http://www.bt.cdc.gov/radiation/evacuation.asp
Centers for Disease Control and Prevention, U.S. Department of Health and Human Services
In the event of a radiation emergency, such as a nuclear power plant accident or the explosion of a radioactive dirty bomb, you may need to leave your home or the immediate area (evacuate). Depending on the direction the radioactive cloud or stream (called the plume) is moving from the incident site (the area where the accident or explosion took place), you may have to evacuate or go to an emergency shelter. This fact sheet will help you decide on the best actions to protect yourself and your family.