A single long international flight exposes you to about a week’s worth of the natural background radiation you would get on the ground, according to Air & Space magazine. That is no health concern for a passenger, even a pregnant one. The people who fly enough for it to matter are the crew.
The reason is the air above you. Earth’s atmosphere shields the planet from cosmic radiation, so the higher you go, the larger the dose. The dose doubles with every 6,000 feet of altitude.
Where the radiation comes from
Cosmic radiation is the collective name for particles and rays from the Sun and from other galaxies, the UK Civil Aviation Authority explains. It is ionising, which means it can knock charged particles from atoms and disrupt molecules in living cells, though cells repair most of that damage.
Route matters as well as height. Earth’s magnetic field deflects charged particles, so doses are higher at high latitudes, towards the magnetic poles. A route nearer the poles therefore carries a higher dose.
The Sun shifts the numbers too. Its output runs on a roughly 11-year cycle, and at solar maximum its magnetic field pushes more galactic radiation away from Earth. Doses then run about 20% below average, and about 20% above it at solar minimum.
What counts as too much
For scale, a person at ground level receives about 2.4 millisieverts of natural background radiation a year. The recommended limit for air crew is 20 millisieverts a year.
The US Federal Aviation Administration recommends, rather than requires, a five-year average of 20 millisieverts a year for crew, with no more than 50 in any one year. Its advisory circular dates from 1994.
Pregnancy is where crew run into the limits fastest. A pregnant flight attendant working a London-to-Chicago route would go past the safe exposure for her foetus after about 100 flying hours, roughly 12 trips.
The UK rule
In the UK the protections are law. The Air Navigation (Cosmic Radiation: Protection of Air Crew and Space Crew and Consequential Amendments) Order 2019 came into force on 7 August 2019. It brought in the parts of the EU’s 2013 radiation safety directive that cover aircrew.
A crew member counts as a relevant crew member if their duties could give them an effective dose of more than 1 millisievert of cosmic radiation in a calendar year. Ground-level radiation does not count towards it.
When the Sun flares
Most of the Sun’s own radiation is deflected and of no direct concern, the CAA says, except during exceptional solar events. Those are rare but can be large. US Department of Energy scientists estimated that the strongest solar event on record, a flare in 1956, would have pushed exposure on a high-flying supersonic jet to 10 millisieverts an hour.
