How series is the increased risk from Fukashima to the world at large?

U.S. reports concerning toxic radioactive substances reaching the U.S. from the Fukashima disaster site emphasize that the toxic doses reaching North America are very miniscule. However, reports of high radiation and continued leakage at the site might leave one to conclude that, over time, if the continued leakage is not stopped or contained, then global levels of toxic radioactive substances will become a serious problem outside of Japan and throughout the world. How realistic is this concern?