As of 11:30 a.m. (EDT) on March 19, EPA’s RadNet radiation air monitors across the U.S. show typical fluctuations in background radiation levels. The levels detected are far below levels of concern. The Nuclear Regulatory Commission has said we do not expect to see radiation at harmful levels reaching the U.S. from damaged Japanese nuclear power plants.
Source: U.S. Department of Environmental Protection
See also: RadNet Map (real-time radiation monitoring)