A plume is an airborne cloud of radioactive gases, particles, and/or vapors released from a plant.

In many ways, the risk from a radioactive plume is similar to that from any cloud of hazardous materials. Members of the public should avoid being immersed in the hazardous cloud, breathing from the hazardous cloud, and entering areas contaminated by the cloud's passage.

Obviously, the major difference is that a plume released as a result of a major reactor accident will be radioactive and not hazardous in other ways (e.g., flammable, corrosive).