We are all exposed to solar and cosmic radiation that penetrates our atmosphere. Aircraft are exposed even more so because at higher altitudes the atmosphere is thinner and so less radiation is reflected back into space. The fuselage of an aircraft offers little protection from this radiation, so when we fly do we put ourselves at risk of cancer?

Firstly, some flights are more exposed to radiation than others. Obviously the duration of a flight has a significant effect but also average altitude and the flight path will change our risk. Flights around the poles will be more exposed to radiation because the atmosphere is thinner. Many studies of radiation exposure on aircraft personnel focus on Scandinavian airlines as a higher risk group.

A 1993 study measured average ionizing radiation exposure for long haul flights to and from the United States. Flying from London to New York would give approximately 42 microSv, with an airtime of 6.8 hours and an average altitude of 37,000 feet. There are slight variations depending on the direction of flight. Although there are no statistics in this paper for New York to London (the flight the tweet refers to), we have measurements for Dallas to London (42 microSv) and London to Dallas (46 microSv). On longer flights the variation is more considerable. New York to Tokyo (13 hours airtime, average 38,000 feet) exposes you to 86 microSv, and the return flight (12.2 hours, 37,000 feet) is 78 microSv.

A chest X-ray exposes you to 20 microSv, a brain scan 7000 microSv (7mSv) and a bone scan 4000 microSv (4mSv). You are exposed to about twice as much radiation on a long haul flight from London to New York as a chest X-ray. Even with the directional variations the measurement would only change by a few microSv, it certainly wouldn’t halve. If you’re planning on booking a holiday with almost exactly the same exposure levels as a chest X-ray may I recommend Los Angeles to Honolulu (5.2 hours, 35,000 feet) at 19 microSv. Alternatively, if you’re pushing the boat out and are looking for a holiday that exposes you to the same levels of radiation as a brain scan try flying from London to Los Angeles (10.5 hours, 37,000 feet, 69 microSv) 101 times.

Does this constitute a health risk? The average annual UK dose from cosmic radiation at ground level is 260microSv. The average annual UK dose from natural background radiation is almost 10 times as high (2200microSv). Exposure to cosmic radiation is unlikely to cause any health problems. Those who you would consider most at risk would be aircraft personnel, but many studies have shown that exposure levels fall well below UK guidelines for occupational dose limits.