous “tube,” or in other words our body plans are a torus.  As a result you are more likely to pass it out within 24 hours or so.  The respiratory system is an “in and out” system, where the lungs form a fractal branching pattern that reaches to the cellular level.  So a piece of dust with radioactive substances can get lodged in the lung.  So an alpha emitter can sit there and do lots of damage.
A: There are a myriad of answers to your question available on every TV and website, but most have vague mass suggestions on how to stay safe.  Its better to focus on the size of the particulates of contamination. See a video referring to a form of radiation here: http://www.youtube.com/watch?v=WgQ79-oDX2o&feature=related Also not to let rain contact your skin directly when theres a report of radioactivity in an area. Radioactive substances are in our everyday life and some reports may be false positives, but always better to practice caution.  Also the length of time of exposure is just as critical. With the Japan leakage, the exposure to the rest of the world is minimal so far. Keep in mind that its an airborne particulate breathed in that is worrisome and anything to reduce that exposure is best.
As for food, we irradiate it already with small doses.
A: Voluminous literature is available on Effects of ionizing radiations on atomic bomb survivors in Japan, and Chernobyl reactor accident held in April 1986. The harmful effects take place on the whole body, and even at cellular level. Some exposed people have developed cancer. The detailed reports are available in Internet. Take the instance of 131-I accumulated in thyroid gland immediately after Chernobyl reactor accident in 1986. Since it has half life of 8.0207 days, and excreted through urine it is not considered as dangerous at low activity levels.
