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Cosmic radiation is a naturally occurring form of ionising radiation that arises from the sun and the galaxies outside the solar system.
The earth’s atmosphere and magnetic field substantially shields the earth from cosmic radiation. People travelling in an aircraft may be exposed to more ionising radiation, because the earth’s atmosphere provides less protection from cosmic radiation at the typical cruising altitudes of commercial aircraft. Exposure also increases the further the flight path is away from the equator. Therefore, the radiation dose will vary between different flights depending on origin, destination, route, flight level pattern and solar activity at the time.
All people are exposed to background radiation at sea level which can come from the local environment, food and drink, medical exposure or building materials. In high doses, radiation can be harmful. However, the doses received at flight altitudes are considered very low. Aircrew and frequent flyers get the most additional exposure because of the extra time they spend at cruising altitudes. 

Cancer is the principal health effect that has been associated with low-dose ionising radiation.  As all air travel is associated with increased exposure to ionising radiation, it therefore follows that there must be some increased risk of developing cancer. The increased risk is however, extremely small, given that naturally occurring cancer accounts for 23% of deaths anyway in many developed countries.  For example, a person who flies direct return flights from Hong Kong to New York every two weeks for 20 years has their risk of death from cancer increased from 23% to between 23.11% and 23.14%. This represents an increase in percentage over the normal of around 0.5%, which most people would probably not consider unacceptable.
Questions have also been raised on the effects of cosmic radiation on pregnancy and the health of the fetus. Based, on current knowledge, the limited radiation dose obtained during occasional air travel during pregnancy confers very small risks to the fetus.

Radiation dose is measured in milliSieverts (mSv). Most Cathay Pacific Airways flights originate or terminate at low latitudes, so we are more fortunate in this respect than airlines based at higher latitude.  An estimate of the radiation dose for a specific flight can be obtained from the following and other websites:
Some values for flights calculated by computer programs are:


Hong Kong - New York High Solar Activity Year 0.0688 mSv
New York - Hong Kong High Solar Activity Year 0.0619 mSv
Hong Kong - New York Low Solar Activity Year 0.0938 mSv
New York - Hong Kong Low Solar Activity Year 0.0817 mSv
Hong Kong - Vancouver High Solar Activity Year 0.0414 mSv
Vancouver - Hong Kong High Solar Activity Year 0.0465 mSv
Hong Kong - Vancouver Low Solar Activity Year 0.0501 mSv
Vancouver - Hong Kong Low Solar Activity Year 0.0551 mSv
Hong Kong - Bangkok High Solar Activity Year 0.0046 mSv
Bangkok - Hong Kong High Solar Activity Year 0.0041 mSv
Hong Kong - Bangkok Low Solar Activity Year 0.0047 mSv
Bangkok - Hong Kong Low Solar Activity Year 0.0043 mSv

Guidelines concerning radiation exposure have been established by international agencies involved in radiation protection.  In general they recommend the following limits for flying:

  • For occupational exposure on commercial air flights (for example pilots and cabin crew) 20mSv per year
  • For the general public on commercial air flights 1mSv per year. This includes flying when pregnant.


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