Japanese officials and outside experts have repeatedly stated that levels of radiation found in areas surrounding the Fukushima nuclear power plant pose no “immediate risk to human health.” This has often been misinterpreted to mean that this level of exposure to radiation is “safe.”

There are two ways that radiation can damage human health – immediately, through exposure to large doses of radiation, and over time, through cumulative exposure.

Most of the press coverage has addressed the Fukushima disaster through the potential of radiation exposure to make people immediately sick. With the exception of the workers at the plant (whose heroism is increasingly, and justifiably, being noted), the risk of exposure to radiation levels high enough to produce immediate health effects appears low.

The Fukushima disaster, however, is likely to cast a long public health shadow. Scientists believe that cumulative exposure to radiation is an indicator of cancer risk. As noted yesterday, exposure to 100 millisieverts of radiation increases the odds of an individual developing cancer within their lifetime by about 1 in 100.

So, when news reports state that radiation levels in Tokyo (150 miles to the south of Fukushima) are 10 times normal levels, but that the risk is “slight,” they are, in one sense, correct. However, there is no known safe level of exposure to radiation, and low-level exposures still have the potential to cause harm, particularly if they persist over a long period of time.

The persistence of contamination from radioactive substances is likely to be a big concern in currently evacuated areas, and possibly in other areas downwind of the troubled reactors. To what degree will the contamination include contamination with radioactive substances – like some isotopes of cesium and plutonium – that remain dangerous for dozens or hundreds of years? To what extent will radiation find its way into the food chain? At what point will it be safe for residents evacuated from the area to return?

These are long-term questions, and the Japanese government can be excused for a bit of short-term thinking - especially as it is in the midst of a massive earthquake recovery operation in which people’s lives hang in the balance minute by minute.

Those of us who are concerned with nuclear policy as it relates to the United States, however, have no such excuse. The long-lasting impact of radiation from Fukushima is very relevant to the question of whether nuclear power plants represent an unwarranted and unnecessary risk to health. For that reason, the fact that the Fukushima disaster has not yet risen to the level of horror of Chernobyl offers very little comfort.