According to the EPA the radioactivity from the accident in Japan has reached the USA. But it is important to bear in mind that just because you can measure something does not mean it is at a level which can threaten human health. I am showing below the data for four sites in the USA where they have high volume air sampling machine.
Radioactivity in air using EPA data
As the ratio of the isotopes looks about the same for all four I think it is reasonable to assume that the radioactivity in the air shown in the chart above is of four sites which are being supplied with radioactivity by a common source. The radioactivity level is low.
For the iodine-131, the San Francisco iodine level is such that if it stayed the same (ignore radioactive decay), then a person who does hard work (digging) for a whole week (no breaks allowed, work all day all night !) then he would be breathing about 604800 L of air (604.8 cubic meters of air). This air would contain about 2.5 mBq per cubic meter. This works out as 1.512 Bq of iodine-131 per week.
It would then take a nuclear worker about 666667 weeks (12821 years) to reach an intake of 1 MBq which is where one limit is for nuclear workers. Now the limit for the public is 20 times lower than it is for nuclear workers, so for the public you could reach one year’s limit for iodine-131 if you dig for 641 years non stop in San Francisco if the iodine level there stayed the same.
