One component of background radiation comes from the earth and cosmic rays. Exposure from this component of background radiation varies depending upon several factors, including the type of material in the earth, elevation, whether there is snow on the ground or how much it rains. The average background exposure from the earth and cosmic rays at sea level is approximately 28 millirem per year. In Denver, Colorado, it is 81 millirem per year. In the Rowe, Massachusetts area, it is 66 millirem per year. These are normal exposures and variations for this component of background radiation because the characteristics of the locations vary.

In fact, background radiation varies by more than 35 millirem per year in the Rowe, Massachusetts area. These background radiation readings were measured by Yankee using a Pressurized Ion Chamber instrument on Tuesday, January 6, 1998. The millirem per year calculation was based on this measurement.