"We know that cosmic radiation at aircraft altitudes is several orders of magnitude more intense than that experienced at ground level because there is less protection from our atmosphere", says Bob Bentley, project scientist at MSSL. But scientists are currently unsure how much of the radiation penetrates into the cabins of aircraft and what risk it presents. The team is also keen to find out if the solar cycle affects levels of radiation inside aeroplane cabins, particularly as the sun approaches a maximum in its activity this year.

Physicists from NPL will interpret radiation readings collected over a range of longitudes and latitudes to establish the 'radiation dose'. "The radiation dose from a transatlantic flight is similar to the dose from a chest x-ray", says Robert Hunter, a doctor working for the CAA. "For frequent fliers and aircrew the accumulated dose may be significant". The 'healthy worker' effect could previously have masked the true incidence of illness in aircrew: levels of illness among workers - who must be relatively healthy to do their jobs - are always lower than those among the general population, which includes people who are too ill to work.

EU regulations introduced in May this year require all member states to investigate the effects of cosmic radiation on air passengers and crew. The investigation comes as aeroplanes are flying at progressively higher altitudes, where they will be less protected by the earth's atmosphere - a trend set to continue with future generations of aircraft.