A permanent bombing
The Earth is continually bombarded by charged particles (protons, atomic nuclei) which travel nearly at the speed of light. They come from our galaxy (supernovae) and sometimes from the Sun. When they impact atomic nuclei in the Earth’s atmosphere, secondary particles are produced that may eventually reach sea level. See the site of NMDB for more information.
The secondary particles are the dominant source of irradiation at the cruise altitude of civil aircraft. The dose rate is the higher, the closer one comes to the magnetic poles and the higher the flight altitude. During a flight from Paris to San Francisco or Tokyo, aircraft are exposed to an overall dose of about 70 micro-Sievert. At sea level in France the average annual dose due to natural and artificial ambiant radio activity ranges from 2 to 3 milli-Sievert (abbreviation : mSv). Flight crew of civil aircraft have to be monitored because of the repetitive exposure to cosmic rays : the dose accumulated over 5 years must not exceed 100 mSv, and it must not exceed 50 mSv in any year. The maximum dose for pregnant women is 1 mSv over the entire duration of their pregnancy.
The monitoring
In France the monitoring is based on cosmic ray measurements. In case of a major solar event the supplementary dose is calculated from measurements of the neutron monitors in Terre Adélie and on the Kerguelen Islands. This is done within the SIEVERT system (Système Informatisé d’Evaluation par Vol de l’Exposition aux Rayonnements cosmiques dans les Transports aériens), which was developed by the Directorate General for Civil Aviation (DGAC), the Institute for Radioprotection and Nuclear Safety (IRSN), Paris Observatory and the Paul-Emile Victor Institute for Polar Research and Technology (IPEV). See the site of LIRA and the site of SIEVERT.