Answer to Question #444 Submitted to "Ask the Experts"

Q

When flying on a commercial jet airliner at high altitudes (i.e., transcontinental or transatlantic), what is the x-ray exposure per hour?

A

The radiation exposure to passengers and crew in high-flying aircraft is caused not only by x rays (photons) but also by a variety of energetic particles such as neutrons, protons, electrons, muons, and pions. These radiation types are produced as a result of the interaction with the Earth's atmosphere of high-energy particles (primarily protons and alpha particles) that come from a variety of cosmic sources in our galaxy, with a lesser contribution from our own sun.

The galactic component of this incoming cosmic radiation is always present; the solar contribution varies in intensity over an approximately 11-year cycle. In fact, the galactic component is greatest at solar minimum and is reduced at solar maximum by solar particle interactions with irregularities in the magnetic field associated with the "solar wind." Additionally, there is a significant variation of dose rate with altitude and to a lesser extent with geomagnetic latitude.

During the last period of "solar minimum," at an altitude of 30,000 feet, the dose rate was about 4 μSv per hour at the latitudes of North America and Western Europe. During solar maximum, which is occurring now, the dose rate fell to around 3 μSv per hour. For the higher altitude of 40,000 feet, the dose rates were about 8μSv per hour at solar minimum and now are about 6.5 μSv per hour. To put this into perspective, the legal value of "maximum permissible dose" for members of the public exposed to radiation originating from ground-based industrial or medical facilities is 1,000 μSv per year. So an airline passenger flying at an average altitude of 35,000 feet for a period of about 160 hours (75,000 miles) during solar minimum would receive an exposure at about the limit of the current acceptable level.

Of course, most people who fly 75,000 miles a year or more do so because of their professional responsibilities as business travelers. It is my contention that the almost 450,000 individuals in the United States who fall into that category should be classified formally as occupationally exposed workers and that they should receive appropriate education about their exposures, particularly if they may be, or are about to become, pregnant. In addition to the general cosmic-ray "background" discussed above, there are rare solar particle events ("solar storms") that can significantly elevate the dose rates at airliner altitudes.

The National Oceanic and Atmospheric Administration (NOAA) estimates that in an 11-year solar cycle there could be up to three events that might produce a dose rate of up to 200 μSv per hour for a few hours at airliner altitudes. The most recent significant particle event occurred on 14 July 2000. Although an exact value of the maximum dose rate has not yet been established, my estimate is that it was at least 50 μSv per hour extending over the relatively long period of almost a full day.

Robert J. Barish, PhD, CHP, DABR, DABMP, FAAPM