noteworthy wrote:

State health officials stressed that on a daily basis, Americans are exposed to radiation from sources like the sun, rocks, and bricks with doses that are 100,000 times higher than what has been detected in the US coming from Japan.

But as my other links state - the concern is not just about the level of radioactivity but also the risk of ingesting radioactive particles. There are some people arguing that the later risk is being underestimated.

RE: State steps up monitoring after radioactivity found in rainwater - The Boston Globe