Another source is environmental background exposure (from cosmic radiation and natural isotopes), which can be significant, particularly at high altitudes; airplane flights result in increased exposure to environmental radiation as follows: From a single coast-to-coast airplane flight: 0.01 to 0.03 mSv From average yearly background radiation exposure in the US: About 3 mSv From yearly exposure at high altitudes (eg, Denver, Colorado): Possibly > 10 mSv Table Radiation may be harmful if the total accumulated dose for a person is high, as when multiple CT scans are done, because CT scans require a higher doses than most other imaging studies.