High-flyers face increased risk of cancer thanks to cosmic rays
Cosmic radiation comes from outside the Earth's atmosphere - and the higher you fly and the further north or south you fly, the more likely you are to be exposed to it.
A flight across the poles exposes you to twice as much cosmic radiation as a flight across the equator because the atmosphere is thinner and the Earth's magnetic field is less effective in deflecting particles.
When you fly at 10,500 metres you are exposed to between 50 and 100 times more radiation than a person on the ground. A transatlantic flight exposes passengers to the equivalent radiation of at least one chest X-ray.
The amount of cosmic radiation at aviation altitudes varies. It follows an 11-year cycle, with the intensity of radiation being at its lowest when solar activity is at its highest, the Sun's stronger magnetic field deflecting the particles. Radiation is a risk to humans because cells may be altered as a result of being irradiated, and may subsequently become cancerous.
On some airlines, flight and cabin crew wear badges to monitor cumulative radiation exposure. On Cathay Pacific flights, data is drawn from general cosmic radiation reading and flight rosters to give readings for individual staff.
Radiation exposure is measured in Sieverts and the yearly normal dosage for the average person is estimated at two to three milliSieverts - or thousandths of a Sievert.
As a guideline for airline passengers, the International Commission on Radiological Protection recommends a limit of 1 milliSievert per year - equating to around 200 hours flying time a year on regular routes.
Studies suggest pilots and flight attendants on some airlines are exposed to the same amount of radiation as nuclear power plant workers.
In Finland, where flights over high latitudes are the norm, a 1995 study found flight attendants were twice as likely to contract breast cancer than women who do not fly regularly.
Cosmic radiation can damage cells' DNA, leading to possible cancers. The body can usually fix the damage, but higher radiation does increase the risk
The high-energy particles are most concentrated at the poles. As the radiation is absorbed by the atmosphere, flying at attitude increases the dosage
Scientists believe cosmic rays originate from exploding stars (supernovas) and pulsars - colapsed, spinning stars. The radiation is made of charged particles moving at near the speed of light and gamma rays