I realized I didn’t fully explain radiation safety. I may have mentioned that the reason radiation is nasty is that it can ionize the atoms that make up your body which is a bad thing. One of the problems with radiation is that it is generally cumulative. This is why astronauts (who are exposed to very high amounts of radiation from cosmic rays while in space) are only permitted to be in space for a limited total amount of time (e.g. seven Shuttle flights). But it is important to also know that radiation affects people differently since it is governed by quantum mechanics and is thus probabilistic in nature.
In any case, people tend to get an average of from below 1 to about 3 millisieverts of radiation per year from natural sources (a Sievert (Sv) is a unit used to measure how much radiation a person is exposed to). People living in areas like New Hampshire tend to get slightly higher amounts since there is quite a bit of radioactive material naturally in the bedrock (e.g. granite can be quite radioactive – the US Capitol building in Washington spits out about 0.85 mSv per year). A chest x-ray exposes a person to 6 and 18 millisieverts in a single burst. One little publicized fact about the Three Mile Island nuclear accident is that, on average, people living within about 10 miles of the plant only received 0.08 mSv with the maximum recorded value of 1 mSv (one-third the value a woman receives from a mammogram). The International Commission on Radiological Protection recommends that workers trying to stave off a nuclear disaster (e.g. what is currently taking place) not receive more than a total of 500 mSv. Workers trying to save lives are recommended not to exceed 1 Sv (1000 mSv).
With that said, it is important to note distinctions in how the numbers are being presented when coming out of Japan. For example, this morning, CNN reported
Radiation levels Thursday hit 20 millisieverts per hour at an annex building where workers have been trying to re-establish electrical power, “the highest registered (at that building) so far,” a Tokyo Electric official told reporters.
By comparison, the typical resident of a developed country is naturally exposed to 3 millisieverts per year.
The company said Friday afternoon, though, that radiation levels at the plant’s west gate, at .26 to .27 millisieverts, have been fairly stable over a recent 12-hour span.
First, it is not clear what the 20 mSv/hr value represents at the annex. It could be measured by a single detector but it also could be a total given by a series of detectors in which case the actual amount an individual worker might have been exposed to could be quite a bit less. Second, recall that these workers are wearing protective gear (though I don’t know how protective). Finally, it is important to put this into perspective on the biological side. For certain, unprotected workers exposed to more than 0.25 mSv in a single day will experience some radiation sickness and possible long-term impact to their bone marrow, lymph nodes, and spleen. But guaranteed death requires exposure to about 6 full Sieverts in a single day. The lowest amount known to be clearly carcinogenic (though not guaranteed to be – again it depends on the person) is 100 mSv per year. Workers responding to the current crisis at the Fukushima plant have been cleared for up to 250 mSv per year.
There are a couple of other factors that play into this. One of the decay products of nuclear reactors is iodine-131 which reacts with the thyroid glands. As such, people often take iodine-127 in the form of potassium iodide which can block the absorption of the more nasty form of iodine by the thyroid. In addition to the iodine issue, plutonium, which is only used in the number 3 reactor at Fukushima Daiichi, but in conjunction with uranium, has different byproducts than uranium, thus increasing the variety of harmful byproducts that people could be exposed to. In nuclear plants, it is often the spent fuel rods that can be a greater health hazard since they present a wider array of byproducts that can interact with human tissue.
Finally, for anyone worried about places downstream (i.e. Hawaii, the West Coast, etc.), while it is clear that some of these particles will reach those locations (indeed, many could circumnavigate the globe if they are lofted into the right wind pattern), they should disperse greatly and thus actual exposure levels should not be anywhere near harmful (if they are even detectable at all).