Background radiation levels in the atmosphere vary in intensity with latitude, altitude and phase of the solar cycle. These background levels are generated primarily be galactic cosmic rays (GCR), consisting of energetic nuclei of all naturally occurring elements, interacting with atmospheric constituents, primarily through atomic and nuclear collisions. Cosmic rays were discovered in 1912, about the same time that aviation began to develop. Over the years, an awareness of the potential hazards faced by flight crews and passengers had developed as air travel becomes more frequent and aircraft fly at higher altitudes for longer periods of time. Estimates of potential exposures of aircrew and frequent travellers, validated by on-board measurements carried out by carriers in the United States, Canada and Europe, indicate that these groups sometimes receive exposures above the limits for the general public, and should be categorized as occupationally exposed.
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