Both cesium137 and iodine 131 have been extensively found within a 50 mile radius of Fukushima, and are likely part of the fission products that caused the U.S. fleet to move a safe distance away from Japan earlier today. Cesium mimics potassium, is water soluble and readily taken up within the body. It will get in muscle, bone and other areas without your body knowing the difference. During the decay process, it emits beta rays (very similar to xrays) at very high energies (between about a half a million electron volts to just above 1 million ev). These are highly energetic photons with a tremendous amount of punch to create free radicals and alter DNA. If ingested, you will be irradiated from within, with not much you can do about it until these products are excreted somehow.
Iodine 131 has a very short half life (roughly 8 days) but is exceedingly dangerous because it targets the thyroid. I've read some local news saying that people should cover their thyroid with betadine, but that is nonsense. To protect the thyroid you will need to ingest a form of iodide that will saturate the thyroid gland internally, so there is literally no room for any radioactive iodine to deposit.
Seawater is being used to cool the reactors. It is also being circulated back out to sea, and with the water full of fission products, well... the implications are obvious. Refrain from eating any fresh seafood caught in waters north of the Philippines till further notice, just to be safe. No need to wait for the government to point it out.
Update 2: I have been paying attention to most of the press releases from local officials, and the vast majority of what is being said is sensible. All claims that Japan will not be another Chernobyl, however, should be avoided. How the government will be able to distribute enough potassium iodide for the entire population is a good question. Hope it doesn't come down to that.
Update 3: There is discussion going on about readings at the Onagawa plant 120 km south of Fukushima, with readings above 80 microsievert/hour (roughly 200millirem/day, whole body). Not an acceptable dose for long term exposure. If this is true, then radioactive release from Fukushima must have been huge. Can't get info on readings further south, or in Tokyo.
Update 4: 3 reactors are now melting down
Update 5: A third explosion took place overnight in reactor 2. The containment dome was breached, so now we can assume the nuclear fuel itself is in the air. Radiation levels have risen fourfold. 400-800 milliSievert/hour readings have been reported That is as high as 2000 rem per day. Human adults who are not radiation workers generally are allowed only a half rem per year. At this rate, they'll have to evacuate the staff.
Update 6: Plant workers are being evacuated. Nobody will be available to cool the reactors. Flights to the U.S. have at least doubled in price upon last check.
Radiation readings 3 times higher in Tokyo. It's working its way south.
Update 7 (3/15@11:05am): Now marketwatch is reporting 23X normal backround in Tokyo. The exposure of .809 microsieverts/hour translates to 0.7 REM per year at the current rate, putting every adult over their recommended yearly maximum permissible dose. This is tolerable for a short time, but it needs to come down asap.
Update 8 (3/15@6:02pm): A new development - Reactors 5 and 6 are now showing signs of overheating. Cooling systems likely not working properly due to lack of power.
Wind conditions still very favorable for the Philippines, as far as satellite imagery goes.