"The radiation levels that will come to North America will not be large enough to cause acute symptoms or immediate health effects.
At this point, we do not believe it will be necessary—or feasible given the expected duration of elevated radiation levels—to take shelter. Nor is evacuation a feasible strategy, since after arriving on North America’s west coast the elevated levels will continue to move to the east. We will post immediately if there is reason to believe that sheltering is necessary.
However, unnecessary exposure to radiation should be avoided when possible. Rain, in particular, can bring radioactive particles to the earth." - Protective Measures from Radioactivity in North America - NIRS (external - login to view)