We are exposed to radiation in a number of different forms, and in a number of different ways. We are bombarded by electrons, positrons, photons (gamma and X-rays), neutrons of varying energies, alpha particles, and fission fragments. For example, we get dental X-rays, and mammograms, CT scans. We are exposed to naturally produced radon, and get radiated when we fly in airplanes. Some of us live near nuclear reactor power stations or, in my case, a coal power station (coal is rich in thorium and uranium). Did you know that sleeping next to a human for 8 hours every night actually gives you a radiation dose? Good grief--how have I lived this long without knowing that? Turns out that our bodies have naturally radioactive potassium! The source of this, and other tidbits (Brazil nuts apparently are the world's most radioactive food due to high radium concentrations!) is PBS.
Radiation is energy, and the amount of radiation (measured in joules) absorbed per kilogram of material is defined in units of "gray." The amount of radiation absorbed per kilogram of human flesh is measured in units of sieverts (Sv). It is the amount of "grays" multiplied by a weighting factor which takes account of the radiation type, and the body type absorbing it (skin, bladder, bone marrow, etc.) Thus, the sievert is tied to a biological response rather than purely physical dosage. The most commonly encountered multiple of a sievert is the millisievert (mSv), 1/1000 of a sievert.
Dosages can be single, hourly, yearly, or maximum. Typical single dosages are: dental radiography (0.005 mSv), mammogram (3 mSv), or chest CT scan (6-19 mSv). Typical average hourly doses are: 0.34 microsieverts/hour for Americans and one-half of this value for Australians. Typical yearly dosages are: 0.0003 mSv/year living next to a coal station; 0.24 mSv/year from cosmic radiation from the sky; 0.28mSv/year from the ground; 0.40 mSv/year from natural radiation in our human bodies; 0.85 mSv/year from the radiation produced by the granite in the U.S. Capitol building. The PBS site mentioned above says that the building is so radioactive due to the uranium in its granite walls, it could never be licensed as a nuclear power reactor site!
Symptoms of acute radiation, within one day, are: 0.25-1 Sv, nausea, loss of appetite, damage to bone marrow, lymph nodes and spleen. 1-3 Sv, mild to severe nausea, loss of appetite, damage to the above organs with recovery probable but not assured. 3-6 Sv, severe nausea, loss of appetite, hemorrhaging, infection, diarrhea, peeling of skin, sterility, death if untreated. 6-10 Sv, above symptoms plus central nervous system impairment, death expected. Above 10 Sv, incapacitation and death.
Although reports from the Fukushima emergency have been conflicting, the highest hourly rate may have been 8217 mSv/hour. The normal recommended average limit for workers in nuclear plants is 20 mSv/year, but this has been raised to 250 mSv/year at Fukushima during this crisis. The report today, if true, is that water found in a tunnel at Fukushima Daiichi is emitting more than 1,000 mSv/hour. I don't have specifics on the allowable levels of radiation from coolants inside a nuclear reactor, but referring to the typical average hourly doses that americans receive (0.34 microsieverts/hour), you can see a basis for the statement that this is at least 100,000 times normal levels.