At the Fukushima reactor I, the Japanese government reports, workers have been in the vicinity of radiation levels in the past day as high as 400 millisieverts per hour. "That's pretty high, it's worrisome," says K. S. Clifford Chao, head of radiation oncology at New York-Presbyterian Hospital. But it's unclear how much time workers, who are presumably using protective suits and masks, have been exposed to the radiation levels in that area.
At that level, says Chao, an hour's exposure is roughly equivalent to 1000 times the radiation that the average American experiences in a full year. One of the chief sources of radiation in previous nuclear accidents has been Iodine-131, which has a half life of 8 days, meaning that half of it decays to other isotopes in that time. So-called radiation sickness can occur, according to Chao, when an adult's cumulative exposure reaches 1 or 2 sieverts, though cumulative doses of more than 300 millisieverts can cause nausea. Workers exposed at the maximum level reported at Fukushima would reach this threshold in a few hours, if they lacked protection. (Here is a good radiation primer.)
External radiation levels reportedly have fallen at the plant—though the containment vessel in the #2 reactor has been badly damaged and could release more radioactive material.
Among the fission products causing the greatest concern are longer-lived isotopes, such as Cesium-137 and several derivatives of plutonium. (Plutonium is one of the elements in mixed-oxide fuel in a reactor that appears to have had a partial meltdown.) Both Cesium-137 and the plutonium derivatives have half-lives of roughly 30 years and can lead to cancer if inhaled or ingested. That's another reason why workers wear protective gear and breathing equipment. The strategy of asking residents to stay indoors and filter their air is aimed at allowing them to avoid the shortest-lived radiation while crews get the situation under control, while also avoiding possible inhalation of the most dangerous isotopes.
At 288 kilometers south of the Fukushima nuclear complex, Tokyo is experiencing a background radiation of 0.8 microsieverts per hour, reports say. Chao says that's about 24 times higher than normal but not dangerous.