There’s acute exposure, and then there’s chronic exposure to radiation. Acute exposure results from short bursts of radiation from an external source and which can cause various maladies or death.
Chronic exposure is the long-term problem. Here’s a good explanation of chronic exposure to radiation
Chronic exposure can be due to exposure to external radiation fields, or can be the result of inhalation or ingestion of radioisotopes which then become fixed in the body through chemical reactions with the tissue protein or, because of the chemical similarity of the radioisotope with normal metabolites, may be systematically absorbed by certain organs and tissues. The results from chronic exposure, such as cancer or genetic mutation, often do not occur until many years after the initial exposure.
This explanation highlights the fact that it will be a long time before the final effects of what is happening in Japan can be known. In addition, it is close to impossible to gauge with any degree of precision what people downwind have been exposed to, as well as what people are eating, drinking, and inhaling.
As result, consideration of chronic exposures should lend some caution to industry PR people who are saying that there’s not much to be concerned about at this stage. Even if things get under control at the reactors, the extent of health effects won’t be known for a long time.