The measured gamma exposure rate around nuclear power plants is due mainly to natural causes and radioactive plumes emitted from the plants: A calculation method based on a puff model has been proposed to identify the latter contribution and evaluate quantities in response to short-term fluctuations of meteorological conditions and the release rate. The calculation results have been compared with the measured exposure rate less the natural background. In general, the calculation exposure rate is one-half to twice as much as the measured exposure rate less the background.